Physics--Chapter 8: Rotational Equilibrium and Dynamics
Supplemental Review Questions (answers at end)
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What occurs when a force is applied to an extended object?
How can an object be in both rotational and translational equilibrium?
Describe Newton’s second law of rotation.
A bucket filled with water has a mass of 23 kg and is attached to a rope that is wound with a
crank around a 0.05 m radius cylinder at the top of the well. The moment of inertia of the
cylinder and crank is 0.12 kg•m2. The bucket and water are first raised to the top of the well
and then released to fall back into the well. What is the rotational kinetic energy of the cylinder
and crank at the instant the bucket is moving at a speed of 7.9 m/s?
a) 2.1 × 103 J
b) 1.5 × 103 J
c) 7.0 × 102 J
d) 4.0 × 102 J
Explain how the operation of a simple machine alters the applied force and distance moved.
How does the use of a machine alter the work done on the object?
When a machine increases the force acting on an object, what happens to the distance moved?
When a machine decreases the force acting on an object, what happens to the distance moved?
A girl pushes a box that has a mass of 650 N up an incline. If the girl exerts a force of 150 N
along the incline, what is the mechanical advantage of the incline?
An iron bar is used to lift a slab of cement. The force applied to lift the slab is 4.0 × 102 N. If the
slab weighs 6400 N, what is the mechanical advantage of the bar?
A force of 255 N is needed to pull a nail from a wall using a claw hammer. If the resistance force
of the nail is 3650 N, what is the mechanical advantage of the hammer?
A boy can move a rock that weighs 95 N by applying a force of 15 N. What is the mechanical
advantage of the lever?
What is the efficiency of a machine that requires 1.00 × 102 J of input energy to do 35 J of work?
A box weighing 210 N is pushed up an inclined plane that is 2.0 m long. A force of 140 N is
required. If the box is lifted 1.0 m, what is the efficiency of the inclined plane?
A force of 1250 N is needed to move a crate weighing 3270 N up a ramp that is 4.55 m long. If
the elevated end of the ramp is 0.750 m high, what is the efficiency of the ramp?
What is the efficiency of a machine that requires 135 J of input energy to do 87.5 J of work?
What is the efficiency of a machine that requires 1463 J on input energy to do 670 J or work?
Answers:
1) The force produces a torque, which causes the object to rotate.
2) There must be zero net force and zero net torque.
3) Net torque is directly related to an object’s moment of inertia multiplied by its angular
acceleration.
4) b
5) A machine can increase or decrease the force acting on an object at the expense or gain of the
distance moved.
Physics--Chapter 8: Rotational Equilibrium and Dynamics
Supplemental Review Questions (answers at end)
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The work done on the object is constant.
The distance moved decreases.
The distance moved increases.
4.3
16
14.3
6.3
35%
75%
43.1%
64.8%
46%