Physics 140 — Midterm Exam 2 Version A — 2014 Nov 7
There are 10 multiple choice questions. Select the correct answer for each one and mark it on thebubble form on the cover sheet. Each question has only one correct answer. (2 marks each)
1. A satellite is in orbit around a planet. Which of
the following quantities would change the speed
the satellite needs to orbit at?
(a) the mass of the satellite
(b) the mass of the planet
(c) the radius of the orbit
(d) Both A and C
(e) Both B and C correct
Let v be the speed of the satellite, M the massof the planet, m the satellite’s mass and R theradius of its orbit. Then
v2
R= G
MR2
v =
�G
MR
v depends on both the mass of the planet andthe radius of the orbit, but the mass of the satel-lite is not in the relation. ,
2. A box sits on the horizontal bed of a moving
truck. Static friction between the box and the
truck keeps the box from sliding as the truck ac-
celerates with acceleration a. The mass of thetruck is M, the mass of the box is m and the co-efficient of static friction is µs. What is the mag-nitude of the force of static friction between the
box and the truck floor?
(a) F f = ma
(b) F f = Ma
(c) F f = µsmg
(d) F f = µsMg
(e) None of the above
The friction F f causes the box of mass m to ac-celerate with acceleration a. It’s just Newton’ssecond law.
3. The box is sitting on the bed of a moving truck in
the previous question while the truck slows to a
stop. The work done by static friction on the box
while stopping is
(a) zero.
(b) positive.
(c) negative. correct(d) will depend on the directions that are de-
fined to be positive and negative.
If the box slows to a stop, the force on the boxis opposite to the direction of motion. Hencethe work done by the force is negative.
4. What is the direction of the dot product of two
vectors �A and �B?
(a) Half way between the direction of each of
the vectors.
(b) arccos
��A·�B|�A||�B|
�
(c) arccos
�|�A||�B|�A·�B
�
(d) Perpendicular to both vectors.
(e) None of the above. Correct
The dot product of two vectors is a scalar andhas no direction.
5. Work is done on an object to triple its kinetic
energy. By what factor does the object’s speed
change?
(a)
√9
(b) 1/9
(c)
√3 correct
(d) 1/√
3
(e) 3
1
2mv2
2
1
2mv2
1
= 3
v2v1=√
3
6. Two balls of masses 2m an m each experience anet force of the same magnitude. The 2m-ballexperiences a net force of magnitude F straightdownward and the m ball has a net force of mag-nitude F on it straight upward. What happens tothe centre of mass while the forces are applied?
2mm
F
F
1
Physics 140 — Midterm Exam 2 Version A — 2014 Nov 7
(a) The centre of mass accelerates straight
down
(b) The centre of mass accelerates straight up
(c) The centre of mass accelerates downward
at an angle
(d) The centre of mass accelerates upward at an
angle
(e) The centre of mass does not accelerate.correct
The net external force is zero because bothforces are of equal magnitude but opposite di-rections.
7. In Case 1, a mass M hangs from a vertical springhaving spring constant k and is at rest in its equi-librium position. In Case 2 the mass has been
lifted a distance D vertically upward. If we definethe potential energy in Case 1 to be zero, what is
the potential energy of Case 2?
(a)1
2kD2 correct
(b) MgD
(c)1
2kD2 + MgD
When measured from the new equilibriumpoint, the gravitational potential energy is in-cluded in 1
2ky2
2
Physics 140 — Midterm Exam 2 Version A — 2014 Nov 7
8. Here is a graph of potential energy as a function
of position of an object moving on the x axis. At
which point is the force greatest in the negative
direction?
U(x)
x
a b c d e
0
(e) is correct. The first is the negative of thederivative of the potetial. Where the slope ofU(x) is positive, F is negative.
F(x) = −dU(x)dx
9. Two boys, Al and Bob, are on the balcony of
an apartment and each throws an object at the
same time. The objects land on the flat parking
lot below at the same time. Al throws his object
upward at 30 degrees to the horizontal and Bob
throws his object downward at an angle of 30 de-
grees to the horizontal. Ignore air resistance.
(a) Al throws his object with initial speed that
is faster than Bob’s.
(b) Al throws his object with initial speed that
is slower than Bob’s.
(c) They both throw their objects at thesame speed. correct
(d) The relative speed of the throws depends on
the relative masses of the objects
This activity is not recommended.
Only vA = vB = 0 will allow the ball to begin andend at the same time.
10. A circus performer of weight W is walking alonga high wire as shown and the wire deviates from
horizontal by an angle θ.
The tension in the wire FT is
(a) FT = (W/2) sin θ
(b) FT = (W/2) cos θ
(c) FT = W/(2 sin θ) correct
(d) FT = W/(2 cos θ)
(e) None of the above
The weight of the performer is in equilibriumwith the vertical components of the tension ofthe rope on the right side and left side: mg =2Ty. When the angle of the rope is small, themagnitude of the tension must be large in or-der for the vertical component to be sufficientlylarge to balance the weight: FT = mg/(2 sin θ).
3