PHYS 2211 Module 12.2

Gravitation Near Earth’s Surface

Recommended Reading

12.2 Gravitation Near Earth’s Surface

Learning Objectives

By the end of this section, you will be able to:

  • Explain the connection between the constants G and g
  • Determine the mass of an astronomical body from free-fall acceleration at its surface
  • Describe how the value of g varies due to location and Earth’s rotation


Practice 12.2.1
Compared to Earth, planet X has twice the mass and twice the radius. This means that compared to Earth’s surface gravity, the surface gravity on Planet X is
(a) four times as much. 
(b) twice as much.
(c) the same.
(d) half as much.
(e) one-quarter as much.
Practice 12.2.2
Suppose Earth had no atmosphere and a ball were fired from the top of Mt. Everest in a direction tangent to the ground.

If the initial speed were high enough to cause the ball to travel in a circular trajectory around Earth, the ball’s acceleration would
(a) be much less than g (because the ball doesn’t fall to the ground).
(b) be approximately g.
(c) be much greater than g.
(d) depend on the ball’s speed.

It’s g. The acceleration of ANY object that is freely falling near the surface of the earth is g. Sideways speed is irrelevant. This goes way back to the very beginning of the class —  it’s Galileo’s observation.  Air resistance would change it a wee bit. Also, “g” at the top of Mt. Everest is a TINY TINY bit smaller than “g” here.