# When at rest on the launching pad, the force of gravity on the space shuttle is quite huge—the weight of the shuttle. When in orbit, some 200 km above.

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When at rest on the launching pad, the force of gravity on the space shuttle is quite huge—the weight of the shuttle. When in orbit, some 200 km above Earth’s surface, the force of gravity on the shuttle is 1. nearly as much. 2. about half as much. 3. nearly zero (micro-gravity). 4. zero. Ch 9-1 (Neglect changes in the weight of the fuel carried by the shuttle.)

When at rest on the launching pad, the force of gravity on the space shuttle is quite huge—the weight of the shuttle. When in orbit, some 200 km above Earth’s surface, the force of gravity on the shuttle is 1. nearly as much. 2. about half as much. 3. nearly zero (micro-gravity). 4. zero. Ch 9-1 Answer: 1 The gravitational force on the shuttle, whether at rest or in orbit, depends on only three things: its mass, the mass of Earth, and its distance from Earth’s center. The only variable is distance. On the launching pad the shuttle is about 6370 km from Earth’s center.When in orbit it is about km from the Earth’s center. In accord with , the 200-km difference in distance means a 0.06 fractional difference in force. Discounting the changes in the fuel, the gravitational force on the shuttle in orbit is 94% as much as when on Earth’s surface—nearly the same. (Neglect changes in the weight of the fuel carried by the shuttle.)

Consider a giant flat plate that touches the Earth at one point and extends out into space. Suppose you slide an iron block along the plane, where it makes contact with the Earth. Suppose also that the plate is perfectly frictionless, air drag is absent, and vo < vescape. The block will 1. continue at constant velocity, in accord with the law of inertia. 2. increase in speed as the force of gravity on it weakens with distance. 3. decrease in speed due to the pull of gravity. 4. oscillate to and fro. Ch 9-2 Thanks to Ken Ford.

Consider a giant flat plate that touches the Earth at one point and extends out into space. Suppose you slide an iron block along the plane, where it makes contact with the Earth. Suppose also that the plate is perfectly frictionless, air drag is absent, and vo < vescape. The block will 1. continue at constant velocity, in accord with the law of inertia. 2. increase in speed as the force of gravity on it weakens with distance. 3. decrease in speed due to the pull of gravity. 4. oscillate to and fro. Ch 9-2 Thanks to Ken Ford. Answer: 4, oscillate to and fro. If you answered c, not bad; but d is the better answer. The force of gravity on the block is perpendicular to the plane only at the point of contact with the Earth. As the block slides farther out on the plane, a component of gravitational force parallel to the plane decreases its speed (it travels somewhat “upward” against Earth’s gravity). But sliding against gravity does more than merely reduce its speed—the block finally comes to a stop. What happens then? It slides back and the process is repeated in cyclic fashion. From a flat-Earth point of view, the situation is equivalent to that shown in the sketch.

If the Sun suddenly collapsed to become a black hole, the Earth would
Ch 9-3 1. leave the Solar System in a straight-line path. 2. spiral into the black hole. 3. continue to circle in its usual orbit.

If the Sun suddenly collapsed to become a black hole, the Earth would
Ch 9-3 Answer: 3 We can see from Newton’s equation, that the interaction F between the mass of the Earth and the Sun doesn’t change. This is because the mass of the Earth does not change, the mass of the Sun does not change even though it is compressed, and the distance from the centers of the Earth and the Sun, collapsed or not, does not change. Although the Earth would very soon freeze and undergo enormous surface changes, its yearly path would continue as if the Sun were its normal size. 1. leave the Solar System in a straight-line path. 2. spiral into the black hole. 3. continue to circle in its usual orbit.

Suppose the gravitational force between the Earth and Moon was turned off and the pull replaced by the tension in a steel cable joining them. Consider the tension in such a cable, and its size. The tensile strength of a steel cable is about 5.0  108 N/m (each square meter cross section can support a 5.0  108–Newton force). The cross-sectional area would be about that of 1. a bass guitar string. 2. a typical vertical cable that supports the Golden Gate Bridge. 3. the Empire State building. 4. Manhattan Island, N.Y. 5. an area greater than New York State. Ch 9-4

Suppose the gravitational force between the Earth and Moon was turned off and the pull replaced by the tension in a steel cable joining them. Consider the tension in such a cable, and its size. The tensile strength of a steel cable is about 5.0  108 N/m (each square meter cross section can support a 5.0  108–Newton force). The cross-sectional area would be about that of 1. a bass guitar string. 2. a typical vertical cable that supports the Golden Gate Bridge. 3. the Empire State building. 4. Manhattan Island, N.Y. 5. an area greater than New York State. Ch 9-4 Answer: 5 From we find the area A of the cable to be That’s about 400,000 square kilometers, which is more than three times the area of New York state! (New York has an area of 129,000 km2.) Although gravitation is the weakest of the fundamental forces, we see that between great masses even long distances apart, it can be enormous.

Ocean tides are produced by the Moon
Ocean tides are produced by the Moon. Since our bodies are mostly water, doesn’t the Moon similarly produce tides in our bodies? 1. Yes, there are biological tides that affect mood and behavior. 2. Yes, but negligible (less than are produced by an apple you hold over your head). 3. No, because the water in our body isn’t free to flow. Ch 9-6

Ocean tides are produced by the Moon
Ocean tides are produced by the Moon. Since our bodies are mostly water, doesn’t the Moon similarly produce tides in our bodies? 1. Yes, there are biological tides that affect mood and behavior. 2. Yes, but negligible (less than are produced by an apple you hold over your head). 3. No, because the water in our body isn’t free to flow. Ch 9-6 Answer: 2 Tides are caused by differences in gravitational pulls by the Moon (or other celestial bodies) that stretch Earth’s oceans. The key to tides is differences in pulls, which is related to differences in distance between various parts of a body and the Moon. Earth’s ocean tides are the result of thousands of kilometers difference in distance between near and far parts of the ocean. Scarcely any tides occur in a lake because no part is significantly closer to the Moon than other parts. Likewise for the fluids in your body. You’re not tall enough for your head to be appreciably closer to the Moon than your feet. The Moon does produce microtides in your body, however. How strong? Less than an apple held a half meter over your head produces!

Would observers on the Moon see the Earth “rise” and “set,” as we here on Earth see the Moon rise and set? Ch 9-8 1. Yes 2. No

Would observers on the Moon see the Earth “rise” and “set,” as we here on Earth see the Moon rise and set? Ch 9-8 Answer: 2 The rotation of the Moon is synchronized with its revolution about the Earth, with the result that the same side of the Moon always faces the Earth. So from the Moon, the Earth always appears at the same point in the sky. (Of course, the Earth will be seen to go through the same phases, from crescent to full, as the Moon does from Earth.) 1. Yes 2. No

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