Less force, more distance
Work stays the same.
The same job three ways
You must raise a 60 N box by 1 m. You can lift it straight up, or push it up a short plank, or push it up a longer plank.
How do you think the work you do compares in the three ways, ignoring friction?
What this lesson covers
The idea
Work done by a machine stays the same: if the force decreases, the displacement increases; ignoring friction, the work put in equals the useful work done on the load, so machines create no energy.
The same job three ways
You must raise a 60 N box by 1 m. You can lift it straight up, or push it up a short plank, or push it up a longer plank.
How do you think the work you do compares in the three ways, ignoring friction?
- It is the same in all three
- It is least with the longest plank
- It is least when lifting straight up
Three ways up
Press Next step to try each way.
The same work
The force went down from 60 N to 30 N to 20 N, while the distance went up from 1 m to 2 m to 3 m. The product stayed 60 J each time. The machine made the force smaller, but it could not make the work smaller, and it did not create any energy.
The work done is the same in all cases: if the force decreases, the displacement increases. Ignoring friction, the work we put in equals the useful work done on the load. Machines do not create energy; they only help us use it more effectively.
- Way | Force (N) | Distance (m) | Work (J)
- Lift straight up | 60 | 1 | 60
- Plank of 2 m | 30 | 2 | 60
- Plank of 3 m | 20 | 3 | 60
Notes
When a machine makes the force smaller, the distance becomes larger: the work stays the same. Machines do not create energy.
Check yourself
Using a plank, a person pushes a box with a force of 30 N through 2 m along the plank (an example). How much work does the person do, in J?
Answer: 60 J
W = 30 N × 2 m = 60 J.
A machine lets you use a smaller force for the same task. What happens to the distance through which you apply the force?
An effort of 20 N is applied through 3 m along a plank. The same task is done by lifting a load with a force of 60 N. Through what height is the load raised, in m? (Ignore friction.)
Answer: 1 m
Work put in = 20 N × 3 m = 60 J = 60 N × h, so h = 1 m.
Do machines create energy?
- It decreases. The work stays the same, so a smaller force acts over a larger distance.
- It increases — correct. Yes!
- It stays the same. If the force decreases and the distance stayed the same, the work would decrease. The work stays the same, so the distance increases.
- Yes, a machine can give out more work than is put in. Ignoring friction, the work put in equals the useful work done on the load.
- Yes, but only when friction is present. Friction does not create energy. The work put in equals the useful work, ignoring friction, and machines do not create energy.
- No, they only help us use energy more effectively — correct. Yes!