7B.S11 
Quiz 8C
DL SECTION
_
Grade:
05/25/2011
No books or notes. Calculators OK. Show all your work below. Answers alone will receive no credit!
This quiz has two sides!
Anna and Bob each sit on opposite ends of a seesaw which is
10 m
long, as shown in the picture. Anna, who has a mass of
50 kg
, is
7 m
from the pivot and Bob is
3 m
from the pivot. Note that the pivot is not
in the middle of the seesaw. When Bob and Anna sit on the seesaw, it
is completely stationary. The seesaw itself has a mass of
80 kg
.
a) What is the
net torque
on the seesaw? What is the
net force
on the seesaw? Very briefly and clearly
explain your reasoning.
The seesaw is not rotating nor is its center of mass moving. This means that neither the linear nor the angular
momentum of the seesaw is
changing
. Therefore the net torque and the net force on the seesaw must both be
zero.
b) Find
Bob’s mass
. Ignore the thickness of the seesaw. (Hint: An extended force diagram may be useful, and
don't forget the force of Earth on the seesaw!). Show your work.
There are four forces in the problem, but the pivot force does not exert a torque since it has no moment
arm. The three nonzero torques are due to Anna, Bob and the Earth. If we add all of the torques, we must get
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 Spring '07
 JOHNCONWAY
 Force, Work, Rotation, net force, seesaw

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