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!
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 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.