Download Exam-3-review

Review for Exam #3
October 26, 2005
Is it possible to do work on an object
that remains at rest?
1. Yes
2. No
A box is being pulled up a rough incline by a
rope connected to a pulley. How many forces
are doing work on the box?
1.
2.
3.
4.
5.
One force
Two forces
Three forces
Four forces
No forces
Consider two elastic collisions: 1) a golf ball with
speed v hits a stationary bowling ball head-on. 2) a
bowling ball with speed v hits a stationary golf ball
head-on. In which case does the golf ball have the
greater speed after the collision?
1. Situation 1
2. Situation 2
3. Both the same
v
at rest
at rest
1
v
2
On a frictionless surface, a sliding box collides
and sticks to a second identical box which is
initially at rest. What is the final KE of the
system in terms of the initial KE?
1.
2.
3.
4.
5.
KEf = KEi
KEf = ¼ (KEi)
KEf = (KEi) /√2
KEf = ½(KEi)
KEf = √2(KEi)
vi
vf
A child on a skateboard is moving at a speed of 2 m/s.
After a force acts on the child, her speed is 3 m/s.
What can you say about the work done by the external
force on the child?
1. Positive work was done
2. Negative work was done
3. Zero work was done
You and your friend both solve a problem involving a
skier going down a slope, starting from rest. The two of
you have chosen different levels for y = 0 in this
problem. Which of the following quantities will you
and your friend agree on?
A) skier’s PE B) skier’s change in PE C) skier’s final KE
1.
2.
3.
4.
5.
Only B
Only C
A, B, C
Only A and C
Only B and C
A box sliding on a frictionless flat surface runs into a
fixed spring, which compresses a distance x to stop
the box. If the initial speed of the box were doubled,
how much would the spring compress in this case?
1.
2.
3.
4.
5.
Half as much
The same amount
√2 times as much
Twice as much
Four times as much
x
Answers
1.
2.
3.
4.
5.
6.
7.
No, the distance is zero.
There are four forces acting on the box – gravity, a normal force,
tension and friction. Tension does positive work, friction and
gravity both do negative work, and the normal force does no work,
so THREE forces contribute to the work.
Situation 2 – in situation 1, the bowling ball will have very little
velocity, so the small ball will have almost the same speed it had
before impact. In situation 2, the bowling ball will have a speed
close to v, so the small ball will rebound with 2v.
½ the initial KE
Positive work was done, since the KE increased.
Only B and C
Twice as much, from energy conservation, with kinetic energy
(~v2) at the beginning equaling the potential energy from a
SPRING (~x2).