At GCSE level, you're
unlikely to be asked anything more complex than these two questions.
Gases create a pressure
on their containers, because of the gas molecules colliding with the walls.
When the area is
small, a moderate force can create a very large pressure. This is why
a sharp knife is good at cutting things: when you push the very small
area of the sharp blade against something, it creates a really large
Ice skates have
sharp edges, and thus a small area in contact with the ice.
This means that your weight creates a very large pressure on the ice,
far more than if you were standing in ordinary shoes.
Ice has an unusual property: it can melt under pressure, even if it's
below 0°C. When you're ice skating, you're actually skating on a
layer of water that you've just melted, which quickly re-freezes when
you move on (you're not skating on ice at all!) This is called
regelation, and means that there's very little friction
as you skate along.
Even a slender
supermodel can damage floors by walking on then in high-heeled shoes.
This is because the area of the heel is small, so you can easily create
enough pressure to cause a dent in the floor.
The pressure can be greater than if an elephant was standing there,
even though the force is much less. So you should be able to figure
out why elephants and camels have large feet.
As you go deeper into a liquid, the pressure increases. You
can feel this on your ears as you swim down to the bottom of a
swimming pool. We need to remember this when designing the walls
of pools and dams: the wall must be thicker at the bottom,
to withstand the increased pressure down there.
We can use
pressure in liquids to move a piston and do useful work. This
is how the hydraulic systems in diggers, car brakes and fairground
rides work: a pump creates pressure in an incompressible
liquid, which acts on a piston. By adjusting the area of the piston,
we can adjust the force we get.
The pressure of
the atmosphere on you right now is around 100,000 Pa. OK, that's just
a number, so think of it this way:
- you probably have a skin area of around 2 square metres,
- Pressure = force ÷ area, so force = pressure x area
thus the force on you = 100,000 x 2 = 200,000 Newtons.
That's about the same force as having over a dozen cars piled on top
Otto von Guericke,
(1602-1686), a German physicist, born in Magdeburg, performed a famous
experiment: the "Magdeburg Hemispheres".
These were two halves of a large, hollow metal ball. When all the
air was sucked out of the ball, two teams of eight horses couldn't
pull them apart, because the pressure of the Earth's atmosphere created
a very large force on the ball.
You may have seen a smaller version of this experiment in school.