Exam 2

A 1 kg chunk of putty moving at 1 m/s collides with and sticks to a 5 kg bowling ball initially at rest. The bowling ball and putty then move with a momentum of

1 kg m/s.

On a balanced see-saw, a boy three times as heavy as his partner sits

1/3 the distance from the center point of the see-saw.

A 2 kg outdoor light fixture is held 4 m above the ground. What is the approximate gravitational potential energy of the light with respect to the ground?

80 J

What does it mean to be weightless?

Being weightless is a sensation of floating in space, however, that's not the reality. The astronauts are actually falling towards the earth. They are falling, along with the spaceship vertically. Since they are falling towards the middle of the earth they are actually going in centripetal force orbiting the earth's curvature.

Two ice skaters, initially at rest, push off one another. What is the total momentum of the two skaters after they push off? Explain.

Since both skaters are initially at rest, the total momentum of the system (which is both ice skaters together) is zero. By conservation of momentum, the total initial momentum for an isolated system must equal the total final momentum for the system. This means that for the ice skaters, their total final momentum must also be zero.

You kick a soccer ball 15 meters without hurting your foot much. You then pump the ball up until it is really hard (the extra air does not significantly change the ball's mass) and again kick it 15 meters. This time it hurts a lot. Using the concept of impulse, explain why it hurts more in the second case.

The change in momentum, and therefore the impulse, is the same in both cases. In the second case of kicking the inflated ball, the time of impact is reduced because the ball is now stiffer. Consequentially, the impact time reduced meaning the the average force must increase to maintain the same change in momentum.

Suppose Larissa throws a ball up in the air and it comes back down to the same place she threw it from. Ignoring times when the ball is in contact with her hand, and ignoring air resistance (friction with the air), identify the point(s) of maximum and minimum potential energy, and maximum and minimum kinetic energy.

The points of maximum and minimum height are the points of maximum and minimum gravitational potential energies. This is because gravitational potential energy depends on height and increases with increasing height. The maximum kinetic energy occurs at the point where the ball is moving fastest, namely right after it leaves Larissa's hand and right before she catches it. The minimum kinetic energy occurs when the ball is moving the slowest, right at the top of the path where it instantaneously stops.

How does the string of a yo-yo get the yo-yo spinning?

The string of a yo-yo is attached away from the yo-yo's center of mass. This means that when the yo-yo is dropped, the tension force exerted by the string on the yo-yo causes a torque. A torque causes an angular acceleration, which means the yo-yo can go from not spinning at all, to spinning about its center of mass.

Which does more work in lifting a grain of rice over its head: an ant or a person? Explain your reasoning.

To lift the grain of rice, the ant and the person must exert a force at least as much as the weight of the grain. The grain of rice has the same amount of mass, and therefore, the same weight for both the ant and the person. Since the person is much more taller than the ant, the person has to move the grain of rice over a greater distance, so it must be that the person has to do more work.

When the brakes on his truck fail, the driver steers it up a runaway truck ramp. As the truck rolls up the ramp, it slows to a stop. If we ignore friction for the moment, the reason for that the truck comes to stop is that

all of the the truck's kinetic energy has been converted into gravitational potential energy.

When doing somersaults, you'll more easily rotate when your body is

balled up.

An airplane is flying due south when it experiences a wind gust that exerts a force on the airplane acting due north. The kinetic energy of the airplane initially

decreases

The force that accelerates a rocket in outer space is exerted on the rocket by the

exhaust gases.

If the mass of the Earth somehow increased with no change in radius, your weight would

increase

The gravitational attract between two objects ______ as the distance between the objects decreases.

increases

A ball is projected into the air with 100 J of kinetic energy which is transformed to gravitational potential energy at the top of its trajectory. When it returns to its original level after encountering air resistance, its kinetic energy is

less than 100 J.

Two objects have the same size and shape, but one is much heavier than the other. When they are dropped simultaneously from a tower, they reach the ground at the same time, but the heavier one has a greater

momentum

If Earth had a radius that was twice as big as its current size, then the acceleration due to gravity at the surface would be

one quarter of its current value.

A rifle recoils while firing a bullet. The speed of the rifle's recoil is small because the

rifle has much more mass than the bullet.

It's possible for two objects of equal mass to have different rotational inertia because

rotational inertia depends on total mass and its location with the object.

It is correct to say that impulse is equal to

the change in momentum.

When you jump from an elevated position you usually bend your knees upon reaching the ground. Doing this reduces the force of impact principally because the

time of impact is increased.

When you fly a kite, there is a time when you must do (positive) work on the kite. This time is

when you're winding the string, bringing the kite in.

Suppose you are helping take groceries inside your house and you carry a heavy bag from the car to the house, keeping the bag at a constant level and walking at a constant speed. During your trip the work you do on the bag is

zero