### PHY 3050C

Excursions in Physics
“Excursions Live!”

Second Hour Exam
May 19, 2000

Possibly useful information:

 v = x / t v = vi + a t a = v / t x = xi + vi t + (1/2) a t2 v = r F = m a F12 = - F21 w = m g g = 9.8 m/s2 10 m/s2 Impulse = F t Impulse = p Ptot,i = Ptot,f KE = (1/2) m v2 W = F s cos Etot = KE + PE P = W / t PEg = m g h Fs = - k x Fg = G Mm/r2 PEs = (1/2) k x2 Fc = m v2 / r Xcm = mi xi / mi Angular Momentum = I Ycm = mi yi / mi
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. Which of the following has the largest momentum relative to Earth?

a) a tightrope walker crossing Niagara Falls.
b) a truck speeding along a highway.
c) a Mack truck sitting in the parking lot. p = zero
d) the Science building on campus. p = zero

2. A moving object on which no forces are acting will continue to move with constant

a) acceleration
b) impulse
c) momentum; constant velocity means constant momentum
d) all of these

3. 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 leads to p12 = - p21 which leads to
Ptot,i = p1,i + p2,i = p1,f + p2,f = Ptot,if

d) International shortages of momentum :-)

4. 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 of the rifle is smaller than that of the bullet.
c) mass of the rifle is much larger than the mass of the bullet.
d) momentum of the rifle is larger than that of the bullet.

5. 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

v is the same for both

With a larger mass m, there will be a larger momentum p

d) all of the above

6. A 5 kg ball has a momentum of 30 kg m/s. What is the ball's speed?

a) 3 m/s
b) 6 m/s
p = m v

30 kg m/s = (5 kg) ( 6 m/s)

v = 6 m/s

c) 15 m/s
d) 150 m/s

7. If two objects collide and do not stick together, their total momentum after the collision is

a) less than
b) the same as
Momentum is always conserved.

c) greater than

their total momentum before the collision.

8. If two objects collide and stick together, their total momentum after the collision is

a) less than
b) the same as
Momentum is always conserved.

c) greater than

their total momentum before the collision.

9. When two objects collide and stick together, this type of collision is known as a

a) totally elastic collision.
KE is also conserved in a totally elastic collision.

The two objects do not stick together in a totally elastic collision.

b) totally inelastic collision.

Momentum is always conserved so momentum is conserved in this totally inelastic collision.

c) totally natural collision.
d) totally impulsive collision.

10. Momentum is conserved in any collision. In a “totally elastic collision” another quantity is also conserved. That other conserved quantity is the

a) velocity, v
b) kinetic energy, KE
c) angular velocity, w
d) work, W

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 = (4 N) (3 m) = 12 J

P = 12 J / 2 s

P = 6 W

Be careful; we are using P for "power" here and we have also used P for "momentum". Also, W means units of "watts" in the last equation here but W means "work" in the first two equations. Never use equations blindly. Equations are "shorthand" for longer, more involved statements.

d) 8.0 W

15. Exert 2,000 J of work in 50 s and your power output is

a) 5 W
b) 10 W
c) 20 W
d) 40 W
P = W / t

P = 2,000 J / 50 s

P = 40 W

16. 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

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) PEg = m g h
At position A, h, the height, is greatest.

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

d) KE = (1/2) m v2
Etot = KE + PE

At position D, the PE is smallest so KE will be greatest.

22. 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
The work done is the same,

W = (10 kg) (10 m/s2) (2 m) = (5 kg) (10 m/s2) (4 m) = W

P = W / t

Since the times are the same, the power is the same in the two cases.

c) twice as much
d) four times as much

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

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

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

h = 1 m

c) 5 m
d) 10 m

25. 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

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

The velocity has increased by a factor of three.

That means the KE has increaseed by a factor of nine.

That means the stopping distance must also increase by a factor of nine.

distance = 20 m x 9 = 180 m

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
p12 = - p21

prifle = - pbulltet

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
c) 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 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

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 Mm/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

Dividing by a smaller number yields a larger number.

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 Mm/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
Fg = G Mm/r2

The distance r at radius of the orbit for the Space Shuttle is nearly the same as at Earth's surface; it is only a few percent larger.

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 Mm/r2

r is now twice Earth's radius

r = 2 REarth

Fladder = ( 1/4 ) Fsurface

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 Mm/r2

r is the distance from the center of the Earth and that is essentially the same for all the apples; r = 6,000 km!

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.
See the homework solution for details!

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 sphere roll down an incline starting at the same time. The one to reach the bottom first will be the

a) ring or hoop (this has the larger "rotational mass"
b) sphere
With a smaller "rotational mass" this one is easier to accelerate so it will move 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
When her arms are extended, her "rotational mass" is larger.

Angular momentum remains the same or is conserved.

Angular momentum = I

I = I

c) remains the same (ie, it 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. 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.
d) 2.00 kg

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 on an object moving in a circle is also known as 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.
It has a period of 24 hours and a radius of 40,000 km (or 25,000 mi).

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
KE = W = F s

W = F s = F s

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.