Question
question #72
chergy to the rotational kinetic energy calculated in the previ- ous problem 0.A potter's flywheel is made of a 5.00-cm-thick, round slab of concrete that has a mass of 60.0 kg and a diameter of 35.0 cm. This disk rotates about an axis that passes through its center, perpendicular to its round area. Calculate the angu lar speed of the slab about its center if the rotational kinetic energy is 15.0 J. Express your answer in both rad/s and rev min 79. Wha 10-kg bar Assume 80*A dr to the to steeringv tia of 0.0 steeringv 71.Sports A flying disk ( 160 g, 25.0 cm in diameter) spins at a rate of 300 rpm with its center balanced on a fingertip. What is the rotational kinetic energy of the Frisbee if the disc has 70% of its mass on the outer edge (basically a thin ring 25.0-cm in ameter) and the remaining 30% is a nearly flat disk 25.0-cm in diameter? SSM 5 The equations for rotational kinematics are almost identical to those for linear motion Suppose a roulette wheel is spinning at 1 rev/s. (a) How long will it take for the wheel to come to rest if it experiences an angular acceleration of -0.02 rad/s? (b) How many rota tions will it complete in that time? 73· *A spinning top completes 6000 rotations before it starts to topple over. The average speed of the rotations is 800 rpm. Calculate how long the top spins before it begins to topple 74·*A child pushes a merry-go-round that has a diameter of 4.00 m and goes from rest to an angular speed of 18.0 rp a time of 43.0 s. (a) Calculate the angular displacement and the average angular acceleration of the merry-go-round. (b) What is the maximum tangential speed of the child if she rides on the 'S acting edge of the platform? 75·Jerry twirls an umbrella around its central axis so that it completes 24.0 rotations in 30.0 s. (a) If the umbrella starts duces c from rest, calculate the angular acceleration of a point on the outer edge. (b) What is the maximum tangential speed of a point on the edge if the umbrella has a radius of 55.0 cm 76. Prior to the music CD, stereo systems had a phono- graphic turntable on which vinyl disk recordings were played 81. the pal muscle i is flexe moves b Figure muscle orce o tive leve cm, Curious all this mu percent studies particular phonographic turntable starts from rest and achieves a final constant angular speed of 335 rpm in a time of 4.5 s. How many rotations did the turntable underge durine syndro
Explanation / Answer
given
initial angular speed = wi = 1 rev/s = 2pi rad/s
acceleration = alfa = -0.02 rad/s^2
from the equation of ratatory motion
wf = wi + alfa*t
0 = 2*pi - 0.02*t
time = t = 314.16 s
+++++++
(b)
theta = wi*t + 0.5*alfa*t^2
theta = 2*pi*314.16 - 0.5*0.02*314.16^2
theta = 986.96 rad = 986.96/2pi = 157.1 rotations
