When a push does work
Show on a big screenYou push hard on a fixed post. Your arms ache. Have you done work on the post?
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- Push a fixed post. It does not move. The work, W, is 0.
- Lift a water can. The force and the movement point the same way. W is positive.
- Carry a bucket sideways. The force is up. W = 0.
- Slow a rolling ball. The force is against the movement. W is negative.
No. The post does not move, so you do no work on it.
Rule
Work = force × distance moved along the force. If there is no movement, the work is zero. Work is negative when the force points against the movement.
- Work — a force that moves something the way the force points. W = F × s. Here W is work and F is force. The letter s is the distance moved.
- Joule (J) — the unit of work. 1 J = 1 N × 1 m. N is the newton, the unit of force.
A 12 N force moves a trolley 4 m along the force. Here F = 12 N and s = 4 m. W = 12 × 4 = 48 J.
A force at right angles to the movement does no work. Carrying a bucket along a level path is one case. Your pull is up, and the bucket moves sideways. It moves no distance along that pull.
A rope pulls a sack up with 200 N. The sack is lowered 1.5 m. The sack moves down, against the pull. So s = −1.5 m. W = 200 × (−1.5) = −300 J.
Try the lab to finish this lesson.