Name:
PHY121 R05
Quiz
A cat walks along a uniform plank that is 4.00m long and has a mass of 7.00 kg. The
plank is supported by 2 sawhorses, one 0.440m from the left end of the board and the
other 1.50m from its right end. When the cat reaches the right end, the plank begins to
tip. Consider the instant that the cat reaches the right end, just before the plank moves.
a) what is the sum of the torques on the plank at this point?
 = 0
b) draw the freebody diagram for the plank when the cat stands at its right-hand end,
and the plank is just getting ready to tip. Label all the forces.
CAT
F1 = 0
mplankg
F2
mcatg
c) Is there a force acting on the left sawhorse when the plank tips?
NO
d) what is the mass of the cat? (hint: consider the best place to put the pivot for
this problem)
put pivot at right hand sawhorse, then d for that force = 0, and
sawhorse = 0
then
mplank g (2.0-1.5 m) – mcat g (1.5m) = 0
mcat = mplank (0.5m)/(1.5m) = 1/3 mplank = 2.33 kg