Physics 101 Review
The staging shown weighs 300 N and supports two painters, one 250 N and the other 300 N. The reading on the left scale is 400 N. what is the reading on the right-hand scale?
450 N The upward forces are (400 N + RH tension). By the equilibrium rule S F = 0, this upward total must equal the downward forces are (250 N + 300 N + 300 N) = 850 N. Hence, RH tension must be 450 N.
Half a second after starting from rest, a free-falling object will have a speed of about 10 m/s 20 m/s 5 m/s 0 None of these
5m/s
A certain car goes from rest to 100 km/h in 12 s = 1/300 h. What is its acceleration?
(100 - 0) km/h/ (1/300) h = 30,000 km/h2
A 2-kg chunk of putty moving at 2 m/s collides with and sticks to a 6-kg bowling ball initially at rest. The bowling ball and putty then move with a speed of 0 0.5 m/s 1 m/s 2 m/s More than 2 m/s
.5m/s Momentum before = momentum after (2kg)(2m/s) +0 = ((2+6)kg) v So v=4/8 = 0.5 m/s
Consider again the 120-lb person who steps on some bathroom scales. What is the net force on the bathroom scales?
0 Because the scales are at rest . There are two forces on the scales: the downward weight of the person, exactly balanced by the support force from the floor.
What is the acceleration of a cheetah that zips past you going at a constant velocity of 60 mph?
0 Constant velocity means no change in velocity i.e. no acceleration
Motorboats cross a river pointing in the three directions shown. The boats all have the same speed relative to the water, and all experience the same water flow. 1. Which boat reaches the opposite shore first? 2. Which boat provides the fastest ride? 3. Which boat ends up straight across from where it started?
1. the one headed straight across, since the velocity of the motor is entirely straight across, not "wasted" by going up or down the river. Note that it will end up downstream 2. The boat headed diagonally downwards because it has the largest resultant velocity vector 3. The one headed diagonally upwards because the resultant velocity is straight across, perpendicular to the current.
A player catches a ball. Consider the action force to be the impact of the ball against the player's glove. The reaction to this force is player's grip on the glove Force the glove exerts on the ball Friction of ground against player's shoes Muscular effort in the player's arms None of these
b) Force of object 1 on object 2 = -(Force of object 2 on object 1) - these two forces make action-reaction pair...
If all people, animals, trains and trucks all over the world began to walk or run towards the east, then the
earth would spin a bit slower The total momentum of people etc + earth must remain the same because there is no change in the external force - the running to the east is an internal force to the people + earth system. Since the people gain momentum to the east, then the earth must lose momentum in the east direction, therefore it spins a bit slower. (Note that the earth spins towards the east)
How much gravitational potential energy does a quarter-pound hamburger vertically raised 1 m have?
grav. PE = mgh = ¼ (4.4N)(1m) = 1.1 J (recall 1-lb is about 4.4 N) So this is the work done in vertically lifting it 1m, and hence the power needed to do this in 1s is Power = W/t = 1.1 J/1s = 1.1 W (c.f earlier statement)
Two smooth balls of exactly the same size, one made of wood and the other of iron, are dropped from a high building to the ground below. The ball to encounter the greater force of air resistance on the way down is ...
the iron ball Air resistance depends on both the size and speed of a falling object. Both balls have the same size, but the heavier iron ball falls faster through the air and encounters greater air resistance in its fall. Be careful to distinguish between the amount of air drag and the effect of that air drag. If the greater air drag on the faster ball is small compared to the weight of the ball, it won't be very effective in reducing acceleration. For example, 2 newtons of air drag on a 20-newton ball has less effect on fall than 1 newton of air drag on a 2-newton ball
If all forces come in pairs, then how come the earth doesn't accelerate towards apples falling from trees?
Actually it does! But it is not noticeable or measurable because the earth's mass is so large
What horizontally-applied force will accelerate a 200-kg box at 1 m/s2 across a floor against a friction force of 400N? 200 N 400 N 600 N 2400 N None of these
600N Fnet = Fapplied - friction = ma So, Fapplied = ma + friction = 200 + 400 = 600 N
As you're sitting in your chair, your weight acts as a downward force on the chair. Why then does the chair not sink into the ground? A) because of its weight B) because it feels an upward directed support force from the ground it is pushing down on C) because of inertia - the resistance to changes in motion. D) because of momentum conservation.
Answer: B, the chair is in equilibrium, there is zero net force on it. The gravitational force is equal and opposite to the upward support force.
When the ball at the end of the string swings to its lowest point, the string is cut by a sharp razor. What path will the ball then follow?
At the moment the string is cut, the ball is moving horizontally. After the string is cut, there are no horizontal forces, so the ball continues horizontally at constant speed. But there is the force of gravity which causes the ball to accelerate downward, so the ball gains speed in the downward direction. The combination of constant horiz. speed and downward gain in speed produces the curved (parabolic) path..
A carnival has a Ferris wheel where the seats are located halfway between the center and outside rim. Compared with a Ferris wheel with seats on the outside rim, your angular speed while riding on this Ferris wheel would be A) more and your tangential speed less. B) the same and your tangential speed less. C) less and your tangential speed more. D) None of the above
B Same # cycles per second i.e. same angular speed, but less distance covered each second, so less tangential speed
A 20-kg girl and her 40-kg big brother are each sitting on a sled, waiting to be pushed across the snow. To provide them with equal horizontal acceleration, we would have to push with A) equal forces B) twice as much force on the boy C) twice as much force on the girl D) four times as much force on the boy E) none of these
B Since a = Fnet/m, an object with twice the mass requires twice the force in order to experience the same acceleration.
Newton's Third Law is most closely related to momentum conservation angular momentum conservation energy conservation inertial resistance to changes in motion
Momentum Conservation Momentum conservation follows directly from Newton's third law as explained directly in class lectures..
A large heavy truck and a small baby carriage roll down a hill. Neglecting friction, at the bottom of the hill, the truck will have a greater A) speed. B) momentum. C) acceleration. D) all of these E) none of these
B They'll have same acceleration and same speed (if they started with the same speed). The truck will have more momentum as its mass is larger.
Padded dashboards in cars are safer in an accident than nonpadded ones because an occupant hitting the dash has A) increased momentum. B) increased time of impact. C) decreased impulse. D) decreased time of impact.
B, same change of momentum to final zero, but done over a longer time means the force is less.
A pen at rest. It is made up of millions of molecules, all pulling on each other - cohesive inter-atomic forces present in any solid. Why doesn't the pen accelerate spontaneously due to these forces?
Because each of these is part of an action-reaction pair within the pen. They always add to zero within the system. The pen will remain at rest unless an external force acts on it.
Earth is moving around the sun at 30 km/sec = 107 000 km/h. So, if I stand near a wall, and jump up in the air for a few seconds, why doesn't the wall slam into me??
Because of inertia. While standing on the ground, I am moving along with the earth at 30 km/s, and when I jump, I (and the air) continue moving (sideways) at 30 km/s.
According to Newton's law of inertia, a rail road train in motion should continue going forever even if its engine is turned off. We never observe this because railroad trains A) move too slowly. B) must go up and down hills. C) are much too heavy. D) always have forces that oppose their motion How about if there was no friction and no air drag - then what would happen once the engine is turned off?
D Then the train would be accelerating while the engine was on, and would stop accelerating once the engine is turned off - i.e would move at constant velocity.
As a 1-kg ball falls, the action force is the 10-N pull of the Earth's mass on the ball. The reaction force is A) air resistance acting against the ball, but less than 10N B) acceleration of the ball C) pull of the ball's mass on the earth, but less than 10-N D) pull of the ball's mass on the earth, equal to 10-N E) None of these
D Every force can be thought of as part of an action-reaction pair: Force that object I exerts on object II is equal and opposite to the force that II exerts on I. So here the reaction force is the gravitational pull on Earth exerted by the ball, and it is 10-N upwards
Consider balancing a hammer upright on the tip of your finger. Would it be easier to balance the head on your finger or the handle on your finger, and why?
Easier with the handle because it has more rotational inertia (heavy part further away from your finger), so is more resistant to a rotational change.
What are the action-reaction pairs once the ball is in the air?
First note there are two interactions: (i) one with the earth's gravity and (ii) the other with the air. Action (or reaction): Earth pulls down on ball (weight) Reaction (or action): Ball pulls up on Earth. (ii) Action: Air pushes ball backwards (air resistance) Reaction: Ball pushes air forwards
An airplane makes a straight back and forth round trip, always at the same airspeed between two cities. If it encounters a mild steady tailwind going, and the same steady headwind returning, will teh round trip take
More Suppose the cities are 600 km apart, and the airspeed of the plane is 300 km/h (relative to still air). Then time each way with no wind is 2 hours. Roundtrip time is 4 hours. Now consider a 100 km/h tailwind going, so groundspeed is (300 + 100) km/h. Then the time is (600 km)/(400km/h) = 1 hour and 30 minutes. Returning groundspeed is (300 - 100) km/h, and the time is (600 km)/(200km/h) = 3 hours. So the windy round trip takes 4.5 hours—longer than with no wind at all
How much more distance do you need to come to a complete stop when you slam on the brakes while going at 90 km/h compared to 45 km/h ? (Note that the frictional force the road exerts does not depend on speed). Half the distance The same Twice the distance Four times the distance Need more information in the problem
Four times the distance W = F d = (Delta)(KE), where F is the friction force. Since, for speeds twice as large, the KE is four times as large, this means the stopping distance is also four times as large.
A large person and a small person wish to parachute at equal terminal velocities. The larger person will have to Jump first from the plane Pull upward on the supporting strands to decrease the downward net force Jump lightly Get a larger parachute Get a smaller parachute
Get a larger parachute If he does nothing, the larger person would accelerate for longer and have a larger terminal velocity. So he needs to do something to decrease this and effectively increase air resistance - i.e. get larger parachute.
In a vacuum, a coin and feather fall side by side, at the same rate. Is it true to say that, in vacuum, equal forces of gravity act on both the coin and the feather?
NO! They accelerate together because the ratio weight/mass for each are equal (=g). There is a greater force of gravity on the coin, but its mass (inertia) is greater too.
Dropping down from a pole. As he dives, PE becomes KE. Always total energy constant. If accounted for air resistance, then how would the numbers change? What happens to the energy when he hits the ground?
In presence of air, some energy gets transformed to heat (which is random motion of the air molecules). Total energy at any height would be PE + KE + heat, so at a given height, the KE would be less than in vacuum. PE would be the same for same height. Just before he hits ground, he has large KE (large speed). This gets transformed into heat energy of his hands and the ground on impact, sound, and energy associated with deformation
When a rock thrown upwards falls back down and passes the same point it was thrown from, its velocity is zero and its acceleration is zero its velocity is zero and its acceleration is about 10 meters per second per second its velocity is about 10 m/s and its acceleration is zero its velocity is negative of the initial velocity it was thrown up with and its acceleration is about 10 meters per second per second.
Its velocity is negative of the initial velocity it was thrown up with and its acceleration is about 10 meters per second per second. Note, acc. in free-fall (on earth) is always g~10 m/s2
The orange fish has mass 4-kg, and the purple one has mass 1-kg. If the orange fish is swimming at 2 m/s towards the purple fish at rest, what is the speed of orange fish after he swallows him? Neglect water resistance. If instead the purple fish sees the orange fish coming, and swims away at 1m/s, then what is the speed of the orange fish, after he catches up and swallows him?
Net momentum before = net momentum afterwards (4 kg)(2 m/s) + (1 kg)(0) = ((4+1)kg) v 8 kg m/s = (5 kg) v . So v = 8/5 m/s = 1.6 m/s Net momentum before = net momentum afterwards 4 kg)(2 m/s) + (1kg)(1m/s) = ((4+1)kg) v 9 kg m/s = (5 kg) v . So v = 9/5 m/s = 1.8 m/s
When in a subway car that suddenly stops, you lurch forward. What's the best explanation for this? Because of Newton's 1st law - you have inertia. Because you have no acceleration Because of action-reaction forces between the car and you Because of the support force
Newton's 1st Law Newton's 1st law says that objects (you) tend to keep moving in a straight line; inertia resists changes in motion
A 10 kg bag of rice weighs one-sixth as much on the moon than on earth because the moon's gravity is one-sixth as much as the earth's. If you tried to slide the bag horizontally across a smooth table to a friend, is it one-sixth as easier on the moon than on earth? (ignore friction)
No! The same horizontal force is needed, since the mass (inertia) of the bag is the same.
A 1900 newspaper editorial stated it is impossible to launch a rocket above the Earth's atmosphere. Do you agree?
No, it propels forward under reaction force to the action of rocket expelling the exhaust gases. Just like the balloon demo, or rifle recoil. The reaction to exhaust gases does not depend on a medium for the gases. In fact, in a vacuum there is no air drag and rocket operates even better.
Which will roll down a hill faster, a can of regular fruit juice or a can of frozen fruit juice? Regular Frozen Depends on the relative sizes and weights of the cans?
Regular The regular fruit juice has an appreciably greater acceleration down an incline than the can of frozen juice: Because the regular juice is a liquid and is not made to roll with the can, as the solid juice does. Most of the liquid effectively slides down the incline inside the rolling can. The can of liquid therefore effectively has very little rotational inertia compared to its mass i.e the liquid doesn't rotate. The solid juice, on the other hand, is made to rotate, giving the can more rotational inertia.
Is the impulse to stop a 10 kg bowling ball moving at 6 m/s less, greater or the same, if it is done in 1s rather than 2s? Is the force you must exert to stop it less, greater, or the same, if done in 1s or 2s? In a general situation, when does impulse equal momentum?
Same, since impulse = change in momentum is the same whatever the time it takes. Twice as great force if you do it in 1s than if you do it in 2s, because change in momentum = impulse = FDt. (so half Dt means twice F) If the object's initial momentum is zero, then impulse = momentum change = final mom. - initial mom. = final mom. Likewise, if object is brought to rest, then impulse = - initial momentum
Since weight = mg = force of gravity on an object, heavier objects experience more gravitational force - so why don't they fall faster than lighter ones ?
The acceleration depends both on the force and the mass -- heavier objects also have a greater inertia (resistance to acceleration), a larger mass. In fact mass cancels out of the equation:
In 2 s, a car increases its speed from 60 km/h to 65 km/h while a bicycle goes from rest to 5 km/h. Which undergoes the greater acceleration? What is the average speed of each vehicle in that 2 s interval, if we assume the acceleration is constant ?
The accelerations are the same, since they both gain 5 km/h in 2s, so acceleration = (change in v)/(time interval) = (5 km/h)/(2 s) = 2.5 km/h.s (note units...) For car: 62.5 km/h For bike: 2.5 km/h
What is the reaction force to a bat striking a ball?
The ball's hit on the bat. Note that we could call either the action and the other the reaction - it doesn't matter which one is which.
Why should you not leave the top drawers of a heavy cabinet open while the others are closed ? The center of mass becomes too high to be stable, so it would tip The center of mass would extend in front of the cabinet, beyond the support base, so it would tip The torque exerted by the gravitational force on the drawers decreases It looks messy
The center of mass would extend in front of the cabinet, beyond the support base, so it would tip Objects are stable if a vertical line dropped from their COM goes through the base of the object. Leaving the top drawers open makes it look like the G shape on previous slide
Which has greater kinetic energy, an adult running at 3 mi/hr or a child of half the mass running at 6 mi/hr? A) the adult B) the child C) Both have the same kinetic energy. D) More information is needed about the distance traveled.
The child KE = ½ mv2, so for child, mass halved but v doubled means KE is doubled.
Can an object have zero acceleration but non-zero velocity ?
Yes, this mean it is moving at constant in a constant direction.
Which encounters greater force of air resistance, a falling elephant or feather?
The elephant There is a greater force of air resistance on the falling elephant, which "plows through" more air than the feather in getting to the ground. The elephant encounters several newtons of air resistance, which compared to its huge weight has practically no effect on its rate of fall. Only a small fraction of a newton acts on the feather, but the effect is significant because the feather weighs only a fraction of a newton. The elephant has larger acceleration.
The earth's gravity pulls on the moon. What is the reaction force to this?
The moon's pull on the earth
Consider the meter sticks shown, one with a glob of clay at the top end. If released from upright position, which reaches the ground first? The one without the clay The one with the clay They both reach the ground together
The one without the clay! Try it! Because the stick-with-clay has more rotational inertia, so resists rotation to ground more.
Two balls are thrown from the same point high on a cliff. One is thrown upwards and the other is merely dropped from rest. Neglecting air resistance, which has the higher acceleration? The ball thrown upwards The ball dropped from rest It depends on how fast the thrown ball was thrown It's the same for each None of these
The same acceleration = g downwards
Since action and reaction are equal and opposite, why don't they cancel to give zero effect?
They act on different objects
We just said that for a ball in the air, an action-reaction pair is Earth's gravity acting on the ball, and the ball pulling the Earth up. Which force is bigger?
They have the same strength Action and reaction pairs are always equal and opposite in direction. Again, remember to distinguish the force from the effect of the force - the Earth's acceleration under the interaction with the ball is much smaller than that of the ball, because Earth's mass is much larger and a = F/m.
When two objects collide, their total momentum is always conserved their total energy is always conserved their total energy and momentum are sometimes conserved their total energy and momentum are always conserved unless heat is generated more than one of the above is true none of the above is true
Total momentum is always conserved A Energy is conserved only sometimes, when the collision is elastic
Two smooth tracks of equal length have bumps - A up and B down, both of the same curvature. If two balls start simultaneously with the same initial speed, the ball the to complete the journey first is along Does the gain in speed at B's bottom equal the loss at A's top?
Track B Although both balls have the same speed on the level parts of the tracks, the speeds along the curved parts differ. The speed of the ball everywhere along curve B is greater or same than the initial speed, whereas everywhere along curve A it is less. So the ball on Track B finishes first No! Speed isn't conserved: energy is. The loss in kinetic energy at the top of A will be equal to the gain in kinetic energy at the bottom of B—if there is enough energy to begin with.
A 1000-kg car and a 2000-kg car are hoisted up the same distance. Raising the more massive car requires Less work Twice as much work Four times as much work As much work More than four times as much work
Twice the work Work done = gain in potential energy = mgh. So twice the mass means twice the PE, means twice the work
Michael Jordan's best hang-time was 0.9 s - this is the time the feet are off the ground. Let's round this to 1 s. How high can he jump?
Use d = ½ g t^2 . For 1 s hang-time, that's ½ s up and ½ s down. So, substituting d = ½ (10) (1/2)^2 = 1.25 m
When the pellet fired into the spiral tube emerges, which path will it follow? (Neglect gravity).
While in the tube, the pellet is forced to curve, but when it gets outside, no force is exerted on the pellet and (law of inertia) it follows a straight-line path - hence, B.
Can an object have zero velocity but non-zero acceleration?
Yes Throw a ball up in the air - at the top of its flight, as it turns around it has momentarily zero speed but is changing its direction of motion, so has non-zero acceleration.
The captain of a high-flying airplane announces that the plane is flying at a constant 900 km/h and the thrust of the engines is a constant 80 000 N. What is the acceleration of the airplane? What is the combined force of air resistance that acts all over the plane's outside surface? Now consider take-off. Neglecting air resistance, calculate the plane's acceleration if its mass is 30 000 kg, and the thrust at take-off is 120 000 N.
Zero, because velocity is constant 80 000 N. Since, if it were less, the plane would speed up; if it were more, the plane would slow down. Any net force produces an acceleration. a = F/m = (120 000 N)/(30 000 kg) = 4 m/s2
Two cars of mass m are move at equal speeds v towards each other. What is their combined momentum after they meet? a) 0 b) mv c) 2mv d) None of these
a) Net momentum after = net mom. before = 0
Say a 120-lb person steps on some bathroom scales. a. How much is gravity pulling on her ? b. What is the net force on her? c. Now suppose she stands on two bathroom scales, with weight evenly divided between them. What will each scale read?
a. 120lbs or 120N since pounds = gravitational force b. 0 since she is at rest c. 60 to evenly distribute the weight.
An automobile and a baby carriage traveling at the same speed collide head-on. The impact force is A) greater on the baby carriage. B) greater on the automobile. C) the same for both.
c) The same for both - action/reaction Similarly, same momentum change for both
Two smooth tracks of equal length have "bumps"- A up, and B down, both of the same curvature. if the initial speed = 2m/s and the speed of the ball at the bottom of the curve on Track B is 3m/s, then the speed of the ball at the top of the curve on Track A is 1m/s >1m/s <1m/s
<1m/s The change in the speed in either track comes from change in the kinetic energy that happens because potential energy changes (change in height) while total energy remains constant. For track B, at the bottom, gain in KE = mgh = ½ m(3^2 -2^2) = ½ m(5). This means for track A, loss in KE = ½ m(5) at the top - but this is greater than the initial KE ½ m (2)^2 = ½ m(4). So the ball actually never makes it to the top of A's curve!
If a projectile is fired straight up at a speed of 10 m/s, the total time to return to its starting position is about A) 2 seconds. B) 10 seconds. C) 20 seconds. D) 1 second. E) not enough information to estimate
A Each second it loses about 10m/s so after 1s, it has zero velocity, i.e. is turning around. Then it gains 10m/s as it falls, so after another second (a total of 2 s) it has -10m/s i.e. same initial speed at which it began, hence returned to the same point.
As an object freely falls downward, its A) velocity increases B) acceleration increases. C) both of these D) none of these.
A It accelerates at either (i) a constant rate =g, if there is negligible air resistance or (ii) gradually less acceleration if there is air resistance.
A girl pulls on a 10-kg wagon with a constant horizontal force of 20 N. If there are no other horizontal forces, what is the wagon's acceleration in meters per second per second? A) 2.0 B) 0.5 C) 20 D) 200 E) None of the above
A since a = F/m = 20/10 = 2 m/s2
In which situation is the object in equilibrium?
A man cycling down a straight road at constant speed. Equilibrium means all forces balance, i.e. add to zero, i.e. zero net force on the object. Then, an object at rest remains at rest, or an object moving at constant speed in a straight line keeps going at constant speed in the same straight line. There are forces acting on the man going at constant speed, but they must cancel to zero since otherwise he would be changing his speed...
Which of the following has the largest momentum relative to Earth? A tightrope walker crossing Niagara Falls A pickup truck speeding along a highway A Mack truck parked in a parking lot The science building on campus A dog running down the street
A pickup truck speeding along a highway Momentum = m v Anything stationary has zero momentum.
A rock weighs 30 N on Earth. A second rock weighs 30 N on the moon. Which of the two rocks has the greater mass? A) the one on the moon B) the one on Earth C) They have the same mass. D) not enough information to say
A) Because weight =mg, and g least on moon, so m must be bigger to get the same weight of 30N
Several balls of different masses are dropped from the 15th floor of the North Building. Neglecting air resistance, the quantity that has the same value for each ball energy acceleration momentum force exerted upon striking the ground All of the above
Acceleration In free-fall all objects fall at the same rate, a = g. The velocity will also be the same for all balls, but the momentum will not, since momentum = mv and mass is different for each ball. Also their energies are different because their mass is different, and the force exerted upon striking the ground will also be different because of this.
As she falls faster and faster through the air, her acceleration (air resistance can not be ignored)
Acceleration decreases because the net force on her decreases. Net force is equal to her weight minus her air resistance, and since air resistance increases with increasing speed, net force and hence acceleration decreases. By Newton's 2nd law,
Whenever an interaction occurs, in a system, forces occur in equal and opposite pairs. Which of the following do not always occur in equal and opposite pairs? Impulses Accelerations Momentum changes All of these occur in equal and opposite pairs None of these do
Acceleration Because time for each interaction part is the same, impulses and momentum changes also occur in equal and opposite pairs. But not necessarily accelerations, because the masses of the interaction may differ. Consider equal and opposite forces acting on masses of different magnitude.
Why do acrobats usually use safety nets?
Because falling into a safety net increases impact time compared to falling to floor, so less force is felt The impulse is the change in momentum, which is however fast the acrobat was going before (s)he came to rest. So this isn't changed by a safety net. The net increases the time for this change in momentum to occur, and since this change = impulse = F t, this means F is less.
Two parachuters, green man heavier than blue man, each with the same size of chute. Let's ask a series of questions: 1)First ask, if there was no air resistance, who would get to ground first? (2) They both begin to fall together in the first few moments. For which is the air drag force greater? (3) Who attains terminal velocity first? i.e. who stops accelerating first? (4) Who has larger terminal veloc so who reaches ground first?
Both at the same time. R depends on area - same for each, and speed - same for each. So initially both experience the same drag force R When R becomes equal to the weight, then there is zero net force. Since blue's weight is less, blue attains terminal velocity first. Green, he reaches his terminal velocity later, after acc. longer, so is faster...
Suppose you and a pair of life preservers are floating down a swift river, as shown. You wish to get to either of the life preservers for safety. One is 3 meters downstream and the other is 3 meters upstream from you. Which can you swim to in the shortest time?
Both require the same time. You, and both life preservers are moving with the current - relative to you before you start swimming, neither of the life preservers are moving.
In the head-on collision between the garbage truck and a mini car: a) Which experiences the greater impulse? Which experiences the greater momentum change? Which experiences the greater acceleration?
Both same (same force over same time interval) Both same (momentum of system conserved, so momentum change of truck is equal and opposite to the momentum change of the car) The car (smaller mass)
A garbage truck and a mini car have a head-on collision. Which vehicle experiences the greater force of impact?
Both same. Newtown's 3rd Law of action and reaction.
How can a skydiver decrease his terminal speed during fall?
By spreading out (increase frontal area) i.e. make body horizontal with arms and legs spread out
A skier covers a distance of 3 m in half a second. What is his average speed? A) 1.5 m/s B) 3 m/s C) 6 m/s D) 9 m/s
C) Average speed =distance/time = 3m/0.5s = 6 m/s
A hockey puck is set in motion across a frozen pond. If ice friction and air resistance are neglected, the force required to keep the puck sliding at constant velocity is equal to the product of its mass times its weight. B) equal to its weight divided by its mass. C) zero. D) equal to its weight
C, since no acceleration - so no net force
Consider a large cannonball and a small ping-pong ball, each being dropped from the same point on a tree. Which experiences the greater air resistance force as it falls? The ping-pong ball The cannonball The same on each
Cannonball Air resistance force is greater for greater speeds and for greater sizes. Cannonball has greater size, and also has greater speed... (Remember to distinguish btn force and its effect -- acceleration)
A marble is rolling down an incline, starting from rest at the top. At what point is its kinetic energy equal to its potential energy? At the top At the bottom Halfway down A quarter of the way down
Halfway From energy conservation: at the top, zero KE and PE = mgh where h is the height of the incline. As it rolls, the marble loses PE which turns into KE, and it speeds up. At half the height of the incline, hh/2 so the PE is halved and, the other half of the PE has become KE.
This poor guy is pushing really hard on the wall but it won't budge. What can you say is true? He exerts a large force on the wall but does zero work on it. He exerts zero force on the wall and does zero work on it. He exerts a large force on the wall and does a large amount of work on it. None of the above is true
He exerts a large force on the wall but it does not move as there is an equally large frictional force from the ground in the opposite direction. As the wall does not move there is no work done (W=F.d) on the wall.
You're standing on a log while a friend tries to knock you off by throwing balls to you. Should you try to catch the ball, or let it bounce off you, in order to try not to fall off the log? (analyze in terms of change in momentum)
If you catch the ball, ball changes its mom. from mv to 0. Whereas, if you let it bounce off you, ball reverses direction of momentum, so change in momentum is twice as large. So it's better to try to catch it, as there is less change.
If all of Earth's inhabitants moved to the equator, what would happen to the length of a day? The day would be longer The day would be shorter The length of the day would remain the same There would be no day, only night
Longer days If mass moves to the equator, it is further from the axis of rotation, so the rotational inertia of the earth increases. Because the angular momentum of the earth is conserved, this means its rotational speed decreases, i.e. the length of the day would be longer.
What can you say about the relative magnitudes of the forces if is moving with unchanging speed across the floor ?
Magnitude of weight = support force. Your push = friction, if speed unchanging. (If it is speeding up, then your push > friction.)
Say the garbage truck weighs 15 000-kg, and the mini car weighs 1000 kg. Let's say the truck is initially moving at 30 km/h and the car is at 60 km/h. If the two stick together after the collision, then what is their speed after the head-on collision?
Momentum conservation means: mom. of truck before + mom. of car before = mom of (car+truck) after i.e. mt*vt - mc*vc = (mt+mc) v (- on left because opp dir) (15000)(30) - (1000)(60) = (16000) v So, v = 24.375 km/h
Consider for simplicity, the earth and moon as the only bodies in the universe. a) What is the net force on the earth? b) What is the net force on the earth+moon system.
The moon's gravitational pull Action and reaction cancel within the system
Tracks A and B are made from pieces of channel iron of the same length. They are bent identically except for a small dip near the middle of Track B. When the balls are simultaneiously released on both tracks as indicated, the ball the end of the track is on.
Track B The ball to win the race is the ball having the greatest average speed. Along each track both balls have identical speeds—except at the dip in Track B. Instantaneous speeds everywhere in the dip are greater than the flat part of the track. Greater speed in the dip means greater overall average speed and shorter time for a ball on Track B. Note that both balls finish at the same speed, but not in the same time. Although the speed gained when going down the dip is the same as the speed lost coming out of the dip, average speed while in the dip is greater than along the flat part of the track.
Consider a 1000-kg car going at 90 km/h. When the driver slams on the brakes, the road does work on the car through a backward-directed friction force. How much work must this friction force do in order to stop the car?
W = (Delta)(KE) = 0 - ½ m v2 = - ½ (1000 kg) (90 km/h)2 (1000 m/3600 s)2 = -312500 J = -312.5 kJ So W = 312.5 kJ
A father pushes his child on a sled on level ice, a distance 5 m from rest, giving a final speed of 2 m/s. If the mass of the child and sled is 30 kg, how much work did he do? What is the average force he exerted on the child?
W = (delta)(KE) = ½ m v2 = ½ (30 kg)(2)2 = 60 J W = F.d = 60 J, and d = 5 m, so F = 60/5 = 12 N
Consider pushing a box across a floor. What forces are acting on the box?
Weight downward, support force upward, your push across, and friction between the floor and the box opposing your push.
She holds the book stationary against the wall as shown. Friction on the book by the wall acts
can't say If she barely pushes the book so that the vertical component of her push is less than the book's weight, then friction acts upward to keep the book stationary. If she pushes so that the vertical component of her push equals the book's weight, then there's zero wall friction on the book. If she pushes harder so that the vertical component of her push exceeds the book's weight, then friction acts downward. So unless we know how the vertical component of her push compares with the weight of the book, we can't specify the direction of friction between the book and the wall.
If no external forces are acting on a moving object it will
continue moving at the same speed in the same direction
A ball is thrown up in the air. What is acceleration as it rises and falls?
g = 9.8 m/s2 downwards all the time The acceleration due to gravity is always g = 9.8m/s2 (near the surface of the earth) and points towards earth. When ball is thrown up, its speed decreases because acceleration (= rate of change of velocity) is in a direction opposite to its velocity. As it falls, it speeds up since acceleration is in the same direction as velocity.
If while floating in outer space you take your shoe off and throw it away, you and the shoe will move away from each other, but at different speeds move away from each other at the same speed move a short distance and then slow down move a short distance and then go faster
move away from each other, but at different speeds The total momentum of you + shoe is conserved, so your change in momentum is equal and opposite to the change in momentum of the shoe. Since the shoe weighs less, its speed is higher.
