In 2010, a large rotary saw blade came loose from a street saw, tore across a la
ID: 1372485 • Letter: I
Question
In 2010, a large rotary saw blade came loose from a street saw, tore across a lawn, and cut into a house. (See YouTube video [+].)
Suppose the saw blade has a diameter of 61 cm, a mass of 8.4 kg, and rotates at 4.2 revolutions/sec as it rolls across the lawn.
What is the saw blade's angular momentum?
Hint: You can look up the formula for the moment of inertia of a solid disk in your textbook.
_____________ J OR m/s OR kg·m²/s OR kg·m/s OR kg/s
When the saw blade encounters the house, a force applied to the edge of the blade slows it down. Suppose the force on the edge of the blade is 23.9 N. How long does it take the saw blade to come to a stop?
Hint: Use momentum considerations.
________ s
Explanation / Answer
a)
M = 8.4 kg
R = d/2 = 61/2 = 30.5 cm = 0.305 m
Moment of inertia of saw blade, I = 0.5*M*R^2
= 0.5*8.4*0.305^2
= 0.39 kg.m^2
w = 4.2 rev/s
= 4.2*2*pi rad/s
= 26.38 rad/s
the saw blade's angular momentum = I*w
= 0.39*26.38
= 10.3 kg.m^2/s <<<<<<<<<-------------Answer
b) Net torque acting on the system = rate of change of angular momentum
Tnet = L/t
t = L/Tnet
= 10.3/23.9
= 0.431 s <<<<<<<<<<-------------Answer
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