Chapter 3 Review

Video Review

Key Concept Summary

TA Summary


The word "gravity" is used to mean several different things. Match the definition with the most relevant term or symbol.

The gravitational force the earth exerts on an object.
Newton's law of gravity.
The symbol representing the acceleration caused by gravity.


A small ball is dropped from the edge of a cliff. One-tenth of a second later a much heavier ball is dropped from the same position. Ignoring the effects of air friction, the second ball will accelerate faster and overtake the first.
Although the moon is so far away, its gravitational pull still affects Earth.
The force of gravity increases between objects as they get farther away from each other.
Because the Moon orbits Earth at a constant speed, it travels in uniform motion.
A cannon ball weighs a lot more than a marble. This means it requires much more force to "fall" towards Earth at the same rate as a marble.


If a feather and a brick are dropped at the same height in a vacuum, then
A rock is dropped from a cliff. When is the force of gravity greatest on the rock?
Jane throws a rock horizontally off a cliff. How fast is the rock traveling downward after 5 seconds (ignore air friction, use 10 m/s2 for g)?
Using the Principle of Position Symmetry, the gravitational force on an object near another planet would
A baseball player throws a baseball horizontally at 100 mph. You drop a penny at the exact same height just as the ball leaves the baseball player's hand. Air friction is negligible. Which of the following is true?

Free Response

  1. Why does an object weigh less on the surface of the moon than on Earth’s surface?
  2. The Sun has much more mass than Earth (about 330,000 times as much). Why aren’t we pulled toward the Sun with 330,000 times as much force as we are toward Earth?
  3. Compare the weight and mass of a baseball in four locations:
    • On Earth’s surface
    • In orbit around Earth at an altitude of about 100 miles.
    • On the Moon’s surface
    • Outside the solar system far from any planet.
  4. Compare the definitions of weight and mass. Why does the weight of an object change from place to place while the mass does not?
  5. A cannonball, originally at rest, and a marble, originally at rest, are dropped in a vacuum from the same height at the same time.
    • What happens when they are dropped? Compare the speed and acceleration of the cannonball with that of the marble.
    • Is the gravitational force of attraction larger on the cannonball than it is on the marble? Justify your answer using a fundamental force law.
    • Does the cannonball require a larger force to provide the same acceleration as the marble? Justify your answer using the Second Law of Motion.
    • Explain how the cannonball with greater mass and a greater force of gravity still has the same motion as a marble with a lower mass and lower force of gravity being dropped from the same height.
  6. How many forces are acting on the Moon? What force keeps the moon moving around Earth?
  7. A penny and a feather fall toward Earth in a vacuum tube. They begin falling at the same time.
    • Describe what would be observed.
    • Which of Newton’s laws apply to this scenario?
    • Use the laws of motion that you listed above to explain what would occur.
  8. A ride at an amusement park straps you into a seat and raises you 400 feet above the ground. Once you reach the top, you are dropped. Near the bottom, a spring slows your descent.
    • What forces act on you during the ride? When is each force acting on you?
    • What laws of motion would be relevant as you travel from the top to the bottom of the ride?
    • What is the rate of your acceleration when you are in the middle of falling? What law tells you the rate of acceleration?
    • In the middle of falling, you feel weightless. Explain why.
    • What is the direction of the net force at the bottom?
  9. Two identical encyclopedias are dropped. Encyclopedia A is dropped from five feet off the ground and Encyclopedia B is dropped from ten feet off the ground. Encyclopedia B is dropped first. When Encyclopedia B reaches exactly five feet above the ground, Encyclopedia A is dropped.
    • Which encyclopedia has the greater force and thus the greater acceleration acting on it?
    • Which encyclopedia hits the ground first?
    • Which encyclopedia is travelling faster when it hits?
    • Both Encyclopedia A’s and B’s covers hit the ground face down. Does Encyclopedia A or Encyclopedia B exert the greater force on the ground?
  10. You take your 10-kg bowling ball on a trip with you to the distant planet Sophia. The planet Sophia has double the mass and half the diameter of Earth. Answer the following questions about the bowling ball.
    • What is the ball’s mass on Sophia?
    • How does the weight on Sophia compare to its weight on Earth?
    • Using the Law of Universal Gravitation, explain how your answers above are consistent with one another.
    • If dropped on the planet Sophia, how does the bowling ball’s acceleration compare to its acceleration on Earth?
    • You and your bowling buddies decide to race your bowling balls by dropping them from the same height somewhere on Sophia. Your friend’s ball weighs a mere fraction of what your ball weighs on Sophia. Which ball hits first?
  11. Newton’s second law says F = ma. But the law of gravity says F = GMm/d2. Explain why you need both laws. Which aspect of a falling rock’s motion is described by F = ma and which aspect is described by F = GMm/d2?