Example 10-1 You and Your Backpack
While you sit studying, what is the magnitude of the gravitational force that your 10-kg backpack sitting on your desk
exerts on you? Assume that your mass is 70 kg and that your center of mass is 0.60 m from the center of your backpack.
Set Up
We’ll use Newton’s law of universal gravitation,
Equation 10-2, to find the magnitude of the
gravitational force.
Newton’s law of universal gravitation:
Fbag on you =
Solve
Substitute the given values of G, mbag = 10 kg,
myou = 70 kg, and r = 0.60 m into the expression
for gravitational force. The masses and distance
are given to only two significant figures, so our
result has only two significant figures.
Fbag on you =
=
Gmbag myou
r2
backpack
(10-2)
r
X
X
Gmbag myou
r2
16.67 * 10-11 N # m2 >kg 2 2 110 kg2 170 kg2
= 1.3 * 10
Reflect
you
-7
N
10.60 m2 2
The gravitational force your bag exerts on you is very small, about one ten-millionth of a newton. This is equivalent to
the weight of a few specks of dust. This force is far smaller than the gravitational force Earth exerts on you (see the next
example). It’s also far smaller than the force of friction between you and your chair should you start to slide. That’s why
we generally neglect the gravitational force between everyday objects.