Peoples Physics Concepts

http://www.ck12.org Chapter 6. Gravity

then, we can rewrite the force of gravity equation as

F~G=Gmearth

r^2 earth

︸ ︷︷ ︸

gearth~

×mob j=mob j×~g

We can do this because the quantity in braces only has constants; we can combine them and call their productg.
Remember,this is an approximation that holdsonlywhen the r in formula [3] is more or less constant.

We call the quantitymgan object’sweight. Unlike an object’s mass, an object’s weight can change and depends on
the gravitational force it experiences. In fact, an object’s weight is the magnitude of the gravitational force on it. To
find the weight of an object on another planet, star, or moon, use the appropriate values in the formula for the force
of gravity.

Simulation

Gravity and Orbits (PhET Simulation)

Explore More

1. Which is greater –the gravitational force that the Sun exerts on the moon, or the force the Earth exerts on the
   moon? Does the moon orbit the Earth or the Sun? Explain.


2. Use Newton’s Law of Universal Gravitation to explain why even though Jupiter has 300 times the mass of the
   earth, on the “surface” of Jupiter you’d weigh only 3 times what you weigh on earth. What other factor has to
   be considered here?


3. Provegisapproximately 10 m/s^2 on Earth by following these steps:
   a. Calculate the force of gravity between a falling object (for example an apple) and that of Earth. Use the
   symbolmoto represent the mass of the falling object.
   b. Now divide that force by the object’s mass to find the accelerationgof the object.
   c. Calculate the force of gravity between the Sun and the Earth. (sun mass= 2. 0 × 1030 kg; average distance
   from sun to earth = 150 million km)