Physics Chapter 9 - Gravity
Inverse-Square Law
G Force
Weightlessness
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
1
2
Force
1
1/4
3
1/9
4
1/16
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?
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

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)

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!

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)
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
Review Questions
1.
Would the springs inside a bathroom scale be more
compressed or less compressed if you weighed yourself in
an elevator that accelerated upward? Downward?
2.
Give an example of when your weight is zero.
3.
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.)