Introducing pressure

Begin with some simple questions in order to find out what students know about pressure and so determine a starting point for the topic.

This series of questions describes four pairs of events. The items in each pair are similar, but with a difference. For example, in pair A, the shoes are different.

A

A girl stands on soft sand in flat shoes

A girl stands on soft sand in high-heeled shoes

How would the marks in the sand be different?

B

A boy presses his thumb on the flat top of a drawing pin

A boy presses his thumb on the point of the drawing pin

What difference would his thumb feel?

C

The flat side of a knife is pressed against butter

The sharp edge of a knife is pressed against butter

What difference would you see?

D

A saucer is carefully placed flat onto water in a bowl

A saucer is lowered edge down into water

What difference would you see?

Illustrate your answers with diagrams if you want.

• "You stand with your bare feet on a smooth concrete floor. Then someone sprinkles gravel around you so that you have to walk across the gravel. Why does the gravel hurt while the concrete does not? (Because the gravel sticks to your feet. Is that the whole answer?)" This should lead to a discussion of load and area comparisons leading on to a need for a new concept: pressure.
• Other examples might include pressing on the bench and then a corner of the bench with the flat of your hand; holding a ruler by squeezing the edges and the flat surface; leaning against a wall with the flat of the palm of your hand and then a finger.