Physics Chapter 9

Why are all tides greatest at the time of a full Moon or new Moon?

At full Moon and new Moon, the tides from the Moon and the Sun add because they are in line with Earth.

Earth's gravitational field is strongest at _______.

Earth's surface

Calculate the force of gravity on the 1-kg mass if it were 1.3×10^7 m above Earth's surface (that is, if it were three Earth radii from Earth's center). F=G m1m2/d^2

F = 1.1 N

In what sense does the Moon "fall"?

It falls away from the straight path that it would follow if there were no forces acting on it.

Tidal friction is causing

The Earth to slow its rotation rate very, very gradually.

Part D Consider Earth and the Moon. As you should now realize, the gravitational force that Earth exerts on the Moon is equal and opposite to that which the Moon exerts on Earth. Therefore, according to Newton's second law of motion __________.

The Moon has a larger acceleration than Earth, because it has a smaller mass

Describe how the gravitational forces from the Sun and the Moon compare from one side of Earth to the other.

The Sun exerts a stronger force on the side of Earth nearest the Sun, and the Moon exerts a stronger force on the side nearest the Moon.

If Earth shrank, but there was no change in its mass, then what would happen to your weight at the surface?

it would increase

The force of gravity acts on all apples on an apple tree. Some apples are twice as far from the ground as others. For the same mass these twice-as-high apples have

practically the same weight.

The Event Horizon of a black hole is best described as

the boarder surrounding a black hole where the force of gravity is so large that light cannot escape

A black hole is

the remains of a giant star that has undergone gravitational collapse.

If the Sun collapsed to a black hole, Earth's gravitational attraction to it would be

the same

One side of the Moon continually facing Earth is

tidal lock.

Part B The following diagrams are the same as those from Part A. Again considering only the two objects shown in each pair, this time rank the strength, from strongest to weakest, of the gravitational force acting on the object on the right.

trongest 1.Asteroid to Sun 2.Asteroid to Earth 3.Asteroid to moon 4.Asteroid to Asteroid 5.Asteroid to hydrogen atom Weakest Newton's third law tells us that the gravitational force exerted on the asteroid on the left by the object on the right will be equal in magnitude, but opposite in direction to the gravitational force exerted on the object on the right by the asteroid on the left. That is why the ranking here is the same as the ranking for Part A.

Tidal forces in general are the result of

unequal forces acting on different parts of a body.

In comparison with other fundamental forces, the universal gravitational constant G indicates that gravity is

very weak

Inside a freely-falling elevator, you would have no

weight

Inside a freely-falling runaway elevator, your

weight is zero

Gravitational lensing

1. Shows us that light can bend around a massive object 2. Allows us to see an image of an object that is directly behind a massive celestial object 3. provided validation of Einstein's theory of gravitation ((All of the above))

In Einstein's Theory of gravity

1. warped space-time affects the motion of objects with mass 2. the gravitational field is portrayed as a warping of space-time 3. forces are not used to describe gravity 4. warped space-time affects the motion of light ((all of the above))

What is the magnitude of Earth's gravitational force on a 1-kg body at Earth's surface?

10 N

A weight watcher who normally weighs 400 N stands on top of a very tall ladder so she is one Earth radius above Earth's surface. How much is her weight there?

100 N

A Texas State Bobcat tosses a ball at an angle to the horizon such that the vertical component of velocity is 30 m/s and the horizontal component of velocity is 20 m/s. Neglect air resistance. About how far from the student does the ball land?

120 m

An asteroid exerts a 360-N gravitational force on a nearby spacecraft. If the spacecraft moves to a position three times as far from the center of the asteroid, the force will be

40 N

What is the force of gravity on a 500-N woman standing on Earth's surface?

500 N

What is the magnitude of the gravitational force between two 1-kg bodies that are 1 m apart?

6.67 × 10 -11 N

Part A Each of the following diagrams shows a spaceship somewhere along the way between Earth and the Moon (not to scale); the midpoint of the distance is marked to make it easier to see how the locations compare. Assume the spaceship has the same mass throughout the trip (that is, it is not burning any fuel). Rank the five positions of the spaceship from left to right based on the strength of the gravitational force that Earth exerts on the spaceship, from strongest to weakest.

Gravity follows an inverse square law with distance, which means the force of gravity between Earth and the spaceship weakens as the spaceship gets farther from Earth.

Part B The following diagrams are the same as those from Part A. This time, rank the five positions of the spaceship from left to right based on the strength of the gravitational force that the Moon exerts on the spaceship, from strongest to weakest.

Gravity follows an inverse square law with distance, which means the force of gravity between the Moon and the spaceship increases as the spaceship approaches the Moon. Now continue to Part C for activities that look at the effects of both distance and mass on gravity.

Part C From Part B, you know that from afar you'll never see the in-falling rocket cross the event horizon, yet it will still eventually disappear from view. Why?

Its light will become so redshifted that it will be undetectable.

Part C The following diagrams show five pairs of asteroids, labeled with their relative masses (M) and distances (d) between them. For example, an asteroid with M=2 has twice the mass of one with M=1 and a distance of d=2 is twice as large as a distance of d=1. Rank each pair from left to right based on the strength of the gravitational force attracting the asteroids to each other, from strongest to weakest.

M=2 d=1 M=2 M=1 d=1 M=2 M=1 d=1 M=1 M=1 d=2 M=2 M=1 d=2 M=1

Which is most responsible for Earth's ocean tides?

Moon

The concept of force is fundamental to

Newton's theory of gravitation.

Which has the higher tides: spring tides or neap tides? Why?

Spring tides are higher, because the tides from the Moon and Sun add together.

Part C The following diagrams are the same as those from Part A. This time, rank the pairs from left to right based on the size of the acceleration the asteroid on the left would have due to the gravitational force exerted on it by the object on the right, from largest to smallest.

Strongest 1.Asteroid to Sun 2.Asteroid to Earth 3.Asteroid to moon 4.Asteroid to Asteroid 5.Asteroid to hydrogen atom Weakest According to Newton's second law, the asteroid with the largest acceleration will be the one that has the strongest gravitational force exerted on it by the object on the right. That is why the ranking here is the same as the ranking for Part A.

The following five diagrams show pairs of astronomical objects that are all separated by the same distance d. Assume the asteroids are all identical and relatively small, just a few kilometers across. Considering only the two objects shown in each pair, rank the strength, from strongest to weakest, of the gravitational force acting on the asteroid on the left.

Strongest 1.Asteroid to Sun 2.Asteroid to Earth 3.Asteroid to moon 4.Asteroid to Asteroid 5.Asteroid to hydrogen atom Weakest Because the distance is the same for all five cases, the gravitational force depends only on the product of the masses. And because the same asteroid is on the left in all five cases, the relative strength of gravitational force depends on the mass of the object on the right. Continue to Part B to explore what happens if we instead ask about the gravitational force acting on the object on the right.

Why is a black hole invisible?

The escape velocity of the black hole is greater than the speed of light.

How does the force of gravity between two bodies change when the distance between them is doubled?

The force decreases to ¼ of its initial value.

What happens to the force of attraction between two planets when the masses of both are doubled?

The force quadruples.

Why do we only see one side of the Moon which is the "Man in the Moon" face of the Moon?

Tidal friction has slowed the Moon's rotation rate until it is already tidally locked in a synchronous orbit.

Part A From the viewpoint of an observer in the orbiting rocket, what happens to time on the other rocket as it falls toward the event horizon of the black hole?

Time runs increasingly slower as the rocket approaches the black hole.

Part E If you were inside the rocket that falls toward the event horizon, from your own viewpoint you would __________.

accelerate as you fall and cross the event horizon completely unhindered

Part D If you were inside the rocket that falls toward the event horizon, you would notice your own clock to be running __________.

at a constant, normal rate as you approach the event horizon

Tides caused mainly by the Moon occur in Earth's

atmosphere. interior. oceans. ((all of the above))

A very massive object A and a less massive object B move toward each other under the influence of gravity. Which force, if either, is greater?

both forces are the same.

Black holes are essentially _______.

collapsed stars

If Earth's radius somehow increased with no change in mass, your weight would

decrease.

Einstein's theory of gravitation, compared with Newton's, _______.

eliminates the role of force

During an eclipse of the Sun the high ocean tides on Earth are

extra high

The force of Earth's gravity on a capsule in space increases as it comes closer. When the capsule moves to half its distance, the force toward Earth is then

four times greater.

According to Newton, the greater the masses of interacting objects, the

greater the gravitational force between them.

Two planets in space gravitationally attract each other. If both the masses and distances are doubled, the force between them is

half as much. twice as much. one-quarter. four times as much. ((none of the above))

You experience weightlessness

in a freely falling elevator. in the absence of a supporting surface. momentarily when you step off a chair. ((all of the above))

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

increase also.

As two objects moving toward each other due to gravity get closer, the acceleration of each

increases

When the distance between two stars decreases by one-third, the force between them

increases to nine times as much.

Two objects move toward each other due to gravity. As the objects get closer and closer, the force between them

increases.

One's weightlessness in space has most to do with _______.

no support force

The main reason ocean tides exist is that Moon's pull is stronger

on water closer to it than on water farther away.

Which rocket would require more fuel?

one going from Earth to the Moon

The force of Earth's gravity on a capsule in space will lessen as it moves farther away. If the capsule moves to twice its distance, the force toward Earth becomes

one-fourth.

According to Newton, when the distance between two interacting objects doubles, the gravitational force is

one-quarter.

When a star collapses to form a black hole, its mass

remains the same.

Part B As the falling rocket plunges toward the event horizon, an observer in the orbiting rocket would see that the falling rocket __________.

slows down as it approaches the event horizon and never actually crosses the event horizon

Which two factors mainly affect ocean tides on Earth?

sun and moon