PHYS 211 Recitation Review Problems: Solutions

Physics 211 Fall 2012 Midterm 2 Recitation Review Solutions
3) Cart
For a lecture demo in front of class, a little cart (mass 0.5 kg) travels
on a long, straight, level air track. Shown at right is a graph of
velocity vs. time of the cart. Let's define "right" to be the positive
direction.
1. What is the net force (with correct sign) on the cart at t=3 s?
A) 0 N
m
B) +3 N
v
6
a s
4
m
2
C) -3 N
s
t
1.5 s
D) +2 N
m
F ma 0.5kg 4 2 2 N
s
E) -2 N
+4 m/s
+2 m/s
-2 m/s
+velocity
time (s)
1 2 3 4 5
2. At t=2.5 seconds, how would you describe what the car is doing?
A) It is moving to the right with steady speed.
B) It is moving to the right with decreasing speed
C) It is moving to the left with steady speed
D) It is moving to the left with decreasing speed
E) None of the above/not enough information
3. At t=5 seconds, the air for the air track is suddenly turned off, so kinetic friction (alone) causes
the cart to grind to a halt in another 0.5 sec. During that half-second when the cart is stopping,
call the force of friction on the cart by the track f CT .
What can you conclude about the frictional force f' TC on the track by the cart during this time?
(assume the mass of the track itself is 5.0 kg, i.e. ten times the mass of the little cart)
A) f' TC = -10 f CT
B) f' TC = +10 f CT
C) f' TC = -f CT /10
D) f' TC = -f CT
E) f' TC = +f CT
Newton’s 3 rd Law: Action-reaction pairs are equal and opposite
4. In the previous question, (during the half-second when the car stops due to friction) what is the
coefficient of kinetic friction between the cart and the track?
A) 0.8
m
B) 0.5
v
2
a s
4 m
2
C) 0.4
s
t
0.5 s
D) 0.2
m
4 2
a s
E) 0.1
F ma mg
m 0.4
g 10
2
s
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