In 2010, a large rotary saw blade came loose from a street saw, tore across a la

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 56 cm, a mass of 7.1 kg, and rotates at 5 revolutions/sec as it rolls across the lawn. What is the saw blade's angular momentum?

When the saw blade encounters the house, a net force applied tangent to the edge of the blade slows its rotation. (A radial force stops the linear motion of its center of mass, but here we are concerned with the tangent force that slows its rotation.) Suppose the net force tangent to the edge of the blade is 21.6 N. How long does it take the saw blade to come to a stop? s

Explanation / Answer

here,
m = 7.1 kg
D = 56cm
R = 56/2 = 28 cm = 0.28 m
w = 5 rev /s * 2 *3.14 = 31.416 rad/s

Moment of inertia of saw blade = 0. 5 mr^2
I = 0.5 * 7.1 * 0.28^2
I = 0.28 kg.m^2

A)
Angular momentum is L = I*w
L = 0.28 * 31.416
L = 0.0779 kg.m2/s

B)
as
w = 31.416 rad/s

we have :
V = w/r
v = 31.416/0.28
v = 112.2 m/s

also from Newton second law :
F = ma
a = F/m
a = 21.6 / 7.1
a = 3.042 m/s^2

also we have
v = u+at
t = v/a
t = 112.2 / 3.042
t = 36.88 s

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