Deck 2: The Laws of Motion, Part 2

Question 1

A spinning Frisbee is thrown between two persons in a park. The motion of the Frisbee is

A) purely rotational.
B) purely translational.
C) rotational and translational.
D) neither rotational nor translational.

Answer 1

rotational and translational.

Question 2

When chewing hard nuts, we use the teeth towards the back of our mouth because we are able to exert a greater _____________ on the nuts.

A) force
B) torque
C) angular velocity
D) lever arm

Answer 2

force

Question 3

A baseball player throws a bat up in the air, so that it spins as it translates. The bat will rotate about its

A) pivot point farthest from the handle
B) center of mass
C) pivot point near the handle
D) long axis

Answer 3

center of mass

Question 4

A baseball player balances a bat on his finger; the bat is laying sideways and not standing on end. The bat will balance

A) near the end of its handle.
B) near the end farthest from the handle.
C) at its middle.
D) at its center of mass.

Question 5

Consider a 100 kg Father and his 20 kg girl. They decide to enjoy balancing on a seesaw, which is a uniform board 4 m long and pivoted exactly at its center. If girl sits at the seesaw's end (2m from the center), how far from the center on the other side should the father sit?

A) Farther out than the board will allow.
B) 0.5 m
C) 0.4 m
D) 1.5 m

Question 6

Suppose you are changing a car tire and loosening a lug nut. You weigh 800N and stand on the end of a 2 m long wrench. How much torque do you exert on the lug nut?

A) 1600 J
B) 1600 N-m
C) 800 N-m
D) 2 N-m

Question 7

When building soap box cars which race by coasting downhill it is better to have solid wheels rather than ones of equal mass with spokes because

A) The solid wheels have a lower moment of inertia and therefore accelerate faster.
B) The solid wheels are easier to paint.
C) The solid wheels have a higher moment of inertia and therefore retain more rotational energy.
D) The wheels with spokes lose too much energy by heating up the air.

Question 8

Consider the earth spinning from west to east on its axis. It is slowing down in its rotation due to friction with the air. Using the right hand rule, in what direction is the frictional torque?

A) S pole to N pole
B) N pole to S pole
C) Counterclockwise along the equator
D Clockwise along the equator

Question 9

Suppose the time it takes for the moon to rotate doubles. By what factor will its rotational kinetic energy change?

A) It will double.
B) It will quadruple.
C) It will decrease to ¼ its original value.
D) It will decrease to ½ its original value.

Question 10

Suppose the moon's angular velocity is halved. By what factor will its rotational kinetic energy change?

A) It will double.
B) It will quadruple.
C) It will decrease to ¼ its original value.
D) It will decrease to ½ its original value.

Question 11

Suppose you are designing a race bicycle and it comes time to work on the wheels. You are told that the wheels need to be of a certain mass but you may design them either as wheels with spokes (like traditional bike wheels) or you may make them as having solid rims all the way through. Which design would you pick given that the racing aspect of the machine is the most important? Please explain.

Question 12

You are a football player. Your coach notices that, when it has been raining and it is slippery outside, the players you tackle are rotating forward too much (their feet slip backwards). Please explain why this would show up when it is slippery outside and why contacting them a little higher when you tackle them will fix the problem.

Question 13

During a football game the ball is kicked and it travels across the field. If the ball is kicked away from its center of mass it will

A) Translate only
B) Rotate only
C) Translate and rotate
D) Vibrate only

Question 14

It is possible for two objects of equal mass to have rotational masses because

A) Rotational mass depends on total mass and mass distribution.
B) The objects may be made out of different materials.
C) Rotational mass is always half the total mass.
D) An object not rotating has no rotational mass.

Question 15

Some climatologists say that our climate could be warm enough to melt the polar ice caps, sending the resulting water down towards the equator. If this were to happen, the length of the day would

A) Increase due to friction.
B) Increase due to angular momentum conservation.
C) Decrease due to mass loss.
D) Decrease due to work done by the ice.

Question 16

Consider a wind turbine having moment of inertia 240,000 kg m². If a 120 kg person were holding on to the edge of the turbine 10 m from its center the torque due to the person is ________ and the angular acceleration of the turbine is ________.

A) 120,000 Nm; 0.05 rad/s²
B) 12,000 Nm; 0.005 rad/s²
C) 240,000 Nm; 0.05 rad/s²
D) 12,000 Nm; 20 rad/s²

Question 17

An object pivoting about its center of mass is

A) Free of net torque
B) Motionless
C) Always in motion
D) Experiencing a torque about its center of mass.

Question 18

Drive shafts in cars are long metal tubes that transfer power to the axles from the engines through spinning very rapidly. Why can it severely damage your car if the drive shaft is not balanced?

Question 19

You are staring at a non-rotating wind turbine and all of a sudden the wind comes up and makes the turbine start rotating clockwise. The net force that causes translational motion is ______ and the torque causing the turbine to rotate is ________.

A) positive; towards you.
B) negative; towards you.
C) zero; to the left.
D) zero; away from you.

Question 20

Suppose the time it takes for a wind turbine to rotate triples. By what factor will its rotational kinetic energy change?

A) It will double.
B) It will quadruple.
C) It will decrease to 1/9 its original value.
D) It will increase to 9 times its original value.

Question 21

Suppose you double the length of the blades of a wind turbine. For the same wind conditions, the torque on the turbine

A) Stays the same
B) Doubles
C) Quadruples
D) Halves

Question 22

What type of friction causes bearings to not waste much energy?

A) Sliding
B) Static
C) Both
D) Neither

Question 23

A child is riding a bike and skids to a stop. What happens to their kinetic energy?

A) It is turned into potential energy.
B) It is conserved.
C) It is dissipated by static friction.
D) It is turned into heat by sliding friction.

Question 24

You operate a 120 W light bulb for 5 minutes. How much energy did you use?

A) 120 J
B) 3600 J
C) 36,000 J
D) 36,000 KW - h

Question 25

Suppose you take off in a car with your physics book on top. If you are accelerating forward and the book rides with you, in what direction does friction act on the book?

A) backwards
B) forwards
C) There is no friction since there is no sliding.
D) The force of friction is infinite.

Question 26

Suppose you are riding a bicycle. If you want to waste as little energy as possible while the bike is in motion, what type of friction should exist between the bike and the road?

A) Static
B) Sliding
C) None
D) Can't tell

Question 27

Power is

A) The amount of energy used to do a particular job.
B) The amount of heat generated when a job is done.
C) The rate at which energy is used, stored, created, dissipated, etc.
D) The amount of energy used multiplied by time.

Question 28

If you need to harness energy in order to do a particular job, the type of energy needed is

A) ordered energy.
B) thermal energy.
C) energy dissipated by friction
D) gravitational power

Question 29

Sliding friction is not desirable in wheel bearings because

A) too much energy is transferred to the wheel.
B) sliding friction will store too much energy.
C) the wheel will lock on the axle and not move
D) work will be converted into thermal energy, and be dissipated.

Question 30

What does friction not depend upon?

A) the type of surfaces in contact
B) the area of contact
C) the force with which an object is pressing against a surface
D) whether the contact surface is wet or dry

Question 31

Consider a bowling ball sliding down the lane just after you have thrown it. As you know, when the ball is sliding, friction is creating a torque on the ball. Identify four things you could alter so as to cause the ball to experience a larger torque.

Question 32

A person is running a potter's wheel, where they use their feet to run pedals that turn a large disc they can put clay on. Suppose the disc is turning and some clay is dropped straight down on the rotating wheel, which slows down a little. Why did the wheel slow down when the clay was dropped on it, and where on the wheel should the clay be placed so that it is easiest for the potter to speed the wheel back up again? Please explain.

Question 33

When sliding a heavy box across the floor, it is often times helpful to lift up on it slightly because

A) You are lessening the friction force by reducing the contact force between the box and the floor
B) You are reducing the weight of the box.
C) You are working against friction by lifting up.
D) You are working against gravity, even if the box does not move upwards.

Question 34

What type of friction could cause a damaged to waste energy?

A) Sliding
B) Static
C) Both
D) Neither

Question 35

When your car slides on an icy road, it is very hard to control because

A) The bumps in the road are magnified due to sliding.
B) Air resistance is the only force acting on the car.
C) Static friction is weaker than sliding friction.
D) Sliding friction is weaker than static friction.

Question 36

When your car goes out of control on an icy road, it is safest to use either the clutch or gear shift to allow your car to coast and not be powered by the engine. Why is this so?

A) Coasting allows the wheels to rotate so as to maintain sliding friction.
B) Coasting allows the wheels to rotate so as to recover static friction again.
C) Coasting allows the bearings in the car to work better.
D) Coasting in this case allows friction to not exert any torque on the car wheels.

Question 37

You are walking with your friend along a beautiful shoreline and begin talking about physics. She says "I completely understand friction from physics class! It always acts against the motion of an object, slowing it down." Is your friend's statement accurate?

Question 38

Suppose your car is traveling at a certain speed on a very slick road. All of a sudden you step on the accelerator and the back wheels begin to spin but the car remains at the same speed. The kinetic energy of the car

A) Remains the same.
B) Decreases.
C) Is all converted to disordered energy.
D) Increases because of the increased rotation of the tires.

Question 39

Suppose you are driving your car down the highway at constant velocity. Which of the following are true?

A) The car's momentum is constant but not its kinetic energy.
B) The car's kinetic energy is constant but not its momentum.
C) Both the car's kinetic energy and momentum are constant.
D) Neither the car's kinetic energy nor momentum are constant.

Question 40

Force and momentum are related in that:

A) Force is a change in momentum per unit time
B) Momentum is a change in force per unit time
C) Momentum and force both have the same units
D) Momentum times force equals a constant.

Question 41

During a July 4^th firecracker party, a pop bottle rocket laying still on the ground explodes, sending fragments everywhere. If the momentum of the pop bottle rocket (ignore friction!) initially at rest is conserved during an explosion, what can be said about the kinetic energy of various fragments after the collision?

A) The KE of various parts of the system are necessarily conserved.
B) The sum of all the KE of the various pieces equals what it was originally.
C) The total KE afterwards is greater that what it was initially.
D) The total KE afterwards is less that what it was initially.

Question 42

Suppose you are a police investigator and are investigating two car accidents. Before you even go to either accident scene, you know (interestingly) that both accidents were identical head - on collisions with identical cars moving at identical speeds, but in one case the cars locked bumpers while in the other case they bounced off each other elastically. From a physics point of view, which accident is more dangerous?

Question 43

Suppose your car's speed doubles. By what factor will its kinetic energy change?

A) quadruple
B) double
C) remain constant
D) Can't tell

Question 44

An 800 kg car moving at 4 m/sec to the right strikes a 400 kg car moving to the left. If the cars stick together and stop moving right after the collision, what was the velocity of the 400 kg car?

A) It was stationary
B) 2 m/sec to the left
C) 8 m/sec to the left
D) 4 m/sec to the left

Question 45

Suppose the earth's polar ice caps melted, sending water towards the equator and increasing the moment of inertia of the earth to 1.1 times its present value. The angular velocity of the earth will be

A) 0.82 times its present value
B) unchanged
C) 0.909 times its present value
D) 1.1 times its present value.

Question 46

If the momentum of a TNT stick (ignore friction!) is conserved during an explosion, what can be said about the momentum of various fragments of the system after it explodes?

A) The momentum of various parts of the system are not necessarily conserved.
B) The momentum of various parts of the system are necessarily conserved.
C) The total momentum afterwards is greater than what it was initially.
D) The total momentum afterwards is less than what it was initially.

Question 47

Suppose you are riding on a merry - go - round and you move out to the edge. Why does the rotational rate of the system decrease?

A) There was a net force on the entire system, slowing it down.
B) There was a net torque on the entire system, slowing it down.
C) Angular momentum is conserved and the moment of inertia increased.
D) Angular momentum is conserved and the moment of inertial decreased

Question 48

Consider a person standing on a raft, which is sitting still on the surface of calm water.
-During a certain time period the person is standing still, then walks to the opposite side of the raft and then stops. Is the momentum of the raft conserved during this entire time? Please explain; ignore friction between the raft and water, air resistance, etc.

Question 49

Consider a person standing on a raft, which is sitting still on the surface of calm water.
-Is the momentum of the raft and person taken together conserved during the time period described in (a)? Please explain. Ignore friction between the raft and water, air resistance, etc.

Question 50

Consider a person standing on a raft, which is sitting still on the surface of calm water.
-Now suppose there is friction between the raft and water. Please discuss how your answer to part (b) will change, if at all.

Question 51

Consider a person standing on a raft, which is sitting still on the surface of calm water.
-During the time period described in (a) what is the net impulse delivered to the raft if friction between the raft and the water, air resistance, etc. are to be ignored?

Question 52

Consider a sailboat with the wind gently blowing the sail on a beautiful, calm day, so the boat is propelled through the water at a constant velocity.
-Is the momentum of the boat conserved? Please explain.

Question 53

Consider a sailboat with the wind gently blowing the sail on a beautiful, calm day, so the boat is propelled through the water at a constant velocity.
-Is the momentum of each atom and molecule that strike the sail conserved? Please explain.

Question 54

Consider a sailboat with the wind gently blowing the sail on a beautiful, calm day, so the boat is propelled through the water at a constant velocity.
-What can be said about any forces acting on the boat other than that from the wind on the sail? That is, must other forces be present?

Question 55

Suppose you are driving your car down the highway and speeding up. Which of the following are true?

A) The car's momentum is constant but not its kinetic energy.
B) The car's kinetic energy is constant but not its momentum.
C) Both the car's kinetic energy and momentum are constant.
D) Neither the car's kinetic energy nor momentum are constant.

Question 56

Suppose you are driving your car down the highway and rounding a curve at constant speed. Which of the following are true?

A) The car's momentum is constant but not its kinetic energy.
B) The car's kinetic energy is constant but not its momentum.
C) Both the car's kinetic energy and momentum are constant.
D) Neither the car's kinetic energy nor momentum are constant.

Question 57

As you come to a stop in your car, its momentum

A) Is dissipated in the form of sound energy
B) Is converted to heat energy
C) Is changed to zero because of external forces on the car.
D) Remains constant

Question 58

For momentum to be conserved in a system it must

A) Experience no net external force.
B) Experience no dissipative forces like drag.
C) Be weightless
D) Not be connected to a motor.

Question 59

Your physics professor was laughing so hard at his own jokes that he slipped into a delirium right there in class! Before class ended he remarked that "The momentum of a larger object carries more force than that of a smaller object - that's why a Mac truck is harder to stop than a VW Bug. Please comment on the physical accuracy of his statement.

Question 60

Please discuss the three conserved quantities in this chapter, the conditions under which they are conserved and real-life examples of their transfer mechanisms.