1. Physics Chapter 9 - Gravity
Inverse-Square Law
G Force
Weightlessness

2. The Inverse-Square Law
Gravity decreases as the square of the distance increases (Inverse-Square Law)
DISTANCE is measured between the centers of the objects!
Distance
Force /4 1/ /16

3. Inverse-Square Example
The gravitational force between a 10,000 kg and a 20,000 kg asteroid separated by a distance of 100 meters is 1.33 x 10-6 N. What would the gravitational force be if the distance were somehow doubled to 200 m?

4. G Force
Force on rides and in planes, etc are often compared to the normal force of gravity (weight)
The comparison is just a multiple of g (9.8 m/s2)
Example: “2 G’s” means the force you feel is due to acceleration being 2 x g, or in other words the force is 2 x your weight

5. Gravity & Weightlessness
When weight is measured, something provides support (like the scale) so we aren’t falling
If you stand or sit on a scale in an elevator or on a ride at 6 Flags, your weight will vary (the force you feel)

6. Gravity & Weightlessness
To feel weight, we need support. Weight is the force on (or by) the support
When falling, we don’t feel gravity because there is no support
If your inside something (car, elevator) that’s falling, you can’t weigh yourself or feel gravity!

7. Weight & Weightlessness
Your weight equals the force with which you press against the supporting floor.
If the floor accelerates up or down, your weight varies (even though the gravitational force that acts on you remains the same)

8. Gravity & Weightlessness
The moon and satellites are falling around the earth-–they are going fast enough not to crash into the earth
The earth and other planets are falling around the sun and when orbiting earth, astronauts are falling around earth!
The velocity needed to stay in orbit can be calculated (by using Potential and Kinetic Energy), which is called Escape Velocity and is about 11,200 m/s for earth

9. Review Questions
Would the springs inside a bathroom scale be more compressed or less compressed if you weighed yourself in an elevator that accelerated upward? Downward?
Give an example of when your weight is zero.
If the Earth somehow expanded to a larger radius, with no change in mass, how would your weigh be affected? How would it be affected if the Earth instead shrunk? (Hint: Let the equation for gravitational force guide your thinking.)