For a beam to balance the turning effect (also called a moment) on the left hand side must be the same as the turning effect on the right hand side.
Another way of saying this is that the combined forces trying to rotate the lever anti-clockwise must be the same as those trying to turn the lever clockwise. The turning effect is a combination of the force being applied and the distance that it is from the pivot (fulcrum) which is the point the beam is turning around.
If the moments/turning effects are unbalanced then the lever rotates to the left or the right.
A lever is balanced when the moments on both sides are equal. The moment (or turning force) can be calculated by multiplying the force (or load) by the distance from the fulcrum (or pivot). The following animation is a good way to explore the behaviour of levers. Have a proper play and make sure you understand what is happening before trying the games. We are going to be covering levers from the start of term so don’t worry if you struggle with this – it is not as easy at it looks!
This beam balances because 5N × 2m (on the left) = 10N × 1m (on the right). If you multiply N by m you get Nm so each moment is equal to 10Nm. The same principle applies if you have more than one force on one side of the beam.
This beam balances because 5N × 2m (on the left) = (4N × 1m) + (3N × 2m) (on the right). The individual moments on the right are calculated first then added together. There is a 10 Nm turning effect/moment on each side of the see-saw.
There are three classes of lever.
A class one lever has the fulcrum between the load and the effort e.g. a see-saw or a pair of scissors.
A class two lever has the load between the fulcrum and the effort e.g. a nutcracker or stapler
A class three lever has the effort between the load and the fulcrum e.g. a fishing rod or a cricket bat
The following is how levers might be hidden in an exam-style question.
In this diagram of a wheelbarrow, the load is between the fulcrum (centre of the wheel) and the lifting force (effort) so it is a second class lever. The weight of the load acts through its centre of mass (or centre of gravity – which is the point through which an object’s mass appears to act). If I can lift with a maximum force of 500N (I am a weakling!) what is the largest load I can lift with this wheelbarrow?
The moment I can create on the right is 500N × 150cm = 75,000Ncm so the maximum load I can lift is 75,000Ncm ÷ 50cm = 1,500N. Another way to look at this is to see that the distance from the fulcrum on the right is 3 times as much as the distance on the left so the load I can lift is 3 times as much as the force I can supply to the wheelbarrow.
Questions…
- What is another word for turning effect?
- How do you calculate the turning effect?
- When does a beam balance?
- How do you find an object’s weight on Earth if you know its mass in kg?
- If I use the wheelbarrow above to move a 900 N rock in my garden, how much force do I need to lift it? (Show your working)
- I describe a cricket bat as a class three lever, where is the fulcrum when the bat is being used?
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