Physics Chapter 9

What two forces act on you while you are in a moving elevator? When are these forces of equal magnitude and when are they not?

The two forces are the normal force and mg, which are equal when the elevator doesn't accelerate, and unequal when the elevator accelerates.

The human body is composed mostly of water. Why does the Moon overhead cause appreciably less tidal effect in the fluid compartment of your body than a 1kg melon held over your head does?

Tides are produced by differences in forces, which related to differences in distance from the attracting proportional difference for the melon out-tides the most massive but more distant Moon. One's head is not appreciably closer to the Moon than one's feet.

Discuss why tides occur in Earth's crust and in Earth's atmosphere.

Tides occur in the Earth's crust and the Earth's atmosphere for the same reason they occur in the Earth's oceans. Both are large enough so there are appreciable differences in distances to the Moon and the Sun which corresponding gravitational differences as well.

Is the acceleration due to gravity more or less atop Mr. Everest than at sea level? Defend your answer.

Less because an object there is farther from Earth's center.

What would be the path of the Moon be if somehow all gravitational forces on it vanishes to zero

The moon would move in a straight-line path instead of circling both the Sun and Earth.

What would be the effect on Earth's tides if the diameter of Earth were very much larger than it is? If earth were at its present size but the moon were very much larger and had the same mass?

Tides would be greater if the Earth's diameter were greater because the difference in pulls would be greater. Tides on Earth would be no different if the Moon's diameter larger. The gravitational influence of the Moon is as if all the Moon's mass were at its CG. Tidal bulges on the solid surface of the Moon, however, would be greater if the Moon's diameter were larger, but not on the Earth

An astronaunt in the International Space Station cannot stand on a weighing scale. But an astronaut inside a rotating space station can stand on a weighing scale. Explain.

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Discuss and explain why this reasoning is wrong "The Sun attracts all bodies on Earth. At midnight, where the Sun is driectly below, it pulls on your in the same direction as Earth pulls on you; at noon, when the Sun is directly overhead, it pulls on you in the same direction as Earth pulls on you; at noon, when the SUn is directly overhead, it pulls on you in a direction opposite to Earth's pull on you. Therefore, you should be somewhat heavier at midnight and somewhat lighter at noon.

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Is there a torque about the Moon's center of mass when the Moon's long axis is aligned when Earth's gravitiation field? Explain how this compares with a magnetic compass.

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Stand on a bathroom scale on a level floor and the reading reading on the scale shows the gravitiational force on your, mg. If the floor is slanted at an angle, the scale reading will be less than mg. Discuss why this is so, and why it is a good idea to measure your weight when the scale is on a horizontal surface.

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Why does a person in free fall experience weightlessness, while a person falling at terminal velocity does not?

A person is weightless when the only force is gravity, and there is no support force. Hence the person in free fall is weightless. But more than gravity acts on the person falling in terminal velocity. In addition to gravity, the falling person is "supported" by air drag.

Some people dismiss the validity of scientific theories by saying that they are "only theories." The law of universal gravition is a theory. Does this mean that scientists still doubt its validity? Discuss and explain.

Any doubts about the theroy have to do with its applications to yet untested situations, not with the theory itself. One of the features of scientific theories is that they undergo refinement with new knowledge.

Consider the lights of mulitple candle flames, each of the same brightness. Rank the light that enters your eye from brightest to dimmest for the following situations. a) Three candles seen from a distance 3 m. b) Two candles seen from a distance of 2 m c) One candle seen from a distance of 1 m

B, C, A

Rank the average gravitational forces from greatest to least between the a) Sun and Mars. b) Sun and the Moon c) Sun and Earth

C, B, A

The intensity of light from a central source varies inversely as the square of the distance. If you lived on a planet only half as far from the Sun as our Earth, how would the Sun's light intensity compare with that on Earth? How about a planet 10 times further away than Earth?

For the planet half as far from the Sun, light would be four times as intense. For the planet ten times as far, light would be 1/100th as intense.

A friend says that above the atmosphere, in space shuttle territory, Earth's gravitational field is zero. Discuss your friend's misconception by using the equation for gravitational force in your explanation.

Force never gets to zero. If it were zero, any space shuttle would fly off in a straight-line path.

Is gravitiational force acting on a person who falls off a cliff? On an astronaut inside an orbiting space shuttle?

Gravitational force is indeed acting on a person who falls off a cliff, and on a person in a space shuttle. Both are falling under the influence of gravity.

If somebody tugged hard on your shirt sleeve, it would likely tear. But if all parts of your shirt were tugged equally, no tearing would occur. How does this relate to tidal forces?

Just as differences in tugs on your shirt will distort the shirt, differences in tugs on the oceans distort the ocean and produce tides.

An astronaut lands on a planet that has the same mass as Earth but twice the diameter. How does the astronaut's weight differ from that on Earth?

Letting the equation for gravitation guide your thinking, twice the diameter is twice the radius, which corresponds to 1/4 the astronaut's weight at the planet's surface

If Earth were hallow but still had the same mass and radius, would your weight in your present location be greater than, less than, or the same as it is now? Discuss and explain.

Letting the gravitational force equation be a guide to thinking, we see that gravitational force and hence one's weight does not change if the mass and radius of the Earth do not change.

Comment on whether or not the following label on a consumer product should be cause for concern: CAUTION: the mass of this product pulls on every other mass in the universe, with an attracting froce that is proportional to the product of the masses and inversely propprotional to the square of the distance between their centers.

Nothing to be concerned about on this consumer label. It simply states the universal law of gravitation, which applies to all products, It looks like the manufactuerer knows some physics and has a sense of humor.

Why aren't high ocean tides exactly 12 hours apart.

Ocean tides aren't exactly 12 hours apart because while the Earth spins, the Moon moves in its orbit and appears at its same position overhead every 25 hours, instead of every 24 hours. So the two-high-tide cycles occurs at about 25 hour intervals, making high tides about 12.5 hours apart.

If you could somehow tunnel inside a uniform-density star, would your weight increase or decrease? If, instead, you somehow stood on the surface of a shrinking star, would your weight increase or decrease? Discuss why your answers differ.

On a shrinking star, all mass of the star pulls in a noncancelling direction and you get closer to the overall mass concentration and the force increases. If you tunnel into a tar, however, there is a cancellation of gravitational pulls; the matter above you pulls counter to the matter below you, resulting in a decrase in the net graviational force.

If the mass of Earth increased, your weight would correspondingly increase. But, if the mass of the SUn increased, your weight would not be affected at all. Discuss why this is so.

Since Earth is in free fall around the sun, the sun contributes nothing to your weight. Earth gravitation presses you to Earth; solar gravitation doesn't press you to Earth.

Earth and the Moon are attracted to each other by gravitiaonal force. Does the more massive Earth attract the less massive Moon with a force that is greater, smaller, or the same as the force with which the Moon attracts Earth?

The Earth and Moon equally pull on each other in a single interaction. The pull of the Earth on the Moon is equal and opposite to the pull of the Moon on the Earth.

Is the force of gravity stronger on a crumpled piece of paper than on an identical piece of paper that has not been crumpled? Defend your answer.

The force of gravity is the same on each because the masses are the same, as Newton's equation for gravitational force verifes.

Another friend says that the Moon's gravity would prevent rocks dropping from the Moon to Earth, but that if the Moon's gravity somehow no longer pulled on its own rocks, then rocks on the Moon would fall to Earth. DIscuss the wrongness of this assumption.

The force of gravity on Moon rocks at the Moon's surface is considerably stronger than the force of gravity of the distant Earth. Rocks dropped on the Moon fall onto the Moon surface.

An apple falls because of the gravitiational attraction to Earth. How does the gravitational attraction to the Earth to the apple compare?

The forces betweent he apple and Earth are the same in magnitude. Force is the same either way, but the corresponding accelerations of each are different.

The planet Jupiter is more than 300 times as massive as Earth, so it might seem that a body on the surface of Jupiter would weigh 300 times as much as on Earth. But it so happpens that a body would weigh scarely 3 times as much on the surface of Jupiter as on the surface of Earth. Discuss why this is so, using the terms in the equation for gravitational force to guide your thinking.

The gravitational force on a body, its weight, depends not only on mass but distance. On Jupiter, this is the distance between the body being weighted and Jupiter's center, the radius of Jupiter. If the radius of Jupiter were the same as that of the Earth, then a body would weigh 300 times as much as Jupiter is 300 times more massive than Earth. But the radius of Jupiter is about 10 times that of Earth, weakening gravity by a factor of 100, resulting in 3 times its Earth weight.

It so happens that an actual increase in weight is found even in the deepest mine shafts. What does this tell us about how Earth's denisty changes with depth?

The increase in weight indicates that the earth is more compressed, more compact, more dense, twoard the center. The weight that normally would be lost when the deepest mine shafts from the upward force of thes urrounding "shell" is more than compensated by the added weight gained due to the closeness to the more dense center of the Earth.

Why does a bungee jumper feel weightless during the jump?

The jumper is weightless due to the absence of a support force.

Whenever the ocean tide is unusually high, will the following low tide be unusually low? Defend your answer in terms of "conversation of water"

Whenever the ocean tide is unusually high, it will be followed by an unusually low tide. This is because for when one part of the world is having an extra high tide, another part must be donating water and experiencing an extra low tide. Or as the hint in the exercise suggests, if you are in a bathtub and slosh the water so it is extra deep in front of you, that's when it is extra shallow in back of you "conversation of water"

If Earth were of uniform density what would the value of g be inside Earth at half its radius

it would be 5 m/s^2.