Excursions in Physics

**Second Hour Exam**

October 8, 2001

Statistics:

High: 96

Mean: 84

Low: 56

Problems 44 and 49 were initially answered **in**correctly 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

1. Which of the following has the largest momentum relative to the earth?

A) a tightrope walker crossing Niagara Falls.

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

3. Impulse is equal to change in

A) kinetic energy

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

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.

The bullet and rifle have

zeromomentum initially. After firing, the total momentum is stillzero.

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 distanceD) four times as much work

W = Force x distance

11. If you push an object just as far while applying

A) half as much work

B) the same amount of 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/s^{2})(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/s^{2})(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 v^{2}

Since the speed issquared, 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 **de**creased with no change in radius,
your weight would

A) increase

**B) decrease**

C) stay the same

36. If the **radius** of Earth somehow **in**creased 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

inverselyas thesquareof the distance.

This time the distance isdoubledso the force isone-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 **out**ward, 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*

(C) 2003, Doug Davis; all rights reserved