Ex2F01

Excursions in Physics
Second Hour Exam
October 8, 2001

Statistics:

High: 96

Mean: 84

Low: 56

Problems 44 and 49 were initially answered incorrectly by me on the solution key. Essentially everyone got those correct so I simply added four points to everyone's score.

For every question, also consider as a possible answer
E) none of the above

1. Which of the following has the largest momentum relative to the earth?
A) a tightrope walker crossing Niagara Falls.
B) a sports car speeding along a highway.
C) a cement truck sitting still in a parking lot.
D) the Science building on campus.

2. A moving object on which the net force is zero will continue to move with constant
A) kinetic energy
B) velocity
C) momentum
D) all of these

3. Impulse is equal to change in
A) kinetic energy
B) momentum
C) potential energy
D) force

4. Conservation of momentum is directly related to
A) Newton’s First Law of Motion
B) Newton’s Second Law of Motion
C) Newton’s Third Law of Motion; F₁₂ = - F₂₁
D) International shortages of momentum

5. A rifle recoils from firing a bullet. The speed of the rifle's recoil is small because the
A) force against the rifle is smaller than against the bullet.
B) momentum is mainly concentrated in the bullet.
C) momentum of the rifle is smaller.
D) bullet has less mass than the rifle.

The bullet and rifle have zero momentum initially. After firing, the total momentum is still zero.
That means the rifle carries as much momentum in one direction as the bullet carries in the opposite direction:

M_rifle v_rifle = m_bullet V_bullet

6. Two objects, A and B, have the same size and shape, but A is twice as heavy as B. When they are dropped simultaneously from a tower, they reach the ground at the same time, but A has a greater
A) speed
B) acceleration
C) momentum

p = m v
Even tho' v_A = v_B, the momenta will be different because of the different masses.
Since m_A = 2 m_B, with v_A = v_B we will have p_A = 2 p_B.

D) all of the above

7. Kepler’s Laws of Planetary Motion were “discovered” or produced by Johannes Kepler, using a lifetime of observational data taken by
A) Nicholas Copernicus
B) Galileo
C) Tycho Brahe
D) Isaac Newton

8. A 4 kg ball has a momentum of 16 kg m/s. What is the ball's speed?
A) 3 m/s
B) 4 m/s
C) 12 m/s
D) 48 m/s

9. A ball is moving at 6 m/s and has a momentum of 48 kg m/s. What is the ball's mass?
A) 4 kg
B) 8 kg
C) 12 kg
D) 192 kg

10. 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 = Force x distance
W = Force x distance

D) four times as much work

11. 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 = Force x distance

W = Force x distance

D) four times as much work

12. Exert 3 N for a distance of 3 m in 3 s and you deliver a power of
A) 0.5 W
B) 1.0 W
C) 2.0 W
D) 3.0 W

P = Work/time
P = [Force x distance]/time
P = [(3 N) x (3 m)]/ (3 s)
P = 9 J / 3 s
P = 3 W

13. Exert 200 J 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 = Work/time
P = 200 J / 50s
P = 4 W

14. 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
D) four times as much energy

15. An object that has kinetic energy must be
A) elevated
B) falling
C) moving
D) at rest

16. An object that has potential energy may have this energy because of its
A) speed
B) acceleration
C) momentum
D) position

17. A person can lift containers a vertical distance of 1 meter or can roll them up a 3 meter-long ramp to the same elevation. With the ramp, the applied force required is about
A) one-third as much

The amount of work done remains the same
W = W
Force x distance = Force x distance

B) the same
C) three times as much
D) nine times as much

18. 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

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

A) position A

PE_grav = m g y

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

D) position D

E = constant
E = KE + PE
KE = max where PE = min
PE = m g y
PE is minimum at position D so that means KE is maximum

21. Which requires more work: lifting a 10-kg sack vertically 2 meters or lifting a 5-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
D) both require the same amount of force

22. According to Kepler’s laws, as a planet gets closer to the Sun,
A) it becomes warmer.
B) its speed increases.
C) its period increases.
D) its mass decreases.

23. A 3 kg mass is held 4 m above the ground. What is the approximate potential energy of the mass with respect to the ground?
A) 8 J
B) 40 J
C) 80 J
D) 120 J

PE = m g y
PE = (3 kg)(10 m/s²)(4 m)
E = 120 J

24. A 5 kg mass has 400 J of potential energy with respect to the ground. Approximately how far is it located above the ground?
A) 2 m
B) 4 m
C) 8 m

PE = m g y
PE = (5 kg)(10 m/s²)(y m) = 400 J
y = 8 m

D) 10 m

25. Using 20,000 J of work, a model elevator is raised from the ground floor to the second floor in 10 seconds. How much power does the elevator use?
A) 20 W
B) 200 W
C) 2 kW

Power = Work / time
P = 20,000 J / 10 s
P = 2,000 J/s
P = 2,000 W
P = 2 kW

D) 20 kW

26. 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
Since the speed is squared, increasing the speed by 3 increases the kinetic energy by 9.

D) 27 times the kinetic energy

27. A car moving at 50 km/hr skids 20 m with locked brakes. How far will the car skid with locked brakes if it is traveling at 150 km/hr?
A) 40 m
B) 60 m
C) 90 m
D) 180 m

Increasing the speed by 3 increases the KE by 9.
With 9 times the KE, we must do 9 times the work to bring the car to a stop.
With locked brakes, this means 9 times the distance or 180 m.

28. 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) momentum
B) kinetic energy
C) velocity
D) all of the above

29. What does an object have when moving that it doesn`t have when at rest?
A) momentum
B) energy
C) mass
D) all of the above

30. If an object has kinetic energy, then it also must have
A) momentum
B) velocity
C) speed
D) all of the above

31. According to Kepler's laws, the paths of planets about the Sun are
A) straight lines
B) parabolas
C) ellipses
D) hyperbolas

32. According to Newton, the greater the masses of interacting objects, the
A) greater the force of gravity, by the product of the masses
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

33. 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

34. What is the force of gravity on a 80-kg man standing on Earth's surface?
A) 9.8 N
B) 80 N
C) 800 N
D) 8,000 N

35. If the mass of Earth somehow decreased with no change in radius, your weight would
A) increase
B) decrease
C) stay the same

36. If the radius of Earth somehow increased with no change in mass, your weight would
A) increase
B) decrease
C) stay the same

37. If Earth's mass decreased to one-half its original mass with no change in radius, then your weight would
A) decrease to one-quarter its original value
B) decrease to one-half its original value
C) remain the same
D) increase to twice its original value

38. The force of gravity acting on the Space Shuttle in orbit is nearly
A) zero because it is weightless
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

39. A woman who normally weighs 400 N stands on top of a very tall ladder so she is one Earth radius above Earth's surface. How much would she weigh there?
A) zero
B) 100 N

The force of gravity varies inversely as the square of the distance.
This time the distance is doubled so the force is one-fourth.

C) 200 N
D) 400 N

40. 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
D) twice the weight

41. 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.
C) Jupiter's radius is 10 times Earth's radius.
D) you are 100 times more weightless there.

42. Horses that move with the fastest linear speed on a merry-go-round are located
A) nearer to the center
B) nearer to the edge
C) always white
D) in front of the slower ones

43. An industrial flywheel has a greater rotational inertia when most of its mass is
A) nearer the axis
B) nearer the rim
C) spread out evenly

44. A cylinder and a ring roll down an incline starting at the same time. The one to reach the bottom first will be the
A) cylinder
B) ring
C) neither; they both reach the bottom at the same time

45. 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) two
B) four
C) eight
D) sixteen

46. When a twirling ice skater extends her arms outward, her rotational speed
A) increases
B) decreases
C) remains the same (or is conserved)

47. To pry open a stubborn lid from a paint can, it is best to use a screwdriver that has a handle that is
A) long and thin
B) short and wide
C) yellow
D) slippery

48. 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 teeter-totter 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; this was one of the homework problems
D) 2.00 kg

49. 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.
C) zero because its speed remains constant.
D) directed outward, away from the center of the curve.

50. The moon travels in a nearly circular orbit at a nearly constant speed.
A) The acceleration of gravity on the moon’s surface is about one-sixth the value at Earth’s surface.
B) The force of gravity between Earth and the moon provides the centripetal force necessary to keep the moon in its orbit.
C) The force of gravity causes the moon to “fall” one-twentieth of an inch each second toward Earth when compared to the straight line path it would take if gravity were suddenly turned “off”.
D) all of the above.

PHY3050G - 10/8/2001

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