PHY 3050C
Excursions in Physics

Second Hour Exam
March 2, 2000

### Statistics:

High: 96

Mean: 80

Low: 54

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.

For every question, also consider the following as a possible answer:
e) none of the above

For every question, also consider the following as a possible answer:
e) none of the above

1. Energy can be

a) measured in units of joules.
b) potential or kinetic.
c) considered as the ability to do work.
d) all of the above.

2. In the context of this course, conservation of energy means

a) energy is scarce and must be used wisely.
b) the difference between PE and KE (ie, PE - KE) is a constant.
c) energy is important or fundamental or conservative.
d) the value of the total energy remains constant.

3. If an object is in stable equilibrium, a small movement will cause its center of mass to move

a) beyond its base of support.
b) lower.
c) higher.
d) closer to its geometric center.

4. Whenever an object moves in a circle, even at constant speed, there is

a) an acceleration tangent to (or along) its circular path.
b) a force tangent to (or along) its circular path.
c) an acceleration directed along the radius, toward the center of the circle.
d) a force due to friction.

5. Consider a solid disk and a hoop that have the same radius and the same mass. The “moment of inertia” or the “rotational mass” of the solid disk is

a) smaller than
b) the same as
c) larger than

that of the hoop.

6. When an ice skater pulls her arms and leg in close to her axis of rotation, she spins faster. This is an example of the conservation of

a) energy.
b) momentum.
c) angular momentum.
d) natural resources.

7. According to Kepler’s Laws of Planetary Motion, planets move faster when they are

a) closer to the Sun.
b) less massive.
c) more massive.
d) farther from the Sun.

8. Kepler’s Laws of Planetary Motion were

a) based upon the Ptolemaic System in which the Sun, the moon, and the planets revolved around Earth.
b) based upon a system in which all the planets, including Earth, revolved around the Sun in perfect circles.
c) based upon a lifetime of careful observation or experimental data of planetary positions
d) in direct opposition to Newtons Laws of Motion.

9. The Space Shuttle is in a low-Earth orbit where the force of gravity is only slightly different from that at Earth’s surface. The period of such a low-Earth orbit is about

a) 9 minutes
b) 90 minutes
c) 24 hours
d) 9 days

10. An effective communications satellite may be in a geosynchronous orbit with a period of 24 hours. Such an orbit has a radius of about

a) 400 km
b) 4,000 km
c) 40,000 km
d) 400,000 km

11. Work involves

a) mass multiplied by acceleration
b) mass multiplied by distance
c) force multiplied by distance
d) force multiplied by time

12. 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.
d) four times as much work.

13. 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.
d) four times as much work.

14. Exert 4 N for a distance of 3 m in 2 s and you deliver a power of

a) 2.0 W
b) 4.0 W
c) 6.0 W
P = W / t

W = F d = (4 N)(3 m) = 12 N m = 12 J

P = 12 J / 2 s

P = 6 W

d) 8.0 W

15. Exert 1,200 J of work in 60 s and your power output is

a) 5 W
b) 10 W
c) 20 W
P = W / t

P = 1200 J / 60 s

P = 20 W

d) 40 W

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

a) one-third as much energy
b) the same amount of energy
c) three times as much energy
PE = m g h

d) nine times as much energy

17. An object that has kinetic energy must be

a) elevated
b) falling
c) moving
KE = (1/2) m v2

d) at rest

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

a) speed
b) acceleration
c) momentum
d) position

19. 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

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

a) greatest PEg

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

d) greatest KE
KE is greatest where PE is least

and PE is least at position D

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

a) one-fourth as much
b) one-half as much
c) the same
d) four times as much
P = W / t

W = m g h

W1 = (10 kg)(10 m/s2)(4 m) = 400 J

W2 = (5 kg)(10 m/s2)(2 m) = 100 J

W1 = 4 W2

t1 = t2

P1 = 4 P2

23. 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
PE = m g h

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

PE = 150 J

d) 300 J

24. 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
PE = m g h

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

h = 1 m

c) 5 m
d) 10 m

25. A car moves twice as fast as another identical car. Compared to the slower car, the faster car has

a) the same kinetic energy
b) one-half the kinetic energy
c) twice the kinetic energy
d) four times the kinetic energy
KE = (1/2) m v2

That means that doubling the speed causes the KE to increase by a factor of four

since 22 = 4.

26. 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

That means that increasing the speed by a factor of three causes the KE to increase by a factor of nine since 32 = 9.

W = F d

Increasing the KE by a factor of 9 means the work done to bring the car to a stop also increases by a factor of 9. Once tires skid, the force of friction is essentially constant so that means the distance must also increase by a factor of nine.

27. 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

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

a) momentum
p = m v

b) energy

An object can have PE and still be at rest.

c) mass

An object always has mass.

d) all of the above

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

a) momentum
b) velocity
c) speed
d) all of the above
KE = (1/2) m v2

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

a) straight lines
b) parabolas
c) ellipses
d) hyperbolas

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

a) greater the force of gravity, by the product of the masses
Fg = G M m / r2

Fg = G M m / 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

32. 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
Fg = G M m / r2

Fg = G M m / r2

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

a) increase
Fg = G M m / r2

Fg = G M m / r2

b) decrease
c) stay the same

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

a) increase
Fg = G M m / r 2

Fg = G M m / r 2

b) decrease

c) stay the same

35. 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
Fg = G M m / r2

Fg = G M m / r2

c) increase to nine times its original value
d) increase to three times its original value

36. 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
This approximation may be called a "low-Earth orbit".

c) about one-tenth its weight at Earth's surface
d) about one-one hundredth its weight at Earth's surface

37. 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
Fg = G M m / r2

If r is increased by a factor of two, the force of gravity is decreased by a factor of four since we are dividing by r2.

c) 250 N
d) 500 N

38. 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
Fg = G M m / r2

r is measured from the center of Earth and r is almost the same for apples at the top of the tree or for apples at the bottom of the tree.

d) twice the weight

39. 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.
This was homework question 8.15
F = G M m / R2

MJ = 300 ME

FJ = 3 FE

FJ = G MJ m / RJ2 = 3 [ G ME m / RE2 ] = 3 FE

FJ = G (300 ME) m / RJ2 = 3 [ G ME m / RE2 ] = 3 FE

G (300 ME) m / RJ2 = 3 [ G ME m / RE2 ]

(300 ME) / RJ2 = 3 [ ME / RE2 ]

(300 ) / RJ2 = 3 [ 1 / RE2 ]

100 / RJ2 = 1 / RE2

RJ2 / 100 = RE2

RJ2 = 100 RE2

RJ = 10 RE

d) you are 100 times more weightless there.

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

a) nearer the axis
b) nearer the rim

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

a) hollow ring
I = M r2

b) solid disk

I = (1/2) M r2

The solid disk has a smaller "rotational mass" (or "moment of inertia").

That means it is easier to accelerate.

Therefore it -- the solid disk -- will reach the bottom faster.

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

42. 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
b) four
c) eight
d) sixteen

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

a) increases
b) decreases
It is the angular momentum that is conserved.

Extending her arms increases her "rotational mass" I

I = I

This means her rotational speed will decrease.

c) remains the same (ie, her angular speed is conserved).

44. 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

45. In Newton’s Law of Universal Gravitation, Fg = G M m/r2, the proportionality constant, “big G”, is very small; G = 6.67 x 10 - 11 N kg2/m2 . This means the force of gravity between two objects of common, ordinary, small masses is incredibly small. Nonetheless, Henry Cavendish devised a very sensitive way to measure these incredibly small forces. This allowed him to measure “big G”, the proportionality constant in Newton’s Law of Universal Gravitation. Once this constant was known, he could readily calculate

b) the mass of Earth.
Cavendish gave his talk or paper the title "Weighing the Earth".

c) the radius of the moon’s orbit.
d) the mass of the moon.

46. 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.
This net force is called the centripetal force.

c) zero because the car is not accelerating.
d) directed outward, away from the center of the curve

47. 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.

48. As your kinetic energy is reduced to zero in an automobile crash, 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
Change in KE = work

work = Force x distance

b) the same as
c) more than

if you were not wearing seat belts.

49. 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.
PE = m g h

d) weight.

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.