chapter 8
A 40 N child and a 80 N child are sitting on a balanced seesaw. The 40 N child is sitting 1.5 m from the fulcrum. How far from the fulcrum is the 80 N child sitting?
0.75 m
Calculate the tension in a horizontal string that whirls a 3-kg toy in a circle of radius 0.5 m when it moves at 2 m/s on a frictionless icy surface.
24 N
What is the relationship between the center of gravity and the support base for an object that is in stable equilibrium?
A vertical line through the center of gravity must pass inside the support base of the object.
Why doesn't the Leaning Tower of Pisa topple over?
A vertical line through the center of gravity passes inside its support base.
What is the law of inertia for rotating systems in terms of angular momentum?
Angular momentum does not change without an applied external torque.
Consider two cases. Case 1: you are balancing a meter stick by supporting the end with the tip of your finger. Case 2: you are balancing a meter stick by supporting the end with the tip of your finger, but now there is a mass attached to the other end. Which is easier?
Case 2
If a skater who is spinning pulls her arms in so as to reduce her rotational inertia by half, by how much will her angular momentum change? By how much will her rate of spin change?
Her angular momentum does not change. Her spin rate doubles.
Is it easier to balance a long rod with a mass attached to it when the mass is closer to your hand or when the mass is farther away?
It is easier when the mass is farther from your hand.
What is meant by the "lever arm" of a torque?
It is the perpendicular distance from the rotational axis to the line along which the force acts.
Why does the rotational inertia of the rod with the attached mass closer to your hand compare the way it does with the rotational inertial of the rod with the attached mass farther away?
Rotational inertia depends on whether the mass is farther or closer to the point of rotation. The farther the mass is, the higher the rotational inertia.
On a rotating turntable, how do tangential speed and rotational speed vary with distance from the center?
Tangential speed increases with distance. Rotational speed is constant.
Where is the center of mass of a baseball? Where is its center of gravity?
The center of mass and the center of gravity are both at the center of the spherical ball.
Inertia depends on mass; rotational inertia depends on mass and something else. What?
The distribution of mass about the axis of rotation
How does the rotational inertia of the rod with the mass toward the bottom compare with the rotational inertia of the mass toward the top?
The rotational inertia of the rod with the mass closer to the top is greater than the rotational inertia of the rod with the mass closer to the bottom.
If you are not wearing a seat belt in a car that rounds a curve, and you slide across your seat and slam against a car door, what kind of force is responsible for your slide: centripetal, centrifugal, or no force?
There is no force as viewed by someone outside the car. To them you move in a straight line.
How do clockwise and counterclockwise torques compare when a system is balanced?
They are equal in magnitude and opposite in direction.
What does a torque tend to do to an object?
Torque tends to twist or change the state of rotation of the object.
When a rock tied to a string whirled in a horizontal circle somehow doubles in mass but keeps the same speed, the string tension _______.
doubles
The torque exerted by a crowbar on an object increases with increased _______.
force and leverage distance
Rotational inertia about the midpoint of an object becomes greater with _______.
increased mass and increased distance to mass concentration
When a rock tied to a string is whirled in a horizontal circle, doubling the speed _______.
quadruples the tension in the string
