FORCES
AND
PRESSURE
Pressure
Blocks A and B on the left are resting on soft ground. Both weigh the same
and exert the same force on the ground. But the force from block B is spread
over a larger area, so the force on each square metre is reduced. The pressure
under block B is less than that under block A.
For a force acting at right angles to a surface, the pressure is calculated like
this:
pressure = f~;~:
In symbols: p = ~
If force is measured in newtons (N) and area in square metres (m-), pressure
is measured in newton/square metre (N/m2). 1 N/m2 is called 1 pascal (Pa):
If a lOONforce is spread over an area of
1m-, the pressure is
100 Pa.
If a 100N force is spread over an area of
2 m-, the pressure is
50 Pa.
If a 100N force is spread over an area of 0.2 m-, the pressure is
500 Pa.
If a 200 N force is spread over an area of 0.2 m-, the pressure is
1000 Pa.
For most pressure measurements, the pascal is a very small unit. In practical
situations, it is often more convenient to use the kilopascal (kPa).
1kPa = 1000 Pa
Increasing the pressure by reducing the area
Reducing the pressure by increasing
The studs on a football
boot have only a small
area of contact with the
ground. The pressure
under the studs is high
enough for them to sink
into the ground, which
gives extra grip
Skis have a large area to
reduce the pressure on the
snow so that they do not
sink in too far.
The area under the edge
of a knife's blade is
extremely small. Beneath
it, the pressure is high
enough for the blade to
push easily through the
material.
Under the tiny area of the
point of a drawing pin,
the pressure is far too high
for the wood to
withstand.
/
Wall foundations have a
large horizontal area. This
reduces the pressure
underneath so that the
walls do not sink further
into the ground.
A load-spreading washer
ensures that the nut is not
pulled into the wood
when tightened up.
the area