Chapter 8

Front 1

Distinguish between tangential speed and rotational speed?

Back 1

Tangential speed is how fast a point on a circular object is moving at a certain distance from the center whereas rotational speed is how many degrees (or radians) a point on the circle goes through in a period of time. Every point on a circle has the same rotational speed. The further out you go from the center, the higher the tangential speed is.

Front 2

What is the Relationship between tangential speed and distance from the center of rotational axis?

Back 2

Tangential speed is directly proportional to rotational speed at any fixed distance from the axis of rotation.

Front 3

Inertia depends on mass; rotational inertia (better known as moment of inertia) depends on mass and something else. What?

Back 3

Moment of inertia, ‘I’ depends on mass and its distance from the center of rotation.
I = "mr 2

Front 4

Consider three axes of rotation for a pencil: along the lead; at right angles to the lead at the middle; at right angles to the lead at one end. Rate these for rotational inertia.

Back 4

Along the lead, least. At the middle, more. At an end, the most.

Front 5

Which will have the greater acceleration rolling down an incline, a hoop or a solid disk?

Back 5

A solid disk has less rotational inertia per mass and will therefore undergo the greater acceleration.

Front 6

What does a torque tend to do to an object?

Back 6

A torque tends to rotate an object, or change its rotational state.

Front 7

What is meant by the “lever arm” of a torque?

Back 7

Lever arm is the shortest distance between an applied force and the axis of rotation.

Front 8

How do clockwise and counterclockwise torques compare when a system is balanced?

Back 8

Balance is achieved when the net torque is zero—when clockwise and counterclockwise torques have the same magnitude.

Front 9

If you hang at rest by your hands from a vertical rope, where is your center of gravity with respect to the rope?

Back 9

Your CG is beneath the rope, between your hands.

Front 10

Where is the center of mass of an empty shoe box?

Back 10

The CM is in the center, where no mass actually exists.

Front 11

Why doesn’t the Leaning Tower of Pisa topple?

Back 11

It doesn’t topple because its CG is above a base of support.

Front 12

In terms of center of gravity, support base, and torque, why can you not stand with heels and back to a wall and then bend over to touch your toes and return to your stand-up position?

Back 12

Unless you have very long feet, when you bend over your CG extends beyond the support base provided by your feet. A torque then exists about the axis at the tip of your toes and over you go.

Front 13

When you whirl a can at the end of a string in a circular path, what is the direction of the force that is exerted on the can?

Back 13

The force on the whirling can is directed toward the center of its circular path, toward your hand.

Front 14

If the string breaks that holds a whirling can in its circular path, 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?

Back 14

When the string breaks, no force (except for downward gravity) acts on the can. In accord with the law of inertia, it flies off tangent to the place it was when the string broke.

Front 15

If you are not wearing a seat belt and you slide across your seat and slam against a car door when the car rounds a curve, what kind of force is responsible—centripetal, centrifugal, or no force? Support your answer.

Back 15

No force. Your tendency to follow a straight-line path in the absence of a force is responsible for the door slamming against you. This is in accord with the law of inertia.

Front 16

Why is centrifugal force in a rotating frame called a “fictitious force?”

Back 16

A real force must have a reaction counterpart. Since centrifugal force has none, we call it “fictitious.”

Front 17

How can gravity be simulated in an orbiting space station?

Back 17

Gravity is simulated by centrifugal force if the station is made to spin.

Front 18

Distinguish between linear momentum and angular momentum.

Back 18

Linear momentum is described by the product mv. Angular momentum is the same multiplied by radial distance r. So angular momentum is mvr.

Front 19

If a skater who is spinning pulls her arms in so as to reduce her rotational inertia to half, by how much will her angular momentum increase? By how much will her rate of spin increase? (Why are your answers different?)

Back 19

Oops—tricky question! Angular momentum doesn’t change at all when she pulls her arms in. What does change is her spin rate, which doubles.

Front 20

Which will roll down fast- a can of water or a can of ice?

Back 20

Both will roll at the same speed because rotational inertia relative to their mass is the same.

Front 21

Is the net torque changed when a partner on a seesaw stands or hangs from her end instead of sitting?

Back 21

Since torque is the product of perpendicular component of the force multiplied by the distance to the pivot, torque is not effected because neither one of these things changes.

Front 22

When you pedal a bicycle, maximum torque is produced when the pedal sprocket arms are in the horizontal position and no torque is produced when they are in vertical position. Explain

Back 22

When one pedals a bicycle, maximum torque is produced when the pedal sprocket arms are in the horizontal position. No torque is produced when they are in the vertical position because torque- lever arm X force. The lever arm is at its maximum when the pedal sprocket arms are in the horizontal position and zero when they are in the vertical position. So the torque produced is maximal in the horizontal position and zero in the vertical position.

Front 23

When a bowling ball leaves your hand, it may not spin. But further along the alley, it begins to spin. What produces the spinning?

Back 23

When a bowling ball leaves the hand, it may not spin. However, farther along the alley it spins because of the torque provided by the air resistance force.

Front 24

Explain why a long pole is more beneficial to a tightrope walker if the pole droops?

Back 24

It's all in the balance and the weight. A longer pole provides more stability for those reasons. Think of it this way. At your now given height, if your arms were only a foot long, could you balance yourself while say walking on top of a narrow wall? or is it easier to do with your arms at their normal length? the poles are made to be flexible and not rigid so that they flex and the walker can adjust the pole while walking. Better center of gravity.

Front 25

The centers of gravity of three trucks parked on a hill are shown by the dots. Which truck(s) will tip over?

Back 25

The center of gravity of Truck A is not above an area of support: The centers of gravity of Trucks B and C are above their areas of support. Therefore only Truck A will tip over.

Front 26

A long track balanced like a seesaw supports a golf ball and a more massive billiard ball with a compressed spring between the two. When the spring is released, the balls move away from each other. Does the track tip clockwise, tip counterclockwise, or remain in balance as the balls roll outward? What principles do you use for your explanation?

Back 26

The center of mass is initially located directly above the pivot. When the two balls spring apart from each other, they do so in a manner that leaves the center of mass where it was. That is, after they spring apart, the center of mass remains directly above the pivot. Therefore, the track remains balanced!

Front 27

Calculate the torque produced by a 50-n perpendicular force at the end of a 0.2 m-long wrench.

Back 27

Torque = perpendicular force x distance from reference point. In a wrench, the reference point is the end (since it stays in the same place, just turns) So Torque = 50N x 0.2m = 10N.m

Front 28

Calculate the torque produced by the same 50-N force when a pipe extends the length of the wrench to 0.5m.

Back 28

T=FxD, or T=FD, or T=F(D), or Torque equals force times distance. 25 N/m torque

Front 29

What is the tangential speed of a passenger on a Ferris wheel that has a radius of 10 m and rotates once in 30 s?

Back 29

- tengential speed (v) = r (radius) * (w) angular speed
v = r w
w = rotates once / 30 sec = angle moved (360 deg)/30
w = 2pi (radian)/30 = 2*3.14/30
--------------
v = 10 (m) [2*3.14/30] (rad/s)
v = 2.09 m/s

Front 30

Neglect the weight of the meterstick and consider only the two weights hanging from its ends. One is a 1-kg weight and the other is a 3-kg weight, as shown. Where is the center of mass of this system? How does your answer relate to torque?

Back 30

Let the cneter of the mass of the system in the above figurebeat a distance x from A then
x = 3(100-x)
4x =300
Then x =75cm
So, the center of mass is at a distance of 75cm from atowards B .
The torque due to 1kg weight is balanced by the torque due to3kg weight.

Front 31

The rock has a mass of 1 kg. What is the mass of the measuring stick if it is balanced by a support force at the one-quarter mark?

Back 31

The center of mass of the measuring stick is at the one-half mark, one-quarter of the stick's length beyond the pivot. The stick is balanced by the 1-kg rock that is one-quarter of its length to the left and by its own weigh which is at the Center of Mass, one-quart of its length to the right. Therefore, the mass of the stick must be one kilogram, 1 kg.