1
CT2-1. A person starts in Boulder, drives to Denver, 50 km away, in 1 hour, stays in
Denver for 1 hour, then speeds back to Boulder in 30 minutes.
50 km in 1 hour
s ta r t
Denv er
fi n i s h
5 0 k m i n 1 /2 h o u r
.
(W a i t 1 h o u r )
What is the average speed of this round trip?
A) zero
B) 67 km/hr
C) 40 km/hr
D) 75 km/hr
E) None of these.
Answer: C Don't forget to include the 1 hour wait in Denver
What is the average velocity of this round trip (same choices)?
Answer: A
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
2
CT2-2. A object starts at x = +5 and moves left along the x-axis at constant speed:
start
finish
x
0
Which graph represents this motion?
x
x
A
B
time
x
time
x
C
time
D
time
E: None of these!
Answer: B
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
3
CT2-3. A person initially at point 3 on the x-axis stays there for a little while and then
strolls along the x-axis to point 1, stays there for a moment and then runs to point 2 and
remains there.
0
1
2
3
x
3
slow
1
fast
2
Which of the following graphs correctly depicts this motion?
x
x
A
B
time
x
time
x
D
C
time
time
E) None of these!
Answer: D
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
4
CT2-4. A train car moves along a long straight track. The graph shows the position as a
function of time for the train.
position
time
The graph shows that the train:
A) speeds up all the time.
B) slows down all the time.
C) speeds up part of the time and slows down part of the time.
D) moves at constant velocity.
E) None of these statements is true.
Answer: B
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
5
CT2-5. A train moves along a straight track and its position vs. time looks like:
x
t
Which graph best depicts the train's velocity vs. time?
v
v
A
B
t
v
t
v
C
t
D
t
Answer: D
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
6
CT2-6. The graph show positions as a function of time for two trains running on parallel
tracks. Which statement is true:
position
train A
train B
tC
time
A) At time tC, both trains have the same velocity.
B) Both trains speed up all the time.
C) Both trains have the same velocity at some time before tC.
D) At some time, both trains have the same acceleration.
E) None of the above statements is true.
Answer: C
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
7
CT2-7. An object's velocity vs. time is:
v
t
Which graph best represents the object's
acceleration vs. time?
a
a
A
B
t
a
t
a
C
t
D
t
Answer: D
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
8
CT2-8. At the moment that the velocity is zero, what is the sign of the acceleration?
A) Positive
B) negative
C) zero
v
a=?
t
Answer: B
Is the acceleration constant all during the time the velocity is changing?
A) Yes, a = constant B) No, a is changing.
Answer: A
CT2-9. A trick question: Which object, A or B, has larger velocity?
x
A
t
B
Answer: B For B, the velocity is positive. For A, the velocity is negative.
A better question: Which object, A or B, has higher speed?
Answer: A
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
9
CT2-10.
C
B
A
A glider on a tilted air track is given a brief push uphill. The glider coasts up to near the
top end, stops, and then slides back down. When the glider is at the highest point of its
path, its acceleration is..
A) straight down
C) upward along the track
B) downward along the track
D) no direction, the acceleration is zero.
Answer: B The direction of the acceleration vector is the direction of the v vector
CT2-11. A ball is thrown straight upward. NO air resistance. At the top of its trajectory,
its acceleration is..
A) zero
B) straight up
C) straight down
D)depends on the mass of the ball
Answer: straight down
CT2-12. If you drop an object in the absence of air resistance, it accelerates downward at
9.8 m/s2. If instead you throw it downward, its downward acceleration after release is….
A) less than 9.8 m/s2.
B) 9.8m/s2
C) more than 9.8 m/s2.
Answer: 9.8m/s2 In free-fall, the acceleration is constant regardless of the velocity.
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
10
CT2-13. A rock is dropped from rest and falls a distance
h (h > 0 ) to the ground. Down is chosen as the positive
direction and the origin is placed at ground level.
What is y0 (the initial position) and what is a (the
acceleration)?
A) y0 = +h , a = – g
B) y0 = –h , a = + g
C) y0 = 0 ,
a=+g
D) y0 = – h , a = – g
E) y0 = 0 ,
a=–g
v0 = 0
h
0
+y
Answer: B
CT2-14. A rock thrown straight down from an initial
height h above the ground, with an initial velocity v0 .
UP is chosen as the positive direction. What is the
correct formula for the velocity in this case?
v  vo  g t
v  vo  g t
B)
v   vo  g t
v0
+y
h
A)
0
C)
v  v  g t
o
D)
E) None of these
Answer: B The initial velocity v0 is a negative number in this case. Putting an extra
minus sign in front of the (negative-valued) v0 would make the equation wrong.
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
11
CT2-15. A rock is thrown downward with an initial speed |v0| from the edge of a cliff.
Assume no air resistance. It falls straight down and strikes the ground after falling a
distance h. A student is asked to compute the final speed of the rock, just before it hits
the ground. Which one formula should she use?
A)
v  vo  a t
B)
x  x o  vo t  (1/ 2) a t 2
C)
D)
E)
v2  vo 2  2a (x  x o )
None of these will work
More than one of these will work OK.
Answer: Formula C
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
12
CT2-16. On planet X, a cannon ball is fired straight upward. The position and velocity of
the ball at many times are listed below. Note that we have chosen up as the positive
direction.
Time(s)
0
1
2
3
4
5
6
7
8
Height(m)
0
17.5
30
37.5
40
37.5
30
17.5
0
Velocity(m/s)
20
15
10
5
0
-5
-10
-15
-20
y
What is the acceleration due to gravity on Planet X?
A) -5m/s2
B) -10m/s2
D) -20m/s2
E) None of these.
C) -15m/s2
Answer: A Use a = v/t with any pair of times.
CT2-17. An object's velocity vs. time graph looks like this:
v
t
What situation produces this kind of motion?
A) A rock is thrown straight up.
B) A rock is dropped from rest.
C) A rock is thrown straight down.
D) A book slides along a table and comes to rest
E) None of these.
Answer: A
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
13
CT2-18. A ball is fired straight downward out of a special spring-loaded gun, which
produces constant acceleration. Upward is chosen as the positive direction. Assume that
air resistance is negligible. Which graph properly represents acceleration of the ball?
a
Ball released
From spring
t
A
a
t
B
a
t
D) None of these
C
Answer: B or C, depending on how the spring loses contact with ball
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
14
CT2-19.
A truck traveling at 50 km/hr approaches a car stopped at a red light. When the truck is
100 m from the back of the car, the light turns green and the car immediately begins to
accelerate at 2.00 m/s2. Which graph below represents this situation?
car
A
x
B
t
truck
C
x
car
t
truck
x
car
D) None of these
t
truck
Answer: B
CT2-20. A ball is fired from a canon straight upward with an initial velocity vo. Assume
no air resistance. To compute the time to reach the apex, which one formula should be
used?
A)
v  vo  a t
B)
x  x o  vo t  (1/ 2) a t 2
C)
D)
E)
v2  vo 2  2a (x  x o )
None of these will work
More than one of these will work OK.
Answer: A or B or E. A is the simplest to use, I think. But you could use B if you
realize that the total time in the air is twice the time to reach the apex.
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder
15
CT2-21. If the initial velocity vo is doubled, the time to reach the apex of the trajectory.
A) doubles.
B) increases by a factor of 4.
C) Neither of these. D) Impossible to tell from the information given.
Answer: A Work out a formula for the time to reach the apex!
CT2-22. To compute the maximum height, which one formula should be used.
A)
v  vo  a t
B)
x  x o  vo t  (1/ 2) a t 2
C)
D)
E)
v2  vo 2  2a (x  x o )
None of these will work
More than one of these will work OK.
Answer: C or, if you know the time by using the result of the previous problem, B. Also,
B can be used without previous results, if you are very clever. So I accept C, B, or E as
correct.
CT2-23. If the initial velocity vo is doubled, the maximum height of the ball..
A) doubles.
C) Neither of these.
B) increases by a factor of 4.
D) Impossible to tell from the information given.
Answer: B Work out a formula for the maximum height!
Phys1120 Spring 2010 Dubson
©University of Colorado at Boulder