# Second Hour Exam

## June 24, 1999

Enter all your answers in the "scantron sheet" or the "bubble sheet". Turn in only that sheet. Anything you write on this exam will not be seen or used or considered or graded. Be sure your name is on the "bubble sheet" you hand in. Be sure your name is bubbled-in. Be sure your answers are recorded correctly.

Possibly useful information:

v = x / t a = v / t

v = vi + a t x = xi + vi t + (1/2) a t2

v = r

F = m a F12 = - F21

w = mg g = 9.8 m/s2 => 10 m/s2

Impulse = F t p = m v Impulse = p

Ptot,i = Ptot,f KE = (1/2) m v2

W = F s cos W = F s Etot = KE + PE

PEg = m g h Fs = - k x PEs = (1/2) k x2

Fg = G Mm/r2 Fc = m v2 / r

Angular Momentum = I

Xcm = mi xi / mi Ycm = mi yi / mi

For every question, also consider the following as a possible answer:

E) none of the above

1. Work involves

A) mass multiplied by acceleration

B) mass multiplied by distance

C) force multiplied by distance

D) force multiplied by time

2. If you push an object twice as far while applying the same force you do

A) half as much work.

B) the same amount of work.

C) twice as much work; W = F s; If distance s is made twice as large, work W will also be twice as large.

D) four times as much work.

3. If you push an object just as far while applying twice the force you do

A) half as much work.

B) the same amount of work.

C) twice as much work

W = F s; If the force F is made twice as large the work W will also be twice as large.

D) four times as much work.

4. Exert 2 N for a distance of 2 m in 2 s and you deliver a power of

A) 1.0 W

B) 2.0 W

W = F s = (2 N) (2 m) = 4 N-m = 4 J

P = W / t = 4 J / 2 s = 2 J/s = 2 W

C) 4.0 W

D) 8.0 W

5. Exert 200 J of work in 50 s and your power output is

A) 0.5 W

B) 1.0 W

C) 2.0 W

D) 4.0 W

P = W / t = 200 J / 50 s = (200/50) J/s = 4 W

6. An object is raised above the ground gaining a certain amount of potential energy. If the same object is raised twice as high it gains

A) half as much energy

B) the same amount of energy

C) twice as much energy; PEg = m g h

D) four times as much energy

7. An object that has kinetic energy must be

A) elevated

B) falling

C) moving; KE = (1/2) m v2

D) at rest

8. An object that has potential energy may have this energy because of its

A) speed

B) acceleration

C) momentum

D) position

9. A clerk can lift containers a vertical distance of 1 meter or can roll them up a 4 meter-long ramp to the same elevation. With the ramp, the applied force required is about

A) one-fourth as much
W = F s; If the distance is made four times as great, the force may be one-fourth as much.

B) half as much

C) the same

D) four times as much

10. When a car is braked to a stop, its kinetic energy is transformed to

A) energy of motion

B) heat energy

C) stopping energy

D) potential energy

11. For which position above does the ball on the end of the string have the greatest gravitational potential energy?

A) PEg = mgh; position A has the greatest height h.

12. For which position above does the ball on the end of the string have the greatest kinetic energy?

D) Etot = PE + KE = constant

Position D has the lowest h for the lowest PE.

Therefore, it must have the greatest KE.

13. Which requires more work: lifting a 5 kg sack vertically 2 meters or lifting a 10 kg sack vertically 4 meters?

A) lifting the 5 kg sack

B) both require the same amount of work

C) lifting the 10 kg sack

W = F s = m g h

W1 = (5 kg) ( 10 m/s2 ) ( 2 m) = 100 J

W2 = (10 kg) ( 10 m/s2 ) ( 4 m) = 400 J

W2 = 400 J > 100 J = W1

D) both require the same amount of force

14. A 10 kg sack is lifted 2 meters in the same time as a 5 kg sack is lifted 4 meters. The power expended in raising the 10 kg sack compared to the power used to lift the 5 kg sack is

A) half as much

B) the same

W = F s = m g h

W1 = (10 kg) ( 10 m/s2 ) ( 2 m) = 200 J

W2 = (5 kg) ( 10 m/s2 ) ( 4 m) = 200 J

W1 = 200 J = 200 J = W2

P = W/t

P1 = W1/t1 = W2/t2 = P2

and t1 = t2

Therefore, P1 = P2

C) twice as much

D) four times as much

15. A 3 kg mass is held 5 m above the ground. What is the approximate potential energy of the mass with respect to the ground?

A) 15 J

B) 75 J

C) 150 J

PEg = m g h

PEg = (3 kg)(10 m/s2)(5 m)

PEg = 150 J

D) 300 J

16. A 5 kg mass has 50 J of potential energy with respect to the ground. Approximately how far is it located above the ground?

A) 0.5 m

B) 1 m

PEg = m g h

PEg = (5 kg)(10 m/s2)(h) = 50 J

1 m = h

C) 5 m

D) 10 m

17. Using 2,000 J of work, a model elevator is raised from the ground floor to the second floor in 5 seconds. How much power does the elevator use?

A) 40 W

B) 400 W

P = W / t = 2000 J / 5 s = (2000/5) (J/s) = 400 W

C) 4 kW

D) 240 kW

18. A car moves 3 times as fast as another identical car. Compared to the slower car, the faster car has

A) the same kinetic energy

B) 3 times the kinetic energy

C) 9 times the kinetic energy; KE = (1/2) m v2

D) 27 times the kinetic energy

19. A car moving at 40 km/hr skids 20 m with locked brakes. How far will the car skid with locked brakes if it is traveling at 120 km/hr?

A) 60 m

B) 90 m

C) 120 m

D) 180 m

KE = (1/2) m v2

At 120 km/hr, with a speed 3 times as great, the car now has nine times the KE

That means nine times as much work must be done

W = F d

Once the tires start to skid, the force can not be increased.

Therefore the distance must be nine times as great

180 m = 9 x 20 m

20. When a rifle is fired it recoils so both the bullet and rifle are set in motion. The rifle and bullet ideally acquire equal but opposite amounts of

A) kinetic energy

B) momentum

C) potential energy

D) all of the above

21. What does an object have when moving that it doesn`t have when at rest?

A) momentum

B) energy; An object can have potential energy when it is at rest.

C) mass; An object has mass when it is at rest.

D) all of the above

22. If an object has kinetic energy, then it also must have

A) momentum

B) velocity

C) speed

D) all of the above

23. According to Kepler's laws, the paths of planets about the Sun are

A) straight lines

B) parabolas

C) ellipses

D) hyperbolas

24. According to Newton, the greater the masses of interacting objects, the

A) greater the force of gravity, by the product of the masses
F = G Mm / r2

B) less the force of gravity

C) greater the force of gravity, by the square of the masses

D) less the force of gravity, inversely as the square of the masses

25. According to Newton, the greater the distance between masses of interacting objects, the

A) greater the force of gravity, proportional to the distance

B) less the force of gravity, inversely as the distance

C) greater the force of gravity, proportional to the square of the distance

D) less the force of gravity, inversely as the square of the distance

F = G Mm / r2

26. What is the force of gravity on a 500-newton woman standing on Earth's surface?

A) 9.8 N

B) 50 N

C) 500 N

D) 5,000 N

27. If the mass of Earth somehow increased with no change in radius, your weight would

A) increase
F = G Mm / r2

B) decrease

C) stay the same

28. If the radius of Earth somehow decreased with no change in mass, your weight would

A) increase
F = G Mm / r2

B) decrease

C) stay the same

29. If Earth's mass decreased to one-third its original mass with no change in radius, then your weight would

A) decrease to one-ninth its original value

B) decrease to one-third its original value

F = G Mm / r2

C) remain the same

D) increase to three times its original value

30. The force of gravity acting on the Space Shuttle in orbit is nearly

A) zero

B) equal to the weight of the Space Shuttle at Earth's surface

C) about one-tenth its weight at Earth's surface

D) about one-one hundredth its weight at Earth's surface

31. A woman who normally weighs 500 N stands on top of a very tall ladder so she is one Earth radius above the earth's surface. How much would she weigh there?

A) zero

B) 125 N

F = G Mm / r2

Increasing r from r = REarth to r = 2 REarth decreases the force to one-fourth the initial value.

C) 250 N

D) 500 N

32. The force of gravity acts on all apples on an apple tree. Some apples are twice as far from the ground as others. These twice- as-high apples, for the same mass, have practically

A) one-fourth the weight

B) one-half the weight

C) the same weight

It is the distance from the center of Earth that goes into Newton's Law of Universal Gravitation, F = G Mm / r2 , not the height above the ground.

D) twice the weight

32. The planet Jupiter is about 300 times as massive as Earth, yet on its surface you would weigh only about 3 times as much. This is because

A) your mass is 100 times less on Jupiter.

B) Jupiter is significantly farther from the sun.

D) you are 100 times more weightless there.

34. Horses that move with the fastest linear speed on a merry-go-round are

A) located nearer to the center

B) located nearer to the edge; v = r ; increasing r increases v.

C) always white

D) in front of the slower ones

35. An industrial flywheel has a greater rotational inertia when most of its mass is

A) nearer the axis

B) nearer the rim

The correct answer is B) but my original key incorrectly had the answer listed as C). All but one of you knew better than that. I have not gone back to Testing Services and had this second hour exam rescored. But I have gone into my grade book and raised your grade 2 pts if you answered this as B). Those new grades are posted outside my office door.

36. A hollow ring or hoop and a sphere roll down an incline starting at the same time. The one to reach the bottom first will be the

A) cylinder

B) sphere;

The sphere has a smaller "rotational mass" (also known as "moment of inertia") so it accelerates more rapidly.

C) neither; they both reach the bottom at the same time

37. Put a pipe over the end of a wrench when trying to turn a stubborn nut on a bolt, to effectively make the wrench handle twice as long, you'll increase the torque by a factor of

A) two
"Rotational force" (or torque) = force x moment arm

B) four

C) eight

D) sixteen

38. When a twirling ice skater extends her arms outward, her rotational speed

A) increases

B) decreases

When she extends her arms, this makes her "rotational mass" larger

angular momentum = (rotational mass) x (rotational speed)

angular momentum = constant

angular momentum = angular momentum

(rotational mass) x (rotational speed) =

= (rotational mass) x (rotational speed)

C) remains the same (or is conserved). It is the angular momentum that is conserved.

39. To turn a stubborn screw, it is best to use a screwdriver that has a handle that is

A) long and thin

B) thick or wide

C) yellow

D) slippery

40. A 1-kg rock is suspended from the tip of a meter stick at the 0 cm mark so that the meter stick balances like a see-saw when the fulcrum is at the 25-cm mark. From this information, what is the mass of the meter stick?

A) 0.25 kg

B) 0.50 kg

C) 1.00 kg.

The "rotational forces" (or torques) of the 1 kg rock and the meter stick must balance; they must be the same.

The center of mass of the meter stick is at its center, at the 50 cm mark. This is 25 cm from the pivot or fulcrum.

Since the moment arms are the same for the two forces causing the two rotational forces, the forces themselves must also be the same.

Therefore the weight of the meter stick must be the same as the weight of the 1-kg rock.

That means the mass of the meter stick must be 1 kg.

D) 2.00 kg

41. A car travels in a circle with constant speed. The net force on the car is

A) directed forward, in the direction of travel.

B) directed towards the center of the curve; Fc = m v2 / r

C) zero because the car is not accelerating.

D) directed outward, away from the center of the curve

42. A driver tries to travel in a circle with constant speed. Due to water on the pavement, the force of friction is reduced. This causes the car to

A) travel in a circle of greater radius.
Fc = m v2 / r

If the force gets smaller the radius will get larger.

B) travel in a circle of smaller radius.

C) suddenly have a greater speed.

D) suddenly have a smaller speed.

43. One end of a long uniform log is raised to shoulder level. Another identical log is raised at its center to the same level. Raising the second log requires about

A) half as much work

B) the same work

C) twice as much work

W = PE = m g h

44. A communications satellite appears stationary to an Earth-based observer. The orbit such a satellite is in is called a

A) low-Earth orbit.

B) polar orbit.

C) geosynchronous orbit.

D) high-Earth orbit.

45. A geological survey satellite, in order to take data from (or pictures of) everyplace on Earth, should be placed in

A) an equatorial orbit.

B) a geosynchronous orbit.

C) a counter-rotating orbit.

D) a polar orbit.

46. If the size of Earth, somehow, suddenly expanded to the size of Jupiter -- but Earth's mass remained the same -- the force of gravity on Earth's new surface would

A) decrease; F = G Mm/r2

B) remain the same.

C) increase.

47. As your kinetic energy is reduced to zero in an automobile crase, use of seat belts means a stopping force is applied to your body over a greater distance. This means the force on you is

A) less than
KE = W = F d

Increasing d means F decreases.

B) the same as

C) more than

if you were not wearing seat belts.

48. As you climb the first hill of a roller coaster, work is done by outside forces. At the top of that first hill, your velocity is nearly zero and you have maximum

A) momentum.

B) kinetic energy.

C) potential energy.

D) weight.

49. As you make a curve on a level highway, the net force on you

A) is supplied by the friction between your car's tires and the highway.

B) is supplied force of gravity.

C) is directed upwards.

D) turns elastic potential energy into heat energy of the brakes.

50. A diver does a double saumersalt and then extends his body so that his rotation nearly stops before going into the water. This is an example of conservation of

A) momentum.

B) kinetic energy.

C) potential energy.

D) angular momentum.