Forces_Concept_7
Pressure
Pressure
Poking and prodding
You're at a party bopping the night away, when another partygoer (a little 'tired and emotional') lumbers over and treads on your feet.. .twice.The first time it's a stockinged foot, but the second time it's a stiletto heel. Why does the second stomp hurt so much more than the first? The person's weight — that is, the force that he or she exerts on your foot — won't have changed, so why the difference? The answer is pressure: the amount of force applied per unit area of a surface. The effect on the surface often depends an the pressure over the area of contact.
Snowshoes are wide, flat shoes for waking over snow Snow is soft and easily penetrated; spreading the force (your weight) over a larger area makes it less easy to penetrate the snow Walking over a damp lawn is difficult in stiletto heels, as the heels will penetrate the surface.
Sometimes it is necessary to concentrate a force into a srnaEl area. Poking a thread through a piece of fabric with your finger rather than a needle wouid be impossibie. Nails are sharp at one end and fiat at the other so that the force of the hammer can be concentrated to a point. If a force is spread over a wider area, you wiil need more force to do the same job.
Push and push back
When you inflate a bailoon, you are increasing the pressure of the air inside the balloon so that it pushes outwards, taking up more space. The ah- inside the balloon would go on expanding until its pressure was equal to that of the atmosphere, were it not for the stretched baitoon exerting an inward pressure on the trapped air 'f you let the balloon go without tying the ends it be able to contract and push the air out rapidly.T he reaction to this air movement wiil mean that the bailoon takes off in the opposite direction.
The concept of pressure will help children to understand how a little force can have a major effect if it is applied over a very small area, such as the blade of a skate or the point of a nail
Pressure force applied per unit area.
Poking sand (exploring, investigating)
A sand tray is useful for this. Ask the children to poke a finger into the sand. Now ask them to try with progressively larger squares of thick, rigid card or plywood. They should discover how much more difficult it is to penetrate the sand when spreading the force over a larger surface area.