Q2.2.a
An object is moving in the +x direction.
1) A only
Which of the following statements about the 2) B only
net force acting on the object could be true? 3) C only
4) A and B
5) B and C
A. The net force is in the +x direction
6) A and C
B. The net force is in the –x direction
7) A, B, and C
C. The net force is zero
Q2.2.b
Cart A moves to the left at nearly
constant speed.
Cart B moves to the left, gradually
speeding up.
Cart C moves to the left, gradually
slowing down.
Which cart(s) experience a net
force to the left?
1) A only
2) B only
3) C only
4) A and B
5) B and C
6) A and C
7) A, B, and C
Q2.3.a:
The x-component of momentum
of an object is found to increase
with time:
t = 0 s px = 120 kg m/s
t = 1 s px = 100 kg m/s
t = 2 s px = 80 kg m/s
t = 3 s px = 60 kg m/s
What can you conclude about the xcomponent of the net force acting on
the object?
1) Fnet,x = 0
2) Fnet,x is constant
3) Fnet,x is increasing with time
4) Fnet,x is decreasing with time
5) Not enough information is given to
determine which is true.
Q2.3.b
A hockey puck is sliding along the ice with
nearly constant momentum
< 10, 0, 5 > kg m/s when it is suddenly
struck by a hockey stick with a force
< 0, 0, 2000 > N that lasts for only
3 milliseconds (3e-3 s).
What is the new (vector) momentum of the
puck?
1) < 10, 0, 11 > kg· m/s
2) < 0, 0, 6 > kg· m/s
3) 14.86 kg· m/s
4) < 16, 0, 11 > kg· m/s
5) < 0, 0, 30 > kg· m/s
Q2.3.c:
You push a book across a table.
In order to keep the book moving
with constant momentum, you
have to keep pushing with a
constant force.
Which statement explains this?
1) A net force is necessary to keep an
object moving.
2) To make the net force on the book
zero, you must push with a force
equal and opposite to the friction
force on the book.
3) The force you exert must be
slightly larger than the friction force.
Q2.3.d:
Inside a spaceship in outer space there
is a small steel ball. At a particular
instant, the ball has momentum
< −8, 3, 0 > kg· m/s and is pulled by a
string, which exerts a force
< 20, −10, 0 > N on the ball.
What is the ball’s (vector)
momentum 2 seconds later?
1) < −28, 23, 0 > kg· m/s
2) < 12, −7, 0 > kg· m/s
3) 36.2 kg· m/s
4) < 32, −17, 0 > kg· m/s
5) < 40, −20, 0 > kg· m/s
Q2.5a
A spring whose stiffness is 30 N/m is 12 cm
(0.12 m) long when relaxed. You push on
the spring, compressing it so its length is
now 10 cm (0.10 m). What is the vector ሬࡸԦ?
1) < 0, 1, 0> m
2) < 0, 0.1, 0 > m
3) < 0, −1, 0 > m
4) < 0, −0.1, 0 > m
5) None of the above
Q2.5b
A spring whose stiffness is 30 N/m is 12 cm
(0.12 m) long when relaxed. You push on
the spring, compressing it so its length is
now 10 cm (0.10 m). What is the stretch s?
1) 0.02 m
2) 0.12 m
3) 0.10 m
1) −0.02 m
2) −0.12 m
3) −0.10 m
Q2.5.c
A spring whose stiffness is 30 N/m is 12 cm (0.12
m) long when relaxed. You push on the spring,
compressing it so its length is now 10 cm (0.10 m).
What is the magnitude of the force the spring now
exerts on your hand?
1) 0.6 N
2) 3 N
3) 3.6 N
4) 30 N
Note to instructors:
Questions 2.5d - 2.5h refer to lecture-demo program 02_BlockSpring_v2_PRIVATE.py
and to the discussion of the iterative prediction of the motion of a block and spring
on pp. 66-69
Q2.5.d. Which diagram shows the directions of momentum and net force
on the block during time step 1?
1.
2.
3.
4.
None of these
Q2.5.e. Which diagram shows the directions of momentum and net force
on the block during time step 2?
1.
2.
3.
4.
None of these
Q2.5.f. Which diagram shows the directions of momentum and net force
on the block during time step 3?
1.
2.
3.
4.
None of these
Q2.5g
In applying the Momentum Principle iteratively to predict the motion of a block on a
spring, what quantities must be recalculated for each time step?
D: the momentum of the block
A: the stretch of the spring
E: the new position of the block
B: the force by the spring on the block
C: the force by the Earth on the block
1) A, B, C, D, and E
2) A, B, D, and E only
3) D and E only
4) A, B, and C only
5) some other combination of quantities
Q2.5h
Which of the following would make the biggest improvement in the prediction of the
motion of the block?
1) Use more significant figures
2) Use a shorter time step and take more steps
3) Use a larger time step and take fewer steps
4) There is nothing we can do to improve this prediction
Q2.6.a:
A ball is initially on the ground, and you
kick it with initial velocity < 3,7,0> m/s.
At this speed air resistance is negligible.
Assume the usual coordinate system.
Which components of the ball's
momentum will change in the next half
second?
1) px
2) py
3) pz
4) px & py
5) py & pz
5) pz & px
7) px, py, & pz
Q2.6.b:
1) < 0, 2.45, 0 > N*s
The mass of the ball is 500 g, and its
initial velocity is < 3,7,0> m/s. What is the 2) < 0, –2.45, 0 > N*s
net impulse acting on the ball during the 3) < 0, 9.8, 0 > N*s
next 0.5 seconds after you kicked it?
4) < 0, –9.8, 0 > N*s
5) < 0, 4.9, 0 > N*s
6) < 0, –4.9, 0 > N*s
The initial momentum of the ball was < 1.5, 3.5, 0 > kg*m/s.
The final momentum of the ball is < 1.5, 1.05, 0 > kg*m/s.
Therefore...
Q2.6.c: Which graph correctly shows py for the ball during this 0.5 s?
1
2
3
4
5
6
Q2.6.e:
Initially the velocity of the ball is
< 3 , 7 ,0 > m/s. After 0.5 s, the ball's
velocity is < 3, 2.1, 0 > m/s.
What is the best choice for the ycomponent of the ball's average velocity
during this interval?
1) 2.10 m/s
2) 4.55 m/s
3) 4.90 m/s
4) 7.00 m/s
5) 9.10 m/s
Q2.7.a
A student is running very fast.
What is the student’s
approximate speed? (Think
about what you know about
track and field events.)
1) 0.1 m/s
2) 1 m/s
3) 10 m/s
4) 100 m/s
5) 1000 m/s
Q2.7.b Two running students collide head-on
One student exerts a force of
magnitude F on the other student.
Suppose we choose BOTH students as
the “system” to which to apply the
momentum principle. What is the net
force acting on this system?
1) < F, 0, 0 >
2) < 2F, 0, 0 >
3) < 0, 0, 0 >
Q2.7.c
Approximately what is the time interval t
from just before the students make contact
to just after?
1) 0.001 s
2) 0.01 s
3) 0.1 s
4) 1 s
5) 10 s