Astronomy 101 Chapter 5 Review

Name another kind of field also associated with action at a distance besides gravity.

Electric

Why did Newton conclude that gravity has to be mutual and universal?

From his third law

How is gravity related to acceleration? Are all accelerations the result of gravity?

Gravity is the force that tells a mass how to accelerate. Other forces can also cause acceleration.

You have the same mass as a person sitting next to you. Are you gravitationally attracted to them? If so, why don't you instantly zoom over and stick to them?

I am gravitationally attracted to them. However, since our mass is fairly small, this attraction is insignificant.

Since Earth is not the center of the Universe, why doesn't Aristotle's explanation of gravity work?

If Aristotle's explanation of gravity were true, then all things would be pulled off Earth and move toward the center of the Universe, which he believed was their proper place.

How does the first postulate of special relativity imply the second postulate?

If the speed of light were not constant, measurements of its speed by different observers could be used to detect which observer was moving

How do planets orbiting the Sun and skaters doing a spin both conserve angular momentum?

The angular momentum of a planet is conserved as it moves faster closer to the Sun. The angular momentum of a skater is conserved as he or she spins faster with his/her arms drawn in near the axis of rotation.

Balance a new pencil on the side of your finger. Where is the center of mass?

The center of mass is a point located within the pencil where it rests on the finger.

According to Newton, what is the force causing the radial acceleration of the orbital motion of planets?

The force causing that acceleration is gravitation, the force bringing masses toward one another.

Why can't a spacecraft go "beyond Earth's gravity"?

The force of gravity is inversely proportional to the square of distance. This force drops to zero at a distance of infinity. Therefore, Earth will always pull on a spacecraft

You put a heavy textbook and your pencil on a ceramic tile floor, and you blow on each. Which has more momentum—the pencil, the textbook, or neither? Why?

The pencil because it has a greater momentum than zero

You put a heavy textbook and your pencil on a ceramic tile floor, and you blow on each. Which has more inertia—the pencil, the textbook, or neither? Why?

The textbook

Today at the beach you see the highest of all high tides during the last month. You see the Moon in the daytime sky. What is the most likely Moon phase?

Waxing crescent

When you ride a fast elevator upward, you feel slightly heavier as the trip begins and slightly lighter as the trip ends. How is this phenomenon related to the equivalence principle?

When the elevator begins to move up, you feel an increase in your apparent weight. You cannot tell the difference between increased apparent weight due to acceleration and increased weight due to increased gravity.

You are sitting next to a person who has twice as much weight. Are you gravitationally attracted to them? If so, is it twice as much as their attraction to you? If not, why not?

Yes, I am gravitationally attracted to them. Our gravitational attraction is equal because it is proportional to the product of our two masses.

Does the car have nonzero velocity

Yes, because the car travels.

From your knowledge of general relativity, would you expect radio waves from distant galaxies to be deflected as they pass near the Sun? Why or why not?

Yes. Radio waves are a form of electromagnetic radiation just like visible light. Since the gravitational force of the Sun bends light, radio waves should be bent as well.

Where is the center of mass of your body?

standing human body is a point within the core of the body and it changes when the position changes.

If you drop a feather and a steel hammer at the same moment, they should hit the ground at the same instant. Why doesn't this work on Earth, and why does it work on the Moon?

Air resistance slows the feather on Earth but doesn't affect the hammer very much. Thus, they land at a different time. On the Moon, there is no air (and no air resistance).

Where is the center of mass of the Earth-Moon system?

Along the line from the center of Earth to the center of the Moon and at a distance of about 4700 km from the center of Earth.

If a planet were to slowly migrate inward toward the Sun while remaining in a stable orbit, would its orbital speed increase, decrease, or stay the same? Would its angular momentum change?

As the planet migrates toward the Sun, its orbital speed would increase and its angular momentum would stay the same.

Why is the period of an open orbit undefined?

In an open orbit the object never completes a revolution around the parent object and does not return to its starting place.

How does the concept of a field explain action at a distance? Use the gravitational field as an example.

It is a representation of an object influencing another one. The mass affects the field which spreads out in all directions causing gravitational attraction of any other mass.

According to Kepler's first law, planets move in elliptical orbits. Why is that considered accelerated motion?

Kepler's second law shows that a planet's speed changes due to radial acceleration. Planets also change direction as they move

According to Aristotle, if earth and water were displaced then they would return naturally to their proper place. Today, what do we call this Aristotelean natural motion

Motion due to gravity.

Why did Newton conclude that some force had to pull the Moon toward Earth?

Newton knew that objects move in a straight line at a constant speed, unless acted upon by an outside force (Newton's first law). However, the Moon followed a curved path around Earth

Which of Kepler's or Newton's laws best describes Aristotelean "violent motions"?

Newton's second law of motion

You plant your friend's forearm on the table, winning the contest. Which of Newton's laws applies to this motion and why?

Newton's second law of motion applies because the force that I applied to the mass of my friend's arm was sufficient to accelerate it to the table.

you are in an arm wrestling contest with each other. Neither person is winning, but each of you is struggling to push the other's forearm over to the tabletop. Which of Newton's laws applies to this scenario and why?

Newton's third law of motion, because the force that I am exerting is equal but opposite in direction to the force that my friend is exerting.

An astronaut is in space with a baseball and a bowling ball. The astronaut pushes both objects in the same direction. If both balls are traveling at the same speed, does the baseball have the same momentum as the bowling ball?

No, the baseball has less momentum than the bowling ball.

An astronaut is in space with a baseball and a bowling ball. The astronaut gives both objects an equal push in the same direction. Does the baseball have the same acceleration as the bowling ball from the push? Why?

No. The baseball has greater acceleration than the bowling ball because the bowling ball has the same force acting on a greater mass.