Problems 6-10 Solutions Outline
Problem 6
Why is it tiring to push hard against a solid wall even though you are doing no work?
No work is being done on the wall by you.
There is, however, work being done inside your arm muscles. Exerting a force via a muscle causes small continual motions in your muscles, which is work, and which causes you to tire. An example of this is holding a heavy load at arm’s length. While at first you may hold the load steady, after a time your arm will begin to shake, which indicates the motion of muscles in your arm.
Problem 7
What
is the correct unit of work expressed in SI units?
A) kg
m/s2
B) kg
m2/s
C) kg
m2/s2
D) kg2 m/s2
Problem 8
Can work be done on a system if there is no motion?
A) Yes, if an outside force is provided.
B) Yes, since motion is only relative.
C) No, since a system which is not moving has no
energy.
D) No, because of the way work is defined.
Problem 9
If you push twice as hard against a stationary brick
wall, the amount of work you do
A) doubles.
B) is cut in half.
C) remains constant but non-zero.
D) remains constant at zero.
Prob
A 50-N object was lifted 2.0 m vertically and is being
held there. How much work is
being done in holding the box in this position?
A) more than 100 J
B) 100 J
C) less than 100 J, but more than 0 J
D) 0 J