Work & Machines
Science: KS3, topic 3.2.20
Work and Machines
Machines have one limit. The limit is the conservation of energy.
Conservation of energy
- The work done (or energy transferred) is given by the force × the distance.
- The distance is how far the object moves.
Force multiplier effect
- Using a force multiplier increases the force.
- So we reduce the distance moved by the object.
- The reduction in distance follows directly from the law of conservation of energy.
The conservation law
- The law of conservation of energy says that energy cannot be created or destroyed.
- The work done is a constant.
Results
- A long lever lets you move an object with greater force.
- But the lever cannot move the object far.
- So doorknobs are far from the door hinges.
- The longer lever makes doors easier to open.
Example
- A 20N force moves an object 10m.
- A lever is then used to move the same object.
- The lever increases the force to 40N.
Calculating the distance
- How far will the object move?
- 20 × 10 = 200.
- 200 ÷ 40 = 5.
- The object moves 5m.
Test yourself on Work & Machines
Why are doorknobs far from door hinges?
Select the correct statement in each pair
A worker uses a long lever to lift a heavy rock with a bigger force. Why can the lever not move the rock very far?