Question 1

A child runs and jumps tangentially onto an initially stationary playground merry-go-round. As a result, the merry-go-round begins to turn. Explain in terms of the child-merry-go-round system.

Accepted Answer

The child has an initial angular momentum about the center of the merry-go-round.The total angular momentum of the system is conserved,so the merry-go-round must rotate when the child is on it.

Question 2

A puck moves on a horizontal air table. It is attached to a string that passes through a hole in the center of the table. As the puck rotates about the hole, the string is pulled downward very slowly and shortens the radius of rotation, so the puck gradually spirals in towards the center. What happens to the puck's angular speed?

Accepted Answer

The tension in the string is directed towards the center,so there is no torque about the center.Thus,its angular speed has to increase to keep the angular momentum constant.

Question 3

Two equal forces are applied to a door at the doorknob. The first force is applied perpendicular to the door; the second force is applied at 30° to the plane of the door. Which force exerts the greater torque?

Accepted Answer

Torque is maximized when the force is applied perpendicular to the lever arm. The first force, being perpendicular, exerts the greater torque compared to the second force applied at an angle.

Question 4

If you stand with your back towards a wall and your heels touching the wall, you cannot lean over to touch your toes. Why?

Accepted Answer

The answer of If you stand with your back towards...

Question 5

The attraction of the Moon causes tides on Earth. Because of friction, tidal action slows the rotation of Earth. What happens to the angular momentum of the Earth-Moon system, and does this affect the motion of the moon?

Accepted Answer

The answer of The attraction of the Moon causes tides...

Question 6

In recent years, many planetary systems have been discovered around distant stars. This has been accomplished, not by direct observation of the planets themselves, but indirectly, by observing a wobbling motion in the star. Why is this wobbling motion evidence of a planet?

Accepted Answer

The answer of In recent years, many planetary systems have...

Question 7

Jane says that the magnitude of the torque exerted by a force of magnitude F is equal to the perpendicular distance from the axis of rotation r_⊥ multiplied by F, while Jason insists that it is equal to the distance from the axis of rotation r multiplied by the magnitude of the perpendicular component of the force, F_⊥. Who is right?

Accepted Answer

The answer of Jane says that the magnitude of the...

Question 8

A puck moves on a horizontal air table. It is attached to a string that passes through a hole in the center of the table. As the puck rotates about the hole, the string is pulled downward very slowly and shortens the radius of rotation, so the puck gradually spirals in towards the center. What happens to the puck's linear speed?

Accepted Answer

The answer of A puck moves on a horizontal air...

Question 9

Two equal forces are applied to a door. The first force is applied at the midpoint of the door; the second force is applied at the doorknob. Both forces are applied perpendicular to the door. Which force exerts the greater torque?

Accepted Answer

The torque depends on both the magnitude of the force and the distance from the pivot point (in this case, the hinge) where the force is applied. The force applied at the doorknob is farther away from the hinge than the force applied at the midpoint, which means it has a larger lever arm and produces a greater torque. Therefore, choice B is correct.

Question 10

When a car decelerates, its front end goes down. Explain.

Accepted Answer

The answer of When a car decelerates, its front end...

Question 11

Why is it recommended that if you are lifting a heavy object you should lift it as close to your body as possible?

Accepted Answer

The answer of Why is it recommended that if you...

Question 12

Some students are discussing why a simple Atwood machine accelerates. Jane says that because of the unequal masses, gravity exerts a constant external torque on the two-mass and pulley system and thus its total angular momentum increases at a constant rate. Is Jane's explanation a valid one?

Accepted Answer

The answer of Some students are discussing why a simple...

Question 13

A child is riding near the outer rim of a freely-rotating playground merry-go-round. What happens if the child walks towards the center?

Accepted Answer

The answer of A child is riding near the outer...

Question 14

State the conditions for static equilibrium.

Accepted Answer

The answer of State the conditions for static equilibrium....

Question 15

If the net force on an object is zero N, does the net torque on the object have to be zero Nm?

Accepted Answer

The answer of If the net force on an object...

Question 16

A puck moves on a horizontal air table. It is attached to a string that gradually winds itself up around a round peg in the center of the table. This shortens the string, so the puck gradually spirals in towards the center. What happens to the magnitude of the puck's angular momentum?

Accepted Answer

The answer of A puck moves on a horizontal air...

Question 17

In a model airplane, the propeller is a significant part of the total mass. A model airplane is flying on a level path, with its propeller rotating clockwise as seen facing the airplane. At a preset time, a signal causes the plane to lower its tail flaps and it goes into a nose-down attitude. When this happens, the nose of the plane also swings towards the right. Explain why this happens.

Accepted Answer

The answer of In a model airplane, the propeller is...

Question 18

Can two different forces, acting through the same point, produce the same torque on an object?

Accepted Answer

The answer of Can two different forces, acting through the...

Question 19

An object in motion with constant speed along a straight line can never have nonzero angular momentum.

Accepted Answer

An object moving in a straight line can have angular momentum relative to a point not on the line of motion, as angular momentum depends on the position vector relative to the point and the linear momentum.

Question 20

Tightrope walkers often employ a long pole to help them retain their balance. What advantage is there to using a very flexible pole so that the ends droop?

Accepted Answer

The answer of Tightrope walkers often employ a long pole...

Quiz 11: Rotational Dynamics and Static Equilibrium