Homework #5 Physics 111-4 S12 Page 1 of 2
Homework #5
Chapter 10: Rotational Motion
Page #170
#1: Do all points on a rigid, rotating body have the same angular velocity? Linear speed? Radial
acceleration?
#3: Why doesn’t it make sense to talk about a body’s rotational inertia unless you specify a
rotation axis?
#11: Given a fixed amount of material, what shape should you make a flywheel so it will store
the most energy at a given angular speed?
#17: A compact disk’s rotation varies from about 200rpm to 500rpm. If the disk plays for 74min,
what’s it’s average angular acceleration in (a) rpm’s and (b) rad/s2?
#20: A 320-N frictional force acts on the rim of a 1.0m diameter wheel to oppose its rotational
motion. Find the torque about the wheel’s central axis.
#25: Four equal masses, m, are located at the corners of a square of side, L, connected by
essentially massless rods. Find the rotational inertia of this system about an axis that (a)
coincides with one side and (b) that bisects two opposite sides.
Page 171:
#33: A the MIT Magnet Laboratory, energy is stored in a huge solid flywheel of mass 7.7x104kg,
distributed uniformly in a disk. (a) What’s it’s rotational kinetic energy at 3500rpm? (b)
What average power must be applied to bring the blade from rest to 3500rpm in 3.2s?
#37: (a) Find the energy stored in the flywheel in #33 when it’s rotating at 360rpm. (b) The
wheel is attached to an electric generator and the rotation rate drops from 360rpm to
300rpm in 3.0s. What’s the average power output?
#39: What fraction of a solid disk’s kinetic energy is rotational if it’s rolling without slipping?
Chapter 11: Rotational Vectors and Angular Momentum
Page #183:
#1: Does Earth’s angular velocity vector point north or south?
#7: Why do helicopters have two rotors?
#13: A car is headed north at 70 km/h. Give the magnitude and direction of the angular velocity
of its 62cm diameter wheels.
#23: A 640g hoop 90cm in diameter is rotating at 170rpm about its central axis. What’s it’s
angular momentum?
#27: A uniform, spherical cloud of interstellar gas has mass 2.0x1030kg, has a radius of
1.0x1013m, and is rotating with a period 1.4x106years. The cloud collapses to form a
start 7.0x108m in radius. Find the star’s rotation period.
Page #184:
#37: As an automotive engineer, you’re charged with redesigning a car’s wheels with the goal of
decreasing the wheels angular momentum by 30% for a given linear speed of the car.
Other design considerations require that the wheel diameter go from 38cm to 35cm. If
the old wheel had a rotational inertial 0.32kgm2, what do you specify for the new
rotational inertia?