Deck 4: Mechanical Objects, Part 2

Question 1

When sitting still on the ground, a tricycle is in a state of

A) unstable dynamic equilibrium
B) dynamic equilibrium
C) stable static equilibrium
D) unstable static equilibrium

Answer 1

stable static equilibrium

Question 2

When moving, a bicycle is in a state of dynamic equilibrium due to

A) gyroscopic effects
B) air resistance
C) the shape of the fork
D) (A) and (C)

Answer 2

(A) and (C)

Question 3

A three - wheeler is

A) statically stable and dynamically unstable
B) statically unstable and dynamically stable
C) both statically and dynamically unstable
D) both statically and dynamically stable

Answer 3

statically stable and dynamically unstable

Question 4

A motorcycle is

A) statically stable and dynamically unstable
B) statically unstable and dynamically stable
C) both statically and dynamically unstable
D) both statically and dynamically stable

Question 5

In studying many martial arts, it is necessary to practice efficient and safe postures. One important concept related to how one should stand is that the feet should be reasonably far apart, because

A) the base of support is wider, thus stabilizing the person.
B) the weight on each foot will be less than when standing normally.
C) the weight on each foot will be more than when standing normally.
D) the friction force increases, giving the person more traction.

Question 6

At various times in the news it has been reported that certain vehicles are susceptible to rolling over. The physical mechanism that is responsible for rollovers is

A) Frictional torques about the driver only.
B) Frictional torques from the tricycle on the road acting below the road's center of mass.
C) Frictional torques from the road on the tricycle about its center of mass, which is below the rider.
D) Strong air currents simply blowing the system over.

Question 7

It is important for cyclists to lean their bicycles while negotiating sharp turns because

A) the torque created by their leaning balances out the frictional torque from the road which would tend to flip them.
B) the torque created by their leaning balances out their weight and holds them up.
C) their tires flatten out, thus getting better traction on the road.
D) they increase their static stability

Question 8

You are riding a multi-speed bicycle up a steep hill that's 100 m high and want to do as little work as possible on your way up the hill.

A) You should choose the largest crank sprocket (attached to the pedals) and the smallest freewheel sprocket (attached to the rear wheel).
B) You should choose the smallest crank sprocket (attached to the pedals) and the largest freewheel sprocket (attached to the rear wheel).
C) You should choose the largest crank sprocket (attached to the pedals) and the largest freewheel sprocket (attached to the rear wheel).
D) Your choice of gears doesn't matter because you'll do the same amount of work climbing the hill no matter which gear you choose.

Question 9

You buy an older motorcycle and notice that, even in low gear, it is difficult to get the bike going. You should replace the sprocket in the rear wheel with one that is

A) smaller so the motor can do more work at the start
B) smaller so the motor won't have to do so much work at the start.
C) larger so the motor won't have to do so much work at the start.
D) larger so the motor can do more work at the start.

Question 10

You go to a quick - fix garage to get your brakes worked on. When your tires are re - installed, the mechanic forgets to put the lug nuts on your tires. Although nobody gets hurt, a tire comes off your car when you are traveling fairly rapidly and it is able to roll through a very large distance, even hitting bumps and objects in the way. Please explain why the tire can roll so far without tipping over.

Question 11

Please explain why tricycles are better at sitting still than bicycles are, but bikes are the vehicles of choice when you want to be moving.

Question 12

Why does a skateboarder lean while negotiating a curve?

A) To keep from tipping over by making sure the net force on them is directed towards the skateboard.
B) It saves energy
C) It lowers air resistance
D) To lessen gravitational torque on their center of mass.

Question 13

A top is

A) Statically stable and dynamically unstable.
B) Statically unstable and dynamically stable.
C) Statically unstable and dynamically unstable.
D) Statically stable and dynamically stable.

Question 14

An object is in stable equilibrium only when

A) Its total potential energy rises whenever it reorients.
B) Its total potential energy lowers whenever it reorients.
C) It is not moving
D) It is moving

Question 15

You can make a bicycle lean by

A) briefly steering it in the direction of a turn
B) briefly steering it in the direction opposite the turn
C) Braking
D) Pedaling faster

Question 16

What is one advantage and one drawback of a very large wheel on a bicycle or tricycle whose operator can pedal the front wheel?

Question 17

What are three drawbacks to a bicycle or tricycle whose front wheel can be pedaled?

Question 18

Astronauts in the space shuttle feel weightless because

A) They are so far away from the center of the Earth.
B) they are in a constant state of free - fall.
C) they are moving very rapidly.
D) there is no atmosphere where they are.

Question 19

Escape velocity at the surface of the Earth is about

A) 250 mph
B) 11,200 mph
C) 25,000 mph
D) 250,000 mph

Question 20

What is the gravitational force that two 100 kg persons exert on each other when standing 1m apart?

A) 6.672x10^-7 N
B) 1 N
C) 6.672x10^-11 N
D) 10,000 N

Question 21

The Earth is about 81 times as massive as the Moon. Suppose the earth and Moon are 10 distance units apart. If you were an astronaut between the Earth and Moon at how many distance units from the Earth will the gravitational attraction on you from the two bodies be equal and opposite?

A) 81 units
B) 9 units
C) 19 units
D) 1 unit

Question 22

The Earth is about 81 times as massive as the Moon. Suppose the earth and Moon are 240,000 miles apart. If you were an astronaut between the Earth and Moon at what distance from the Earth will the gravitational attraction on you from the two bodies be equal and opposite?

A) 216,000 mi
B) 2963 mi
C) 24,000 km
D) 24,000 miles

Question 23

Suppose you are spending your weekend on roller skates trying to get momentum from somewhere. Which of the following will give you momentum in the north direction?

A) Throwing your keys southward.
B) Throwing your keys northward.
C) Bending down and throwing your keys straight up.
D) (B) or (C).

Question 24

You are substitute teaching for your spouse and are using balloons to demonstrate how rockets work. The balloons are blown up and released so that the air can leave them rapidly. Why do the balloons fly all around, even though the balloon does not have a surface to push against?

Question 25

Suppose your instructor staggers into class the morning after an all-night excellence in classroom humor workshop. He proceeds to do a demonstration in class where he measures a 6.67 N gravitational force of repulsion between two particles each having a mass of 1 kg and separated by 1m. Moreover he cuts the distance to 0.1 of what it originally was and then claims the force changes to a new value of 0.667 N. Specific to the scientific content of his demonstration please briefly describe three important concepts he incorrectly illustrated (there's more than three, trust me). One of your three points must be quantitative - involving a force calculation of some sort.

Question 26

Suppose you have two identical particles that attract each other with a certain gravitational force. Now you move them so they are three times as far apart as they were originally, but the force between them stays the same. What is one way in which the masses might change so the force could remain constant?

Question 27

Consider astronauts orbiting the earth in the space shuttle.
-The acceleration of gravity in the orbiting space shuttle is over 90% that on the surface of the earth but the astronauts still feel weightless (no apparent weight) please explain. You may assume the orbit is perfectly circular.

Question 28

Consider astronauts orbiting the earth in the space shuttle.
-Suppose that the astronauts are traveling at a speed of 1000 m/sec. What must the radius of their circular orbit be if their acceleration is 8.5 m/sec²?

Question 29

You and a friend are on roller skates. You are in a room and have a basketball and are standing in between two parallel walls. By touching only the ball, describe how you would make yourselves come together.

Question 30

Compared to planets close to the sun, and based upon Kepler's third law of planetary motion, the length of the year of planets farther out from the sun are

A) the same
B) less
C) larger
D) more information is necessary.

Question 31

Comets have very eccentric orbits around the sun. Please explain why it is that they spend so little time near the earth.

Question 32

The rest energy of a 60 kg person is

A) 8.4x10¹⁸ J
B) 5.4x10¹⁸ J
C) 1.8x10¹⁰ J
D) 9x10¹⁶ J

Question 33

What mass has a rest energy of 100J?

A) 1.1x10^-15 kg
B) 3x10^-8 kg
C) 3.3x10^-7 kg
D) 0.01 kg

Question 34

The relativistic energy of a 1000 kg car moving at 0.95 times the speed of light is

A) 1000 J
B) 2.8x10¹⁷ J
C) 2.9x10²⁰J
D) 9.3x10¹¹ J

Question 35

The relativistic momentum of a 1000 kg car moving at 0.95 times the speed of light is

A) 9.13x10⁸ kg m/s
B) 9.13x10¹¹ kg m/s
C) 3200 kg m/s
D) 2.85x10¹¹ kg m/s

Question 36

The speed at which a car must move so that its relativistic momentum is twice its nonrelativistic momentum is

A) 0.9 times the speed of light
B) ½ the speed of light
C) ¾ times the speed of light
D) times the speed of light

Question 37

The speed at which a car must move so that its relativistic energy is three times its rest energy is

A) 1/9 times the sped of light
B) 1/3 times the speed of light
C) times the speed of light
D) 8/9 times the speed of light

Question 38

The maximum speed a rocket can achieve is

A) the speed of light
B) the speed of sound.
C) equal to the speed of its exhaust plume.
D) less than the speed of its exhaust plume.
E) greater than the speed of its exhaust plume but less than the speed of light.

Question 39

All current rockets eject stages or booster rockets as they climb from the earth's surface up to orbit. A rocket that did not eject any stages or booster rockets while climbing from the ground to orbit would

A) experience too much drag while in orbit and would be unable to remain in orbit for more than a day or two.
B) not have anything beneath the rocket on which to push and would be unable to propel itself upward after it left the launch pad area.
C) clutter the space around the earth with orbiting debris.
D) have to have a launch weight that was about 90% rocket fuel.

Question 40

Astronauts in a space shuttle accelerating uniformly in a straight line far away from any planet or star would

A) feel weightless.
B) feel just like there is local gravity
C) feel like they are moving very rapidly.
D) feel nothing

Question 41

Rockets receive their forward propulsion through

A) pushing against the ground and objects in space
B) mass conservation of the rocket and fuel
C) pushing against the exhaust
D) small aerodynamic forces

Question 42

How far apart would two 100 kg persons need to be so that the force they exert on each other is equal to 1N? You can assume they are point masses, having mass but no size.

A) 1m
B) 6.672x10^-7 m
C) 8.17x10^-4 m
D) 100 m

Question 43

Suppose you have two identical particles that attract each other with a certain gravitational force. Now you move them so they are one quarter as far apart as they were originally, but the force between them stays the same. What is one way in which the masses might change so the force could remain constant?

Question 44

Ideal physics theory would tell you that, if you climb into a basket at a store you cannot go from one side of the store to the other. In fact you can do this, and physicists often take breaks from late night test writing to do such things. Please explain why ideally we should not be able to do this but in reality we can.

Question 45

Consider the Moon orbiting the Earth. _______ is the centripetal force for the orbit, and one could therefore measure the mass of the ________ by studying the Moon's orbit.

A) Drag; Earth
B) Tension; Moon
C) Gravity; Moon
D) Gravity; Earth