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💬 Lifting bags
Lift one wheat bag to a height, then three bags, then one bag three times higher. When have you done "more work" — and can we pin that down with a number?
How is work done by a constant force defined?
Work Done by a Constant Force
Force applied
the constant push or pull, in newton (N)
Displacement
how far the object moves
in the direction of that force
, in metre (m)
↓
Work
= force × displacement in the direction of the force.
W = F × s
(7.2). No displacement → no work.
- Lifting 3 bags (instead of 1) to the same height needs 3 times the force over the same distance → 3 times the work. Lifting 1 bag 3 times higher needs the same force over 3 times the distance → again 3 times the work.
- So: work done = force applied × displacement in the direction of the force. W = F × s
- Applying a larger force over the same distance, or the same force over a larger distance, both do proportionally more work.
What is the SI unit of work, the joule?
Joule (J)
1 newton
the force applied
1 metre
the displacement, along the force
↓
1 joule
= 1 N × 1 m. It is the SI unit of
both
work and energy. 1 J = 1 kg m² s⁻².
- The SI unit of work is the joule (J) . Since force is in newton (N) and displacement in metre (m): 1 J = 1 N × 1 m
-
One joule of work is done when a force of 1 N displaces an object 1 m in the direction of the force. As
1
N
= 1 kg m s -2 , we get 1 J
= 1 kg m 2 s -2 . -
Work can also be read as the
area under a force-displacement graph
(e.g.
10
N
× 1
m
= 10 J ); this even works when the force is not constant.
Important Definitions
- Work — work done on an object by a constant force is the force applied multiplied by the displacement in the direction of the force.
- Joule (J) — the SI unit of work; 1 J of work is done when a force of 1 N moves an object 1 m in the direction of the force.
✎ Note
- While describing work done, it is important to specify the force (or agency) doing the work and the object on which the work is done.
