Physics Quiz Ch 00-01-02-03

After a car is raised in a service station, we say the car has increased _____

in neither elastic potential energy, chemical energy, or kinetic energy

Consider an apple falling from a tree. If the system is the apple alone, momentum _____ conserved. If the system is apple + Earth, momentum ____ conserved. (Ignore tiny effects of the sun.)

isn't / is

The force that propels a rocket is provided by

its exhaust gases

A fast-moving ball on a bowling alley gradually slows as it rolls. The horizontal force acting on the ball is

friction

Calculate the net force on a crate that is pushed to the left with a 100-N force, and pushed to the right with a 50-N force, while friction on the crate is 30 N.

100 N - 50 N - 30 N = 20 N to the right.

Nellie weighs 300 N and stands on a pair of bathroom scales. If she leans so that one-third of her weight is on the left scale, what is the reading on each scale?

100 N on the left scale; 200 N on the other.

A lever is used to lift a heavy load. When a 50-N force pushes one end of the lever down 1.2 m, the load rises 0.2 m. Show that the weight of the load is 300 N.

(F x d)in = (F x d)out 50 N x 1.2 m = W x 0.2 m W = [(50 N)(1.2 m)]/0.2 m = 300 N

In raising a 5000-N piano with a pulley system, the workers note that, for every 2 m of rope pulled down, the piano rises 0.2 m. Show that ideally, the force required to lift the piano is 500 N.

(F x d)in=(F x d)out F x 2 m = 5000 N x 0.2 m F = [(5000 N)(0.2 m)]/2 m = 500 N.

When you walk at an average speed of 4 m/s, in 5s you'll cover a distance of

20 m

Explain why trapeze artists are better protected by nets than by a hard floor, even though the impulse delivered by either is the same when they're brought to a stop.

Although the impulses of hitting a net and the floor are the same (because they both bring the momentum to zero), the forces are very different. When the trapeze artists hits the floor, the time of impact is relatively short and the force correspondingly large. With a net, however, more time is taken to reduce the momentum of fall, resulting in less force. For momentum change, more time means less force.

Why are automobiles that collapse in collisions safer than more rigid cars?

As the car collapses, more time is given for occupants inside to reduce momentum. More time means less force, which means more safety for occupants.

In terms of impulse and momentum, why will a cannon with a long barrel fire a cannonball faster than the same cannon with a shorter barrel?

As the exploding gas inside the barrel expands, the force on the cannonball decreases, but there is still some force being exerted when the cannonball reaches the end of the barrel. More time means the force of exploding gunpowder imparts a greater impulse to the cannonball. More impulse means more momentum change, which means a greater velocity as the cannonball leaves a barrel.

If you toss a ball horizontally while standing still on a skateboard, you'll recoil. Will you recoil if you go through the motions of tossing the ball, but instead hang onto it? Defend your answer.

As you move the hand holding the ball back and forth, you may recoil slightly, first in one direction, then in the other. But overall, from beginning to end, there is no net impulse on you and therefore no change of your momentum. Whether you throw the ball or not, the total momentum of you and the ball remains zero. If you do through the ball, its momentum in one direction is balanced by your opposite recoil momentum. If you don't throw the ball, its momentum and your momentum separately remain zero.

This question is similar to some on driver's-license exams: A car moving at 50 km/h skids 15 m with locked brakes. Show that with locked brakes at 150 km/h the car will skid 135 m.

At three times speed, KE is 9 times (3^2) as much. So will skid 9 times as far. 9 x 15 m = 135 m.

Wonder Woman sprints around a 1320-m long racetrack in 2 minutes. Calculate her average speed.

Ave speed = distance/time = (1320m)/(120s) = 11 m/s.

You apply a 150 N push on a frictionless sled over a period of 2 seconds. Show that the sled's momentum changes by 300 kg·m/s.

Change in mmt = impulse = Ft = (150N)(2s) = 300 N·s = 300 kg·m/s

True or False? A race car can round a curve at a constant velocity.

False

True or False? An egg tossed into a sagging sheet rather than against a wall will not break because the momentum change is less for the sagging sheet.

False

True or false? A heavy object will naturally fall faster than a lighter object even in the absence of air resistance.

False

True or false? A low-mass molecule and a high-mass molecule moving with the same kinetic energy have equal speeds.

False

True or false? A pair of forces, each 10N, can combine to produce a force greater than 20N.

False.

When falling in air, where air drag is significant, why do heavy objects fall faster than lighter objects? In other words, why do heavier objects have greater acceleration of fall? And tie your explanation to terminal speed.

For an object falling in air, acceleration depends on (weight-air drag)/mass, with different ratios for balls of different weighs. In fact, when air drag builds up to equal the weight of a falling object, this ratio becomes zero. That means the object will not accelerate any more as it falls, and will fall at its terminal speed. Terminal speed is reached much more quickly for a falling tennis ball for a falling bowling ball. That's why a bowling ball will fall faster than a tennis ball dropped at the same time in air.

A car crashes into a wall at 25 m/s and its contents are brought to rest in 0.1 s. Calculate the average force exerted on a 75-kg test dummy by the seat belt.

From Ft = 'mv, [(75 kg)(25 m/s)]/0.1 s = 18,750 N.

A braking force is needed to bring a 1200-kg car moving at 25 m/s to rest in 20.0 s. Show that the braking force is 1500 N.

From Ft=△(momentum) = mv ⇒ F = mv/t = ((1200 kg)(25 m/s)) / 20.0s = 1500 kg * m/s^2 = 1500 N

A 20-N block falls freely from rest from a point 1.5 m above ground level. Show that when hitting the ground it has a kinetic energy of 30 J.

Gain in KE = loss in PE = (mg)h = (20 N)(1.5 m) = 30 J. (KE will equal 30 J when PE is zero).

Some people speak of "having the courage of their convictions." But in science, changing one's mind is beneficial. Why?

Growth comes from change. The scientist who changes his or her mind when confronted with evidence for doing so, is better at advancing knowledge. More courage is exhibited in admitting to a change of mind than in keeping fixed convictions. A mind that never changes doesn't grow.

If you stand on a pair of bathroom scales explain how the reaction change as you shift your weight gradually from side to side. What rule governs the readings on the scale?

However you shift your body while standing on scales, the sum of the scale readings will equal your weight. If you stand with weight distributed evenly. the reading of each scale will be half your weight. If you stand with more of your weight on one scale, the other scale will read less, in such a way that the sum of the readings will equal your weight. Your weight is balanced by the support force of the scales, according to the equilibrium rule .ΣF=0.

A 10-kg bowling ball moves at 6 m/s. How much impulse is needed to stop it?

I = 'mv = (10 kg)(6 m/s) = 60 N·s

A golf club exerts an average force of 300 N on a golf ball during its 0.020-s contact time. Calculate the impulse on the ball.

I = Ft = (300 N)(0.020 s) = 6 N·s.

Suppose you push gently on a box of pies across a table at constant speed. What comparison can you make with your pushing force and the force of friction on the box?

If the box doesn't slow down or gain speed, but continues at a constant speed, the box is in equilibrium. This means the net force on it is zero, which further means your push and the force of friction on the box are equal and opposite in direction.

Distinguish between inertia, mass, and weight. Use these terms to discuss a glass of water.

Inertia is a property of all matter to resist changes in motion. Mass is a measure of inertia, expressed in kilograms. Weight is the force due to gravity on an object and is expressed in newtons. A glass of water has mass, as evidenced by the fact that moving it requires a force of some kind. Place it on a balance and you can measure its mass in kilograms. Place it on a weighing scale and you can measure its weight in newtons (or in the US, commonly, pounds, where 1 kg weighs 10 N = 2.2 pounds.)

What does it mean to say that ΣF=0 for an object in equilibrium?

It means that all the forces acting on it combine to zero. The net force on any object in equilibrium is zero.

A physicist says that a car moving along the road is being propelled by the road. Defend this statement in terms of Newton's third law.

It's true. Tires are made to rotate by the engine, and the tire surface is the only point of contact with the road. The turning tired push backward on the road. They can only do this is the road simultaneously pushes forward on the tires. This forward force is the only force that propels the car along the road. Air drag also occurs, but this counteracts the propulsion by the road.

Can precision machines multiply forces? Energy? Defend your answer

Machines can certainly multiply forces, but while decreasing distance. Energy input and energy output are the same for a 100% efficient device. But no way can a machine put out more energy than is put in. The conservation of energy states that useful energy output can at most be equal to the amount of energy input. For real machines, energy output is less than energy input. So real machines are less than 100% efficient.

Lillian (mass 40.0 kg) standing at rest on slippery ice catches her leaping dog (mass 15 kg) moving horizontally at 3.0 m/s. What is the speed of Lillian and her dog after the catch?

Momentum(dog) = momentum(Lil) + (dog) (15kg)(3.0 m/s) = (40.0 kg + 15 kg)v 45 kg m/s= (55 kg)v v=0.8 m/s

A bowling ball falls at the same speed as a tennis ball when both are dropped from rest in a vacuum (or whenever air drag is negligible.) Why is it true? In other words, why are the accelerations of fall the same for all free-falling objects, regardless of weight or mass?

Newton's second law provides the answer: acceleration of fall equals force of gravity/mass. So, acceleration equals the ratio weight/mass. Mathematically, letting eight = mg, we can see that a=F/m=mg/m=g. The masses "cancel" and we see the rations are equal, g. So whatever the weight or mass of a freely falling object, the acceleration of fall is the same, 10 m/s^2.

Show that when the acceleration of an object is zero that the second law of motion reduces to the equilibrium rule, ΣF=0, which was treated in the previous chapter.

Newton's second law says that the acceleration of an object will be equal to the net force on it divided by its mass. In equation form, this is a=F/m, where a is acceleration, F is net force, and m is mass. This can be reexpressed as F=ma when a is zero, then the net force F is zero. This is what is stated in the equilibrium rule; when there is no acceleration the net force is zero. In math notation, ΣF=0.

In the simplest sense, a force is a push or pull. How does Newton's third law further define force? (Use Earth's pull on the Moon as an example.)

Newton's third law views force as part of an interaction between one thing and another thing. Rather than say, for example, Earth pulls on Moon, we say Earth and Moon are interacting with each other - one interaction with counterparts - each equal and oppositely directed..

Calculate the power expended when a 20-N force pushes a cart 3.5 m in a time of 0.5 s.

P = W/t = Fd/t = [(20N)(3.5m)]/(0.5s) = 140 W.

Distinguish between science andtechnology.

Science has to do with the connections of nature; technology has to do with how these connections are useful in a practical way. Science can also be a way of thinking about the world; technology has to do with the tools for improving the world.

What is the key difference between science and pseudoscience?

Science rests on testing by experiment, whereas pseudoscience has no tests for its validity.

Distinguish between speed, velocity, and acceleration. Then give n example of these terms for the case of an object in free fall.

Speed is a measure of distance traveled in a certain time; how fast something goes. Velocity is the speed with direction stated or implied. Unlike speed velocity is a vector quantity (having amount and direction). Acceleration is a measure of how quickly speed or velocity changes. It is also a vector quantity. In free fall, speed increases uniformly at the rate of 10 m/s for each second of fall. We can say the same for velocity, where the speed has direction in this case downward. A falling object in free fall picks up 10 m/s for each second of fall. This is expressed as 10 m/s^2.

How can your speed be zero and 30 km/s at the same time? [Hint: zero relative to what? 30 km/s relative to what?]

Speed is a relative quantity. If you stand at rest on Earth, your speed relative to Earth is zero; but 30km/s is relative to the sun.

A friend wonders if Earth pulls on communication satellites that circle the Earth. What evidence can you site that Earth does indeed pull them?

The fact that the communication satellites circle about the Earth is evidence that a force acts on them. If this force suddenly disappeared, they would fly off on a straight line and no longer circles Earth - in accord with Newton's First Law of Motion.

A falling boulder experiences a force and it accelerates. What is the reaction to this force? How can this question be better phrased so that it is in accord with the rule for identifying action and reaction?

The force that acts on the boulder is Earth pulling down on it. The reaction is the boulder pulling up on Earth. A better-phrased question would be: Earth exerts a downward force on a falling boulder - what is the reaction to this force? Then the answer follows the rule: "A acts on B, B reacts on A." Or, the boulder exerts an upwards force on Earth.

What does it mean to say energy is conserved?

To say that energy is conserved is to say that it cannot be created and cannot be destroyed. It can be transferred to another location, or transformed to some other form, but the total amount remains the same.

True or False? A helicopter gets lifting force by the reaction of it pushing air downward.

True

True or False? When falling on a concrete floor as compared to falling on a wooden floor, force of fall is likely greater on the concrete floor.

True

True or false? A cart traveling at the same speed on the Moon's surface has just as much momentum as the same car at the same speed on Earth.

True

True or false? An explanation of the swinging ball apparatus, wherein two balls released always results in two identical balls popping out, involves both the conservation of momentum and the conservation of energy.

True

True or false? When a vehicle moves along a highway at a constant velocity, the net force on the vehicle is zero.

True.

You toss a ball back and forth in a high-flying airplane. Does the kinetic energy of the ball relate to the speed of the airplane? Defend your answer.

Velocity is a relative quantity. If you hold the ball still in the airplane, the ball will have considerable kinetic energy relative to the ground it flies over, but none relative to your hand. So kinetic energy is a relative quantity, depending on the location from which it is measured. We say it depends on the "frame of reference."

Calculate the work done in lifting a 500-N barbell 2.2 m above the floor. (What is the potential energy of the barbell when it is lifted to this height?)

W = Fd = 500 N u 2.2 m = 1100 J; Also the gain in PE is 1100 J.

What change in kinetic energy does an airplane experience on takeoff if it is moved a distance of 500 m by a sustained net force of 5000 N?

W = Fd = 5000 N x 500 m = △KE = 2,500,000 J

The second floor of a house is 4 m above street level. Relative to street level, show that the potential energy of a 300-kg refrigerator on the second-floor level is 12,000 J.

W = △E = △mgh = 300 kg x 10 N/k x 4 m = 12,000 J.

The weight of an object is given by the equation, Weight = mg, where m is the mass in kg and g is the acceleration of gravity. Interestingly g =10 m/s2 and equivalently = 10 N/kg. Calculate the weight in newtons of a 123-kg block of iron.

Weight = mg = (123 kg)(10 N/kg) = 1230 N

Discuss the positions of a pendulum bob that swings to and fro from a string attached to a ceiling. Relate these positions to the pendulum's state of energy.

When the bob is at its maximum height, its PE is maximum. At its lowest point, PE is a minimum and KE is a maximum. At all points the sum of PE and KE is a constant. At half height, for example, PE relative to its lowest point is half and KE half the maximum. So a loss in PE is a gain in KE, and vice versa, in accord with the conservation of energy.

If a frog jumps horizontally or at an angle onto a horizontally moving skateboard, the skateboard will change speed, in accord with momentum conservation. If a frog instead drops vertically onto a moving skateboard, the skateboard will slow down. That's because upon contact there is an impulse on the board parallel to the skateboard's horizontal motion. Identify and explain that horizontal impulse.

When the frog lands, friction between its feet and the skateboard surface keeps the frog aboard for the ride. Friction keeps the frog moving forward with the skateboard. The reaction to the forward friction on the frog's feet is backward friction on the skateboard—which provides the backward impulse that slows the board. So the board slows and its momentum after the jump, carrying the frog, is the same as its momentum before the frog joins the skateboard.

A 50-kg block of ice is pulled sideways with a net force of 200 N. Show that its acceleration is 4 m/s2.

a = F/m = (200 N)/(50 kg) = 4 m/s2.

A ball rolling down an inclined plane starts from rest and reaches a speed of 12 m/s in 3 s. Show that it has an acceleration of 4 m/s2.

a = △v/△t = vf-v0/t = (12 (m/s) - 0 (m/s)) / 3s = 4 (m/s^2)

A railroad car of mass m and speed v collides with an identical railroad car that is stationary. The cars couple and move together. a. What is the speed of the coupled cars after the collision? (Hint: Use momentum conservation.) b. What is the kinetic energy of the coupled cars? c. Calculate the kinetic energy of the coupled cars if each of them has a mass 104 kg and the initial speed of the moving car before collision was 5 m/s. d. How does the kinetic energy of the coupled cars compare with the initial kinetic energy of the first car? e. What explanation can you offer if there is an energy difference?

a. From mv + 0 = 2mV; V = mv/2mv = v/2, half speed. b. KE = 1/2 (2m)(v/2)^2 = 1/4 mv^2. c. KE = 1/4 10^4(5 m/s)^2 = 6.25 u 10^4 J. d. Half as much. The KE of the coupled cars, 1/4 mv^2, is half the KE of the first car (1/2 mv^2) e. The "missing" KE transforms to thermal energy (also some sound energy).

Lonnie's truck of mass m moves at speed v into a headwind of speed V. The temperature of the road surface is T. a. Write an equation for the momentum of the truck. b. Lonnie's 3500-kg truck moves at 18 m/s while driving into a 5 m/s headwind, and when the temperature of the road is 24°C. Show that the momentum of the truck is 63,000 kg.m/s. c. How does this problem show the importance of letting the symbols in equations guide your thinking?

a. Momentum of the truck = mv b. mv = (3500kg)(18 m/s) =63000 kg* m/s c. The equation for momentum doesn't have symbols for the temperature of the road or the headwind speed. Therefore we don't have to worry about them in our solution to the problem.

Belly-flop Bernie dives from atop a tall flagpole into a swimming pool below. His potential energy at the top is 10,000 J and at the water surface is zero. a. Show that when his potential energy reduces to 1000 J, his kinetic energy will be 9000 J. b. Compared with the height of the flagpole, how far above water level will Bernie be when his kinetic energy is 9000 J?

a. PE + KE = Total Energy, 10,000 J; KE = 10,000 J - 1000 J = 9000 J. b. 1/10th: When Bernie's KE is 9000 J, his PE is reduced to 1000 J, which places him 1/10 the flagpole height above water when his KE is 9000 J.

In the hammer throw contest at a track and field event, Rog whirls a hammer of mass m in a circle and releases it at speed v. a. What impulse acted on the hammer? b. The mass of the hammer is 7.3 kg and it is released at a speed of 28 m/s. Show that the impulse that acted on it was about 200 N.s

a. The impulse Ft =△(mv) = mv . The impulse on the hammer is the same as the hammer's total change of momentum—from zero until the moment it is released. b. The impulse = mv = (7.3 kg) (28 m/s) = 200kg*m/s = 200N*s

Suzie exerts a horizontal force F on a box of mass m that sits on a frictionless horizontal surface. a. Write an equation for the acceleration of the box. b. Suppose her push is 15 N and the mass of the box is 10 kg. Show that the acceleration of the box is 1.5 m/s2.

a. a = (Fnet)/(m) = (f)/(m) b. a= (f)/(m) = (15 N)/ (10 KG) = 1.5*(kg (m/s^2))/(kg) = 1.5*(m)/(s^2)

Joshua exerts a horizontal force F on a puck of mass m on a horizontal surface where the force of friction on the puck is f. a. Begin with Newton's second law and write an equation for the block's resulting acceleration. b. When his push is 5.0 N, the mass of the puck is 0.50 kg, and the frictional force is 1.0 N, show that the acceleration is 8.0 m/s2.

a. a = (Fnet)/m = (F-f)/(m) b. a = (F-f)/m = (5.0 N - 1.0 N)/(0.50 kg) = 8.0*( kg* (m/s^2))/kg = 8.0 m/s^2

A block of ice of mass m is pulled horizontally with a steady force P against a force of friction f. a. Write an equation for the acceleration of the block. b. If the pull on the block is 100 N, friction is 20 N, and the mass is 20 kg, show that the acceleration of the block is 4 m/s2.

a. a=(P-f)/(m) b. a = [ 100N - 20 N]/20kg = 4 m/s^2

Megan rides her bicycle along a straight road at an average velocity v. a. Write an equation showing how far she travels in time t. b. If Megan's average speed is 7.5 m/s for a time of 5.0 minutes, how far does she travel?

a. d = vt b. d = vt = (7.5m/s)/(300 s) = 2250 m

A car with a mass of 1000 kg moves at 20 m/s. A braking force brings the car to a halt in 10 s. a. Use the impulse-momentum equation and solve for the amount of needed braking force. b. Use Newton's second law to solve the same problem.

a. from Ft=△mv,F= △mv/t = [(1000 kg) / (20m/s)] / 10 s = 2000N

A 100-kg quarterback is traveling 5 m/s and is stopped by a tackler in 1 s. Calculate a. the initial momentum of the quarterback. b. the amount of impulse imparted by the tackler. c. the average amount of force exerted by the tackler.

a. mv = (100 kg)(5 m/s) =500 kg.m/s b. 500 N.s (because impulse is equal to the change in momentum). c. Ft= △mv,F = △mv/t = (500 N s) / (1s) = 500 N

The new Taipei tower in Taiwan, of height h, has the world's fastest elevators. An elevator rises from the ground floor and reaches the top at an average elevator speed of v. a. Write an equation that shows the time it takes for the elevator to make its upward trip. b. For an average speed of 15 m/s to a height of 508 m, show that the time of the upward trip is nearly 34 s.

a. t = h/v b. t = h/v = (508m)/(15m/s) = 33.8 s

A tennis ball is served and travels the length of the court L in time t. a. Write an equation for the ball's average horizontal speed. b. Calculate the average speed of a ball traveling the length of the 24-m court in 0.6 s.

a. v = L/t b. v = L/t = (24m)/(0.6s) = 40 m/s.

A race car races on a circular racetrack of radius, r. a. Write an equation for the car's average speed, v, when it travels a complete lap in time, t. b. The radius of the track is 400 m and the time to make a lap is 40 s. Show that the average speed of the car around the track is nearly 63 m/s.

a. v = d/t = 2 S r/t b. v = 2 S r/t = 2 S (400m)/(40s) = 62.8 m/s.

An F-14 Tomcat fighter jet goes from rest to a speed of 75 m/s in 9 s. Show that the acceleration it undergoes is 8.3 m/s2.

a= △v/△t = vf - v0/t = (75 (m/s) - 0 (m/s)) / 9s = 8.3 (m/s^2)

Leslie jogs in the park and maintains an average speed of 4 m/s. What distance does she cover in 10 minutes?

d = vt = 4m/s u 600 s = 2400 m.

As mass is added to a pushed object, its acceleration

decreases

When a chicken is being hatched and pokes its way out of its shell, the chick is at

the beginning of new existence

The force that propels a heavy truck along a highway is provided by

the highway pushing back on the truck tires

Science is concerned mainly with the

the natural world

Raising a heavy load requires work. Raising it twice as high requires ______

twice the work