Physics

1.

1 point

Curly hair behaves like a weak spring that can stretch under its own weight. Why is a hanging curl straighter at the top than at the bottom?

At the top the distortion is greater.

At the bottom the spring constant is higher.

The top part has more weight hanging from it, so it is more stretched out.

Different segments of a given strand of hair have different amounts of curliness.

2.

1 point

When you lie on a spring mattress, it pushes most strongly on the parts of you that stick into it the farthest. Why doesn’t it push up evenly on your entire body?

The upward push is equal to your weight.

The matress obeys Hooke's Law.

Parts of the mattress that are distorted more have a higher spring constant.

Actually, it does push up evenly.

3.

1 point

If you drop a steel marble on a wooden floor, why does the floor receive most of the collision energy and contribute most of the rebound energy?

The floor is softer and more massive than the marble.

The steel marble has a higher coefficient of restitution than the wooden floor.

The floor is softer than the marble.

The floor is more massive than the marble.

4.

1 point

People falling from a high diving board feel weightless. Has gravity stopped exerting a force on them? If not, why don’t they feel it?

Gravity still exerts a force on them, but they don't feel it because their apparent weight has become zero.

Gravity still exerts a force on them, but they don't feel it because they're falling.

Gravity still exerts a force on them, but they don't feel it because of air resistance.

Gravity no longer exerts a force on them while they're falling.

5.

1 point

When you step on a scale, it reads your weight plus the weight of your clothes. Only your shoes are touching the scale, so how does the weight of the rest of your clothes contribute to the weight reported by the scale?

Actually, the scale only reports the weight of your shoes since only your shoes touch the scale.

The scale reports the weight of your shoes if they are heavy.

The scale responds to the entire downward force exerted on it.

Your weight is transmitted through your shoes.

6.

1 point

If you put your bathroom scale on a ramp and stand on it, will the weight it reports be high, low, or correct?

More information is needed.

Correct

High

Low

7.

1 point

The starter motor in your car is attached to a small gear. This gear meshes with a large gear that’s attached to the engine’s crankshaft. The starter motor must turn many times to make the crankshaft turn just once. How does this gearing allow modest forces inside the starter motor to turn the entire crankshaft?

The work done by the starter motor roughly equals the work done on the crankshaft, and work equals torque times angle rotated.

The gearing increases the torque from the starter motor.

The gearing increases the force from the starter motor.

The gearing increases the work from the starter motor.

8.

1 point

Astronauts learn to tolerate weightlessness by riding in an airplane (nicknamed the “vomit comet”) that follows an unusual trajectory. How does the pilot direct the plane in order to make its occupants feel weightless?

In an upside-down circular curve.

In a curve that follows the path of a thrown object with only gravity exerting a force on it.

Straight up then straight down.

In a clothoid curve like that of a roller-coaster loop.

9.

1 point

Sprinters start their races from a crouched position with their bodies well forward of their feet. This position allows them to accelerate quickly without tipping over backward. Explain this effect in terms of torque and center of mass.

When the runner takes off there will be a large frictional force of the ground against their feet, which will produce a large torque rotating them backward. The fact that their center of mass is well forward of their feet means that there will be a counterbalancing torque rotating them forward, which will keep them from toppling.

When the runner takes off there will be a large frictional force of their feet against the ground, which will produce a large torque rotating them forward. The fact that their center of mass is well forward of their feet means that there will be a counterbalancing torque rotating them backward, which will keep them from toppling.

When the runner takes off there will be a large frictional force of the ground against their feet, which will produce a large torque rotating them forward. The fact that their center of mass is well forward of their feet means that there will be a counterbalancing torque rotating them backward, which will keep them from toppling.

When the runner takes off there will be a large frictional force of their feet against the ground, which will produce a large torque rotating them backward. The fact that their center of mass is well forward of their feet means that there will be a counterbalancing torque rotating them forward, which will keep them from toppling.

10.

1 point

When a moving hammer hits a nail, it exerts the enormous force needed to push the nail into wood. This force is far greater than the hammer’s weight. How is it produced?

The hammer has a lot of kinetic energy.

The hammer has a lot of momentum.

The hammer converts gravitational potential energy into kinetic energy

Time of contact is small so force is large.

11.

1 point

If the bottom of your bicycle’s front wheel becomes caught in a storm drain, your bicycle may flip over so that you travel forward over the front wheel. Explain this effect in terms of rotation, torque, and center of mass.

The center of mass is above the drain and so it produces a torque which rotates the bicycle forward.

The force of the drain on the front wheel is very large and the direction of the force passes beneath the center of mass, so it produces an enormous torque rotating the bicycle forward.

As you move your torque is forward, so when the front wheel locks up your center of mass rotates forward.

The front wheel locks up and the momentum of the bike causes it to rotate forward.

12.

1 point

Why does it hurt less to land on a soft foam pad than on bare concrete after completing a high jump?

The energy is less in a foam pad.

The foam pad has a smaller spring constant.

The foam pad is softer.

Padding means greater distortion means the collision lasts longer, and impulse equals force times time.

13.

1 point

When you turn while riding a bike, you must lean in the direction of a turn or risk falling over. If you lean left as you turn left, why don’t you fall over to the left?

The torque produced by leaning left counterbalances the torque produced by the ground on the tires, so there is no net torque about the center of mass.

The torque produced by leaning left counterbalances the torque produced by the tires on the ground, so there is no net torque about the center of mass.

The torque produced by leaning left is what causes the bikes to rotate as it turns to the left.

You don't fall over to the left because the momentum of the bike is to the left.

14.

1 point

Padded soles in running shoes soften the blow of hitting the pavement. Why does padding reduce the forces involved in bringing your foot to rest?

Padding means greater distortion means the collision lasts longer, and impulse equals force times time

The padded shoe is softer.

The energy is less in a padded shoe.

The padded shoe has a smaller spring constant.

15.

1 point

Why must the surface of a hammer be very hard and stiff for it to drive a nail into wood?

The coefficient of restitution is higher.

If it was soft it couldn't drive a nail.

A stiffer surface means less distortion, which takes less time, which means the force on the nail is greater.

The spring constant is higher.

16.

1 point

You board an elevator with a large briefcase in your hand. Why does that briefcase suddenly feel particularly heavy when the elevator begins to move upward?

The briefcase is accelerating downward, so the resulting apparent weight is upward.

The briefcase is accelerating upward, so the resulting apparent weight is upward.

The briefcase is accelerating downward, so the resulting apparent weight is downward.

The briefcase is accelerating upward, so the resulting apparent weight is downward.

17.

1 point

Most racing cars are built very low to the ground. While this design reduces air resistance, it also gives the cars better dynamic stability on turns. Why are these low cars more stable than taller cars with similar wheel spacings?

The car's low center of mass means the frictional force with the ground produces more torque.

The car's low center of mass makes the car more stable.

The car's low center of mass means a smaller lever arm for the frictional force with the ground.

The car's low center of mass means the torque due to the center of gravity is greater.

18.

1 point

The crank of a hand-operated kitchen mixer connects to a large gear. This gear meshes with smaller gears attached to the mixing blades. Since each turn of the crank makes the blades spin several times, how is the force you exert on the crank handle related to the forces the mixing blades exert on the batter around them?

there is no specific relationship between the force you exert on the crank and the force on the batter.

The force you exert on the crank is greater than the force on the batter.

The force you exert on the crank is equal to the force on the batter.

The force you exert on the crank is less than the force on the batter.

19.

1 point

As your car reaches the top in a smoothly turning Ferris wheel, which way are you accelerating?

Downward

Backward

Upward

Forward

20.

1 point

As you swing back and forth on a playground swing, your apparent weight changes. At what point do you feel the heaviest?

At the top of your swing going backward.

At the top of your swing going either forward or backward.

At the lowest point of your swing.

In the middle of your swing going forward, but not yet at the lowest point.

21.

1 point

A salad spinner is a rotating basket that dries salad after washing. How does the spinner extract the water?

The water is pushed outward by its acceleration through the holes in the basket.

The spinner's inertia means it naturally tends to moves in a straight line.

The apparent weight of the water is outward, forcing it through the holes in the basket.

The water is pushed outward by centrifugal force through the holes in the basket. The water is pushed outward by centrifugal force through the holes in the basket.

22.

1 point

To weigh an infant you can step on a scale once with the infant and then again without the infant. Why is the difference between the scale’s two readings equal to the weight of the infant?

The infant's weight using this approach will be wrong if the infant was squirming a lot.

The spring in the scale distorts more with the additional weight of the infant.

The scale responds to the downward force exerted on it.

The weight of the infant changes the scale's spring constant.

23.

1 point

A hammer’s weight is downward, so how can a hammer push a nail upward into the ceiling?

The hammer converts gravitational potential energy into kinetic energy.

The force stopping the hammer is downward so the force on the nail is upward.

The hammer has a lot of momentum.

The hammer has a lot of kinetic energy.

24.

1 point

While you’re weighing yourself on a bathroom scale, you reach out and push downward on a nearby table. Is the weight reported by the scale high, low, or correct?

More information is needed.

Correct

High

Low