Pushing a wall
When no work is done.
A tiring push
A student pushes hard against a rigid wall for a whole minute and feels very tired.
In science, how much work do you think she did on the wall?
What this lesson covers
The idea
No work is done on an object if the force on it is zero, or if it is not displaced whatever the force; one may still feel tired because muscles use up the body's internal energy.
A tiring push
A student pushes hard against a rigid wall for a whole minute and feels very tired.
In science, how much work do you think she did on the wall?
- A lot, because she pushed hard
- None at all
- A little, because she got tired
Push, push the wall, slide
Do the three tasks and watch the work.
Zero times anything
Work is F × s. If the force is zero, the work is zero. If the displacement is zero, the work is zero, however large the force. The wall did not move, so no work was done on it. Still, the muscles repeatedly expand and contract to apply the force, and they use up the internal energy of the body, so you feel tired.
No work is done on an object if the force on it is zero, or if the object is not displaced, whatever the force. You may still feel tired, because your muscles use up the body's internal energy.
Notes
No force or no displacement means no work. Feeling tired is the muscles using up the body's internal energy.
Check yourself
You push a rigid wall with a large force. How much work do you do on the wall?
Why do you feel tired after pushing against a wall?
In which case is the work done on the object zero?
A force of 12 N acts on a rigid wall which does not move. What is the work done on the wall, in J?
Answer: 0 J
W = 12 N × 0 m = 0 J.
- A large amount, because the force is large. Work is force × displacement. The wall does not move, so the work is zero.
- It depends on how long you push. Pushing for longer does not move the wall. With no displacement the work stays zero.
- None, because the wall is not displaced — correct. Yes!
- Your muscles use up the internal energy of your body — correct. Yes!
- Because you did work on the wall. No work is done on the wall, since it is not displaced. The tiredness comes from the muscles using up energy.
- Because your muscles were not used. The muscles repeatedly expand and contract to apply the force, and this uses up the body's internal energy.
- A force of 10 N moves the object 2 m in the direction of the force. The work is 10 N × 2 m = 20 J, so work is done.
- A force of 10 N acts and the object is not displaced — correct. Yes!
- A force of 3 N moves the object 5 m in the direction of the force. The work is 3 N × 5 m = 15 J, so work is done.