A lever consists of a bar which pivots at a fixed point known as the fulcrum. In the example shown the fulcrum is at the center of the lever. This lever provides no mechanical advantage and the force needed to lift the weight is equal to the weight itself.

 

However, if you want to lift a weight that is heavier than the force applied you can move the fulcrum closer to the weight to be lifted. This affects the force required in the following way:

w x d1 = f x d2

Where: w = weight   d1 = distance from fulcrum to weight    f = force needed   d2 = distance from fulcrum to point where force is applied

In this example the fulcrum has been moved towards the weight so that the weight is 1 meter from the fulcrum. This means that the force can now be applied 2 meters from the fulcrum.

If you needed to calculate the force needed to lift the weight then you can rearrange the formula.

w x d1 = f x d2  can be rearranged to f = (w x d1)/d2

f = (10 x 1)/2 (10/2 is the same as 5/1, the force required is 5 Kg)

Example Questions
1. How much force is required to lift the weight?

·         40lbs

·         50lbs

·         60lbs

·         70lbs

Answer
1. C – 60lbs is needed to lift the weight. It can be calculated like this:

f = (w x d1)/d2
f = (80 x 9)/12

f = (720)/12
f = 60 lbs

In practice, levers are used to reduce the force needed to move an object, in other words to make the task easier. However, in mechanical aptitude questions it is possible that you will see questions where the fulcrum has been placed closer to the force then the weight. This will mean that a force greater than the weight will be required to lift it.

You may see more complex questions involving levers, where there is more than one weight for example. In this case you need to work out the force required to lift each weight independently and then add them together to get the total force required.

2. How much force is required to lift the weights?

• A) 25lbs
• B) 35lbs
• C) 40lbs
• D) 45lbs

Answer
2. B – 35lbs is needed to lift the weight. It can be calculated like this:

f = (w1 x d1) + (w1a x d1a)/d2

f = (20 x 10) + (30 x 5)/10
f = (200 + 150)/10
f = 35 lbs