Problem #1 A disc rotates from rest for 8.00 s, until it reaches a rotational sp

Question

Problem #1 A disc rotates from rest for 8.00 s, until it reaches a rotational speed of 5.00 rev s . The instant it reaches this speed it begins to slow to rest over a period of 12.0 s. Calculate the number of revolutions the disc turns while it is in motion.

Problem #2 A car traveling on a flat (unbanked), circular track accelerates uniformly from rest witha tangential acceleration of a. The car makes it one-quarter of the way around the circle before it skids off the track. From these data, determine the coefficient of static friction between the car and the track.

Explanation / Answer

wo = 0

w1 = 5 rev/s = 5*2*pi rad/s = 31.4 rad/s

during acceleration

angle rotated = theta1 = average velocity *t = (31.4/2)*8 = 125.6 rad

while slowing

theta2 = average velocity *time = (31.4/2)*12 = 188.4 rad

total displacement = 125.6+188.4 = 314 radians = 50 revolutions

#2)

speed v = sqr(2*a*pi*r/2)

v = sqrt(a*pi*r)

ar = u*g

v^2/r = u*g

a*pi = u*g

u = a*pi/g

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