physics test 3
A stone is thrown upward at an angle. What happens to the horizontal component of its velocity as it rises? As it falls?
Rising or falling, it does not change. Horizontal remain constant.
When your car moves along the highway at a constant velocity, the net force on it is zero. Why then, do you continue running your engine?
See the answer to question
Suppose that a freely falling object were somehow equipped with a speedometer. By how much would its speed reading increase with each second of fall?
10 m/s every second
What is the acceleration of an object in free fall at the earth's surface?
10 m/s heading downward due to gravity.
4. If you exert a horizontal force of 200 N to slide a crate across a factory floor at a constant velocity, how much friction is exerted by the floor on the crate? Is the force of friction equal and oppositely directed to your 200-N push? Does the force of friction make up the reaction force to your push? Explain.
200 N of friction is exerts by the factory floor. Its oposite to the directions of my force Although the frictional force is equal and opposite to the applied force it does not make up the reaction force to the applied push. "Pairs" of forces according to Newton's 3rd law always act on different objects.
17. How many forces are required for a single interaction?
A single interaction always requires two force since forces are created in pairs. When one object exerts a force on a second object, the second object exerts and equal an opposite force back on the first.v
Can you think of a reason why the acceleration of the object thrown downward through the air in the preceding exercise would actually be less than 10 m/s2 ?
If air resistance were present, the acceleration would be less than 10 m/s2, since the air resistance increases as the speed increases and act opposite the force of gravity.
Does the speedometer on a vehicle show average speed or instantaneous speed?
Instantaneous Speed It changes throughout the drive.
A stone is thrown upward at an angle. What happens to the vertical component of it velocity as it rises? As it falls?
It decreases while rising, but it increases while falling.
26. What law of physics is inferred when we say you cannot touch without being touched?
Newton's third law; every action has a reaction
19. A boxer can hit a heavy bag with great force. Why can't he hit a sheet of newspaper in midair with the same amount of force?
This is due to the Paper having less mass/inertia
7. For a freely falling object dropped from rest, what is the acceleration and velocity at the end of the fifth second of fall? At the end of the tenth second?
acceleration is -10 m/s. fifth second is -50 m/s tenth is -100 mls negative to show its moving down
9. If the mass of a sliding block is kept constant while the net force is doubled, how does the resulting acceleration compare to the original acceleration? How about if the net force where reduced by ½ instead?
double the accelration 1/2 the acceleration these are directly proportional
5. What is the name of the property of objects to maintain their states of motion?
inertia
A friend claims that bullets fired by some high-powered rifles travel for many meters in a straight-line path before they start to fall. Another friend disputes this claim and states that all bullets from any rifle drop beneath a straight-line path a vertical distance given by ½ g t2 and that the curved path is apparent for low velocities and less apparent for high velocities. Now it's your turn: Will all bullets drop the same vertical distance in equal times?
it will always fall at .5 then 2.0 then 4.5 then more. so no not same vertical distances. Horizontal will be different though.
9. One kilogram weighs 9.8 N on earth. Would it weigh more or less on the moon?
it will weight less, but the mass will stay the same.
Why do we say velocity is a vector and speed is not?
speed (scaler) doesn't account for direction
3. Which has more mass, a 2-kg fluffy pillow or a 3-kg small piece of iron? More volume? Why are your answers different?
the iron has more mass because 3 kg is more than 2 kg. The pillow has more volume because it's dimensions are bigger than the small piece of iron, although the iron has more mass. Volume = length x width x height.
21. Consider hitting a baseball with a bat. If we call the force on the bat against the ball the action force, identify the reaction force.
the reaction force is the force of the ball from the bat
10. When you place a heavy book on a table, the table pushes up on the book. Why doesn't this upward push cause the book to rise from the table?
the table uses a force to push up on the book. the book uses a force (gravity) to push down on the table. the two forces are Equal and cancel out
What is the acceleration of an object that moves at a constant velocity? What is the net force on the object in this case?
Acceleration is 0 m/s/s, Net force is zero. An object moving at constant velocity does not accelerate and does not have an unbalanced force acting on it.
6. Is acceleration directly proportional to mass, or is it inversely proportional to mass? Give an example.
Acceleration is inversely proportional to mass. Increasing mass means decreasing acceleration, for example
1. State the law of inertia
An object will continue in a state of rest or moving at a constant speed in a straight line unless acted on by a net force.
14. What is the acceleration of a falling object that has reached its terminal velocity?
At terminal velocity, the air resistance is exactly equal to the force of gravity and the object's velocity no longer changes. If there is no net force and no change in velocity, the acceleration must be zero.
17. A bowling ball sits at rest. Another ball rolls down a lane at constant speed. Which, if either, is in equilibrium? Defend your answer.
Both are in equilibrium. The ball at rest has zero acceleration as well as the ball moving at a constant speed in a straight line
Two golfers each hit a ball at the same speed, but one at 60 degrees with the horizontal and the other 30 degrees. Which ball goes farther? Which hits the ground first? (Ignore air resistance.)
Both balls have the same range (look at range fomula). The ball with the initial projection angle of 30 degrees, however, is in the air for a shorter time and hits the ground first.
11. What two quantities are necessary to determine a vector quantity?
Both magnitude and direction.
4. Which dominated Galileo's way of extending knowledge - philosophical discussion or experiment?
Experiment Aristotle used philosophical
1. When a ball rolls down an inclined plane, it gains speed because of gravity. When rolling up, it loses speed because of gravity. Why doesn't gravity play a roll when it rolls on a horizontal surface?
Gravity does not play a roll in horizontal motion because the force of gravity is always directed down in the vertical direction.
As you leap upward from the ground, how does the force that you exert on the ground compare to your weight?
If you move upward from the ground, the force that you exert on the ground must be greater than your weight since the ground must be pushing back up on you with the same force. If you move upward, this force must be greater than your weight.
If you drop an object, its acceleration toward the ground is 10 m/s2. If you throw it down instead, would its acceleration after throwing be greater than 10 m/s2? Why or why not?
In a perfect world (without air resistance) the acceleration after throwing would be the same, 10 m/s2. This is after the throw.
10. What is the net force that acts on a 10-N freely falling object?
In free fall, the only force acting is the force of gravity. The net force would be 10 N directed downward.
Why does the vertical component of velocity for a projectile change with time, whereas the horizontal component of velocity doesn't?
In projectile motion, gravity acts on the object in the vertical direction but there is no acceleration in the horizontal direction. Since there is no acceleration in the horizontal direction, the velocity in that direction does not change and the object covers the same horizontal distance each second that passes. In the vertical direction, the object slows down on the way up and speeds up on the way back down.
3. What kind of path would the planets follow if suddenly their attraction to the Sun no longer existed?
In the absence of a force, the planets would follow straight-line paths due to no net forces
8. A hockey puck slides across the ice at a constant velocity. Is it in mechanical equilibrium? Why or why not?
It is in mechanical equilibrium because the net force on it is zero. Constant velocity means that there is no acceleration, and no acceleration means no force
7. If the mass of a sliding block is tripled at the same time that the net force on it is tripled, how does the resulting acceleration compare to the original acceleration?
It remain the same for example tripling the force max 3 x the acceleration triple the mass mean 1/3 the acceleration
2. What physical quantity is a measure of how much inertia an object has?
Mass
24.Is it correct to say that action and reaction force always act on different bodies? Defend your answer.
Yes. In order to have a pair of force according to Newton's 3rd law there must be an interaction between two objects and the forces act on each object. Two forces that act on the same object CANNOT be a "pair" of forces according to Newton's 3rd law.
As you stand on a floor, does the floor exert an upward force against your feet? How much force does it exert? Why are you not moved upward by this force?
Yes. The floor exerts an upward force against your feet.The amount of force is equal to your weight. You are not moved upward because the net force on the person is zero. The upward push from the floor cancels out the downward force of gravity.
2. What is the net force on an apple that weighs 1 N when you hold it at rest above your head? What is the net force on it when you release it? a) Identify all the forces acting on the apple and their reaction forces when you hold the able above your head. b) Identify all the forces acting on the apple and their reaction forces when the apple is released and falls.
a) When you hold the apple above your head, the net force on the apple is zero. The apple is at rest. There are two forces acting on the apple. The support force (lets say the force of the hand acting an the apple) and the weight (the force of gravity which is the force the Earth exerts on the apple). Each of these has a reaction force. The reaction force to the force the hand exerts on the apple is the force the apple exerts back on the hand. The reaction force to the force the Earth exerts on the apple is the force the apple exerts back on the Earth b)When the apple is released the support force disappears. The net force is no longer zero but equal to the force of gravity (the force the Earth exerts on the apple). The reaction force is the force the apple exerts on the Eart
What idea of aristotle did Galileo discredit with hi incline-plane experiments?
an object will continue in a state of constant motion along a straight line unless a force acts on it to change its motion.
9. If you push horizontally on a crate and it slides across the floor, slightly increasing speed, how does the friction acting on the crate compare with your push?
f the speed of the crate is increasing (even slightly), this means that the crate has acceleration. When a system is accelerating the net force is NOT zero. Thus, the applied force and the frictional force do not cancel. If the object is speeding up this means that the applied force is larger than the frictional force.
Two 100 N weights are attached to a spring scale as shown. Does the scale read 0 N, 100 N, or 200 N, or does it show some other reading?
it would read as 100
When a ballplayer throws a ball straight up, by how much does the speed of the ball decrease each second while ascending? In the absence of air resistance, by how much does its peed increase each second while it is descending? How much time is required for its ascent as compared to its descent?
its will decrease by 10 m/s. it will descend by 10/s finally it will take the same amount of time to go up as it takes to go back down
6. Which depends on location - weight or mass? Which depends on gravity - weight or mass?
location - weight gravity - weight Mass is always constant
6. Two people of equal mass attempt a tug-of-war with a 12 m rope while standing on frictionless ice. When they pull on the rope, each person slides toward the other. How do their accelerations compare, and how far does each person slide before they meet?
s they pull on the rope, the rope pulls back on the people. Both individuals will have the same force applied. Since there mass is identical this means that they will have the same acceleration. Each person would thus slide the same distance of 6 m.
8. If the mass of a sliding block is doubled while the net force is kept constant, how does the resulting acceleration compare to the original acceleration? How about if the mass was reduced by ½?
the acceleration will be 1/2 the acceleration will be 2 times these are indirectly proportional
Someone standing at the edge of a cliff throws a ball straight up at a certain speed and another ball straight down with the same initial speed. If air resistance is negligible, which ball has the greater speed when it strikes the ground below? Which ball will reach the ground quicker?
the ball thrown upwards will return to its release height (cliff height) with the same speed it had when thrown upward. At this point it's moving with the same velocity as the ball thrown downward, so each ball hits ground with the same speed. Throw down ball reaches first
Suppose that the freely falling object in the preceding exercise were also equipped with an odometer. Would the readings of distance fallen each second indicate equal or unequal distances of fall for successive seconds? Explain.
unequal distance. since you start a 10, then you gonna be going 20, then your going 30. THis is making the gap between each bigger and bigger to find this you do 1/2 (gravity.... 10m/s)(time)
A projectile is launched upward at an angle of 75 degrees from the horizontal and strikes the ground a certain distance downrange. For what other angle of launch at the same speed would this projectile land just as far away? Would both projectiles be in the air for the same amount of time? If not, which would be in the air longer?
we are looking for the sweet spot to ge these 2 to be in teh middle of 45. In this case that would mean it being at 15 degrees. THey would both have the same range or distance. 75 in the air longer
10. What is the net force on a box that is being pushed to the right with 50 N of force, while it is also being pushed to the left with 20 N of force?
30N to the right
12. What is the net force that acts on a 10-N falling object what it encounters 4 N of air resistance? 10 N of air resistance?
4+(-10) = -6 or 6N downward 10-10 = 0N
2. Did Aristotle believe that forces are necessary to keep moving objects moving, or did he believe that, once moving, they'd move of themselves?
A force, push or pull, is required to maintain the motion of an object
5. Consider a pair of forces, one with a magnitude of 25 N and the other with a magnitude of 15 N. What maximum net force is possible for these two forces? What minimum net force is possible?
Maximum net force, if they are in the same direction, is 40N. Minimum net force, if they are in opposite direction, is 10N.
Why is it that a cat that falls from the top story of a 50- story building will hit a safety net below no faster than if it fell from the twentieth story?
The cat will have reached terminal velocity before hitting the ground. This means that there is a point when the air resistance is exactly equal to the weight of the cat. The net force is zero and the cat stops accelerating reaching terminal velocity.
12. In order to slide a heavy cabinet across the floor at constant speed, you exert a horizontal force of 600 N. Is the force of friction between the cabinet and the floor greater than, less than, or equal to 600 N? Explain your answer.
The force of friction will be equal to the horizontal force of 600N because in order for it to slide at a constant speed, there must be equilibrium between forces. no acceleration
16. Why does a heavy parachutist fall faster than a lighter parachutist who wears the same size parachute?
The heavy parachutist has a greater force of gravity but the same amount of air resistance. Thus, at a given speed the heavier parachutist has a greater net force since he/she weighs more. With a greater net force, there is a greater acceleration and thus he/she falls faster
What were the two main classifications of motion in Aristotle's view of science?
The natural state of an object near the surface of the earth is to be at rest A force, push or pull, is required to maintain the motion of an object
A race car travels along a raceway at a constant velocity of 200 km/h. What horizontal forces act, and what is the net force acting on the car?
The net force on the car is zero because it is traveling at constant velocity and has zero acceleration. In order to keep the car in motion, the gas petal must be applied. Thus, there is a horizontal force in the forward direction. Since it is moving with a constant velocity, there must be friction or air resistance acting in the opposite direction with an equal amount of force.
Suppose one person in the previous example has twice the mass as the other. How does their accelerations compare? Which person will have traveled a greater distance when they meet?
The person with the larger mass will have a smaller acceleration. The person with the smaller mass will have a larger acceleration and will travel farther when they meet.
7. Where would your weight be greater - on the earth or on the moon? How about your mass?
The weight of an object would be greater on the Earth because the Earth has a greater gravitational acceleration as compared to the moon. The mass of the object would be the same on the Earth as on the Moon since mass is independent of the location of the object.
14. What does F = 0 mean?
This means that all of the forces acting on an object must add up to zero if we take into account the direction. When the net force on an object is zero this means that the object is in equilibrium and the motion is not changing (zero acceleration).
Correct your friend that says, "The dragster rounded the curve at a constant velocity of 100 km/h." In other words, what is wrong with this statement.
Velocity incorporates both magnitude (how fast) and direction. Since the dragster is going around a curve, its direction is changing and thus its velocity is also changing.
A ball is thrown straight upward with a speed of 30 m/s, using a figure, conceptually explain that the time it takes the ball to reach the top is 3 seconds. You should consider drawing the ball during its upward motion each second along with the speed that it has.
every second it will loose 10m/s one second 20 2 second 10 3 seconds 0
projectile is launched vertically at 100 m/s. If air resistance can be neglected, at what speed will it return to its initial level?
in 20 seconds it will return at 100m/s
2. Is inertia a property of matter or a force of some kind?
property of matter that causes it to resist changes in velocity
13. What two principal factors affect the force of air resistance on a falling object?
speed and the size/shape of the object.
Distinguish between speed and velocity
speed is magnitude(how fast).... scaler velocity is magnitude and direction... vector
22. If the forces that act on a cannonball and the recoiling cannon from which it is fired are equal in magnitude, why do the cannonball and cannon have very different accelerations?
the cannon has more mass than that of the cannonball making its acceleration less
18. If we push a crate at a constant velocity, how do we know how much friction acts on the crate compared with our pushing force?
the crate moves with a constant velocity, the force of friction must be the same size as the pushing force but oppositely directed. If the velocity is constant, there is no acceleration and thus the net force on the crate must be zero.
19. How does the direction of the friction force compare with the direction of the velocity of a sliding object
they are going opposite directions
20. If you push to the right on a on a heavy piece of furniture and it slides, what is the direction of the friction on the furniture?
to the left
8. What are the units of measurement for weight and for mass?
weight = N Mass = kilograms
21. Suppose you push to the right on a heavy piece of furniture, but not hard enough to make it slide. Does a friction force act on the furniture?
yes, its opposite to your push
What is the acceleration of a car that moves at a steady velocity of 100 km/h for 100 seconds? Explain your answer.
zero since the car is moving at a constant veolocity
5. If a Mack truck and a motorcycle have a head-on collision, upon which vehicle in the impact force greater? Which vehicle undergoes the greater change in its motion? Explain your answer.
Two colliding objects exert equal and opposite forces on each other (Newton's Third Law). However, acceleration experienced by an object is inversely related to its mass: the smaller the mass, the larger the acceleration for a given force . Therefore the motorcycle will undergo a much greater change in its motion than the truck.
Distinguish between velocity and acceleration.
Velocity measures how the displacement of an object changes with respect to time. Acceleration measures how the velocity changes with respect to time.
18. When you push against a wall with your fingers, they bend because they experience a force. Identify this force
When you push against a wall, the wall push back on you with the same amount of force but oppositely directed.
20. State Newton's third law.
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
At what point in its trajectory does a batted baseball have its minimum speed? If air resistance can be neglected, how does this speed compare with the horizontal component of its velocity at other points?
Minimum speed occurs at the top, which is 0m/s. But the horizontal speed is always the same
When you jump upward, your hang time is the time your feet are off the ground. Does hang time depend on your vertical component of velocity when you jump, your horizontal component of velocity, or both? Explain.
No, your hang time does not depend on the horizontal component of your velocity. Horizontal and vertical motion are independent of each other.
23 Can a "pair" of forces, according to Newton's 3rd law, act on the same object and appear on the same free body diagram.
No. "pairs" of forces must act on different objects
Free fall is motion in which gravity is the only force acting. Is a skydiver who has reached terminal speed in free fall?
No. A skydiver experiences air resistance and the acceleration of the skydiver will be less that gravity. In this case the skydiver has reached terminal velocity which means his/her velocity is not longer change and the acceleration is zero.
4. Gravitational force on the moon is only 1/6 that of the gravitational force on the earth. What would be the weight of a 10-kg object on the moon and on the earth?
On the Earth a 10 kg object would weigh 100 N (10 kg X 10 m/s2). On the moon a 10 kg object would weigh 1/6 (100 N) = 16.7 N The mass on the Earth and the moon would be 10 kg.
13. How much tension is there in a vertical rope that holds a 20-N bag of apples at rest?
Since the apples are at rest, the downward for of gravity of the apples (20 N) must be equal and opposite to the upward force that is holding it. This means that the vertical rope must have a tension of 20 N
15. If two objects of the same size fall through the air at different speeds, which encounters the greater air resistance?
THe faster an object falls the more air resistance there will be.
12. Name the force that occurs in a rope when both ends are pulled in opposite directions
Tension is how we define force that is transmitted through a rope or cord.
11. Why doesn't a heavy object accelerate more than a light object when both are freely falling?
The acceleration due to gravity is independent of the mass of the object
5. Is acceleration directly proportional to force, or is it inversely proportional to force? Give an example.
The acceleration is directly proportional to th e force. If you apply twice as much force to two identical objects. The object that experiences twice the force will have twice the acceleration.
4. State Newton's Second Law.
The acceleration of an object is directly proportional to the net force applied and indirectly proportional to the mass.
25. How does a helicopter get its lifting force?
The helicopter blades turn and in the process push on the air. The air pushes back on the helicopter creating a lift force
3. Suppose two carts, one twice as massive as the other, fly apart when the compressed spring that joins them is released. How the of the heavier cart compare to the lighter cart after they are released from the spring. (see figure below)
The spring exerts the same amount of force on both carts but the acceleration of the cart with mass 2m will have an acceleration that is ½ that of the cart with mass m.
11. An empty jug of weight 5 N rests on a table. What is the support force (normal force) exerted on the jug by the table? What is the support force when water of weight 2 N is poured into the jug?
The support force is 5 and the goes to 7
15 Why is the support force on an object often called the normal force?
The support force is often called the normal force because the support force is always directed perpendicular to the area of contact between the two surfaces that are in contact. In mathematics, normal is another way to express perpendicular.
16. When you weigh yourself, how does the support force of the scale acting on you compare with the gravitational force between you and Earth?
The support force will be equal to gravitational force from the Earth that is acting on the person. If we consider a person standing on a scale, the two forces that act on the person are the downward force of gravity or weight of the person and the upward support force. If the person is at rest then these two force must be the same size and act in opposite directions.
Which team wins a tug-of-war- the team that pulls harder on the rope, or the team that pushes harder against the ground? Explain.
The team that pushes harder against the ground. The tension in the rope is the same for teams but whichever team pushes against the ground harder will have the ground push back harder on them and thus allow them to win the tug-of-war.
1. Suppose you roll a ball off a tabletop. Will the time to hit the floor depend on the speed of the ball? In other words, will a faster rolling ball take longer to hit the floor? Explain.
The time to drop is the same, the speed only moves it forward, the rate of accent is the same. It will change the distance traveled though
13. Consider your desk at rest on your bedroom floor. As you and your friend start to lift it, does the support force on the desk provided by the floor increase, decrease, or remain unchanged? What happens to the support force on the feet of you and your friend?
The upward support force from the table is exactly equal and oppositely directed to the force of gravity acting on the object. These two force cancel one another out when you consider the net force.