# A lever is used to lift a heavy load. When a 52 N force pushes the left end of the lever down 0.71 m, the load rises 0.3m. If the lever is only 78% efficient, what would be the weight of the load it could lift with this output?

##### 1 Answer

#### Explanation:

Applying the conservation of energy we can say that the work done on the load is equal to the work done by the effort.

Since work done = force x distance moved in the direction of the force we can write:

The term

The term

So we can write:

This assumes that the machine is 100% efficient and that there are no energy losses due to friction etc. However, we do not live in an ideal world and machines are rarely 100% efficient.

The ratio MA/VR is used as a measure of efficiency and is usually expressed as a percentage:

Efficiency =

Now we can apply this to this problem:

You don't need to go through this derivation every time you get a problem like this.

Just remember these 3 points:

1.

2.

3.

Efficiency =