Pressure between two solid surfaces
The pressure between two solid surfaces depends on:
(a) the force exerted at right angles between the two surfaces
(b) the area of contact of the two surfaces
Pressure = Force/Area (units Pascal where 1 Pascal (Pa) = 1 Nm-2)
Examples of high pressure: pencil point, drawing pin point, S hook and coin, running spikes,
stiletto heels
Examples of low pressure: large tractor tyres for use in boggy fields, drawing pin head, snow
shoes, caterpillar tracks, buildings built on a concrete raft, lying down on quicksand or using
a ladder on ice to increase surface area and so reduce pressure, ladder across weak roofs.
One of the simplest and best demonstrations to show that the pressure depends on the area
of contact is to hold a sharpened pencil between you thumb and forefinger and press. The
end with the point digs into your finger while the base in contact with your thumb does not
hurt at all. The force is the same but the areas of contact are different!
Police could use the depth of footprints in soft earth to get an idea of the weight of a burglar!
Example problem
The elephant and the girl!
A girl of mass 45 kg stands on one stiletto heel of area 0.25x10-4 m2
and an elephant of mass 5000 kg stands on one foot of area 2.5x10-2 m2.
Calcuate the pressure that each exerts on the ground.
Pressure = F/A
For the girl:
Pressure = 45x9.8/0.25x10-4 = 18x106 Pa
For the elephant:
Pressure = 5000x9.8/2.5x10-2 = 2x106 Pa
The pressure exerted by the girl is nearly ten times that exerted by the elephant!!
Stick slip motion
When one solid metal object is placed on top of another they do not touch over the whole of
their surfaces but only at a small number of points. This is because they are not really totally
smooth. This means that the whole weight of
the top object will be supported on the very
cold welded joint
small area of the top of these points. The
pressure there will be large and so the two
metals are “cold welded” together. If the top
surface is pulled sideways it will not move until
these welds are broken. When it does it will
jerk forward and the welds will then reform.
Pulling again will break these new welds and
so on.
A stick-slip motion results.