Forces and Pressure
Pressure and how it is shared
Work out pressure from a force and an area, and explain why the same force presses harder when it is spread over less of a surface.
Megha and Pawan walk to a picnic carrying bags of the same weight. Pawan keeps shifting his; it is hurting his shoulders. Megha's is not. The bags weigh the same, so the weight cannot be the difference. What differs is the straps: his are narrow, hers are broad. The same pull is landing on a much smaller patch of his shoulder.
Pressure
Pressure is the force per unit area — the force divided by the area it acts over. The SI unit of force is the newton and of area the metre squared, so pressure is measured in newton per metre squared (N/m²). That unit has its own name: the pascal, written Pa.
Worked example
Why is a load easier to carry on the head with a round pad of cloth underneath it?
Notice what has not changed: the load is the same, so the force pressing down on the head is the same.
If the force had changed, the pad would be doing something quite different. Holding the force fixed is what lets the area do the explaining.
Try it together
An elephant stands on four feet. Each foot covers 0.25 m² of ground, and the elephant's weight is 20000 N. Find the pressure it puts on the ground.
Answer with a bare number each time.
1.First the area. Four feet, each covering 0.25 m². What is the total area, in square metres?
Have a go
Have a go on your own. A sack presses on the floor with a force of 250 N, spread over an area of 5 m². What is the pressure, in N/m²? Answer with a number.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.