Physics 8
Suppose we replace the mass in the video with one that is four times heavier. How far from the free end must we place the pivot to keep the meter stick in balance?
90 cm (10 cm from the weight)
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 can't you stand with your heels and back to a wall and then bend over to touch your toes and return to your stand-up position?
A vertical line through the center of gravity of your body is in front of the tips of your toes, which is outside your support base.
Why doesn't the Leaning Tower of Pisa topple over?
A vertical line through the center of gravity passes inside its support base.
How can gravity be simulated in an orbiting space station?
All of the above
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.
Where is the center of mass of a hollow soccer ball?
In the center of the ball
As distance increases between most of the mass of an object and its center of rotation, how does rotational inertia change?
It increases.
Why is centrifugal force in a rotating frame called a "fictitious force"?
It is not a fundamental force of nature. Instead, it is a force that only appears in an accelerating frame of reference.
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.
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
Consider someone in a rotating space habitat. Rotational motion provides _______.
a support force
The center of gravity of a basketball is located _______.
at its geometrical center
Where is the center of gravity of the broom that Dr. Hewitt holds up?
at the balance point
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.
How do clockwise and counterclockwise torques compare when a system is balanced?
They are equal in magnitude and opposite in direction.
How does the torque due to the weight of one side of the broom exerted around the balance point compare with the torque exerted by the weight of the other side of the broom around the balance point?
equal
The torque exerted by a crowbar on an object increases with increased _______.
force and leverage distance
The tangential speed on the outer edge of a rotating carousel is _______.
greater than toward the center
Consider someone in a rotating space habitat. The outward force felt by the person _______.
has no reaction counterpart
Rotational inertia about the midpoint of an object becomes greater with _______.
increased mass and increased distance to mass concentration
What are the units of measurement for tangential speed? For rotational speed?
m/s for tangential, RPM for rotational
Distinguish between linear momentum and angular momentum.
Angular momentum depends on the distribution of mass, whereas linear momentum depends on the total mass.
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 three axes of rotation for a pencil: along the lead, at right angles to the lead at the middle, and at right angles to the lead at one end. Rate the rotational inertias about each axis from smallest to largest.
Axis along the lead, through the center, through one end
Why does bending your legs when running enable you to swing your legs to and fro more rapidly?
Bending your legs shortens them, thus reducing rotational inertia.
What is required to change the angular momentum of a system?
External torque
If you toss a stick into the air, it appears to wobble all over the place. Specifically, about what place does it wobble?
It will rotate about the center of mass.
How is a flywheel constructed to maximize its rotational inertia?
Most of the mass is concentrated far from the axis.
If you hang at rest by your hands from a long vertical rope, where is your center of gravity with respect to the rope?
Somewhere straight below where your hands grab the rope
Is it an inward force or an outward force that is exerted on the clothes during the spin cycle of an automatic washing machine? What about the direction of the force on the water?
The force is inward on clothes and there is none on water.
When Dr. Hewitt cuts the broom right through the center of gravity, how do the weights of the two sides of the broom compare?
The shorter side, where the bristles of the broom are, has a greater weight than the handle.
e the greater acceleration rolling down an incline: a hoop or a solid disk? Why?
The solid disk will because the mass is closer to the axis of rotation.
How does the tapered rim of a wheel on a railroad train allow one part of the rim to have a greater tangential speed than another part when it is rolling on a track?
The tangential speed is proportional to the radius. The inside of the wheel rolls on a larger radius than the outside of the wheel.
A tapered cup rolled on a flat surface makes a circular path. What does this tell you about the tangential speed of the rim of the wide end of the cup compared with that of the rim of the narrow end?
The tangential speed of the wide end is faster.
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.
What does a torque tend to do to an object?
Torque tends to twist or change the state of rotation of the object.
When you whirl a can at the end of a string in a circular path, what is the direction of the force you exert on the can?
Toward the center of the circle
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
When the rotational speed of a rotating system doubles, its angular momentum _______.
doubles
When a rock tied to a string is whirled in a horizontal circle, doubling the speed _______.
uadruples the tension in the string
If the string that holds a whirling can in its circular path breaks, what kind of force causes it to move in a straight- line path: centripetal, centrifugal, or no force? What law of physics supports your answer?
No force; Newton's first law
Which is easier to get swinging: a baseball bat held at the narrow end or a bat held closer to the massive end (choked up)? Also answer for a short bat versus a long bat.
The easiest is choked up and a short bat.
