Astronomy Study Guide - Hw

The acceleration of gravity on Earth is approximately 10 m/s2 (more precisely, 9.8 m/s2). If you drop a rock from a tall building, about how fast will it be falling after 3 seconds?

30 m/s

Which person is weightless?

A child in the air as she plays on a trampoline.

If you are in an elevator that is accelerating upward, you will feel __________ while the elevator __________. (Hint)

A force pressing you to the floor; moves upward with increasing speed.

According to the universal law of gravitation, if you triple the distance between two objects, then the gravitational force between them _________.

decreases by a factor of 9

Increasing speed means acceleration, and when the elevator is accelerating upward you will feel a force pressing you to the floor,

making your weight greater than your normal (at rest) weight.

Consider the statement "There's no gravity in space." This statement is:

Completely false

True or False? As you accelerate upward, you have more mass than when you are stationary.

False

True or False? While you accelerate downward, Earth exerts a smaller-than-normal gravitational attraction on you.

False

If you want to make a rocket turn left, you need to:

Fire an engine that shoots out gas to the right.

Suppose you are in an elevator. As the elevator starts upward, its speed will increase. During this time when the elevator is moving upward with increasing speed, your weight will be __________.

Greater than your weight at rest.

Why is Newton's version of Kepler's third law so useful to astronomers?

It can be used to determine the masses of many distant objects.

Choose the correct definition and two examples of kinetic energy.

Kinetic energy is the energy an object has due to its motion. Two examples are a car driving down the highway or the movements of the molecules of liquid in a cup of hot coffee.

Suppose you are in an elevator that is moving upward. As the elevator nears the floor at which you will get off, its speed slows down. During this time when the elevator is moving upward with decreasing speed, your weight will be __________.

Less than your normal weight at rest

Choose the correct definition and two examples of potential energy.

Potential energy is energy that is stored. Two examples of potential energy are breakfast cereal, which has chemical potential energy, and a heavy book on the top shelf at the library, which has gravitational potential energy.

Choose the correct definition and two examples of radiative energy.

Radiative energy is energy in light. Sunlight carries this form of energy, as do other forms of light including X-rays and radio waves.

Which of the following examples describes a situation where a car is experiencing a net force?

The car is making a gradual turn

In Part A, you found that your weight will be greater than normal when the elevator is moving upward with increasing speed. For which of the following other motions would your weight also be greater than your normal weight?

The elevator moves downward while slowing in speed.

When you are standing on a scale in an elevator, what exactly does the scale measure?

The force you exert on the scale

Even though the elevator is still moving upward, the fact that its speed is slowing means that the acceleration is downward.

The situation is rather like that of a ball that is still on its way up after you throw it: the ball slows as it goes upward because of the downward acceleration of gravity. Because the acceleration of the elevator is downward, your weight is lower than normal.

If two objects collide, what will be the same before and after the collision.

The total energy of the objects The total momentum of the objects The total angular momentum of the objects

Suppose that two objects collide. Which of the following things is NOT the same both before and after the collision?

The total temperature of the objects

Suppose you are in an elevator car when the elevator cable breaks. Which of the following correctly describes what happens and why.

You float weightlessly within the elevator car because you and the elevator both begin to accelerate downward at the same rate.

Suppose you lived on the Moon. What could be said of your weight and mass?

Your weight would be less than your weight on Earth, but your mass would be the same.

Suppose you throw a ball straight up. It begins going upward quickly (as fast as you throw it), but it gradually slows down until it comes to a stop at the top of its flight, and then starts heading back down. During the time that the ball is on its upward path, its acceleration is __________. (Hint)

downward

As you can see in the video, you become weightless in free-fall. What kind of acceleration do you have in free-fall on Earth? (Hint)

downward acceleration, meaning your downward speed continually gets faster

When a spinning ice skater pulls in his arms, he spins faster because _________.

his angular momentum must be conserved, so reducing his radius must increase his speed of rotation

In a downward-moving elevator, the elevator can be slowing only if it has an upward acceleration. As you know from Part A, an upward acceleration will give you an increased weight. Therefore...

in an elevator that moves downward while slowing in speed, the acceleration is upward and your weight is greater than normal.

Your presence in an elevator cannot change either your mass or the gravitational force exerted on you by Earth. The scale measures the force that is exerted on it...

which in an elevator is a combination of the force due to gravity and a force due to the elevator's acceleration.

Once the cable breaks, you and the elevator car both fall with the acceleration of gravity. This means you are no longer pressing against the scale or the elevator floor, so...

you float weightlessly within the car -- though only until you and the car hit the ground!