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Mechanical Systems
Topic 4: Force, Pressure, and Area
 When force acts over a certain
area it is called pressure.
If the force is constant (the
same) but if the area changes,
so does the pressure.
 i.e. snow shoes do not push
down with any less force, but
by increasing the area you
decrease the pressure.
 An equation for this can be written as:
 Pressure = _Force_ = _F_
 Units:
 Force is measured in Newtons, N.
 Area is measure in meters squared, m2.
 Therefore pressure is measure in Newtons per meter
squared, N/m2.
 1 N/m2 = 1 Pascal (Pa), usually measured by the thousand,
Pascal’s Law
 Pascal’s law states that if pressure is exerted on a
contained fluid than it will be transmitted in all
directions throughout the fluid and perpendicular to
walls of the container.
 A hydraulic lift uses these principles to lift heavy
objects. Fluid pressure is used in a closed system (or
self contained collection of parts)
 Your circulatory system is a closed hydraulic system.
 A hydraulic system consists of a small cylinder, and a large
cylinder, a connecting pipe, and fluid (usually oil). Each
cylinder will also have a type of platform closing it off.
 In this hydraulic lift, pressure applied to a small piston is
transmitted to a large piston by means of a hydraulic fluid.
 By manipulating the pressure equation to be:
p x A= F
You can understand the mechanical advantage of hydraulics.
 If you applied 500N of force to a 5cm2 piston it
would produce 100Pa of pressure in the fluid.
p=F/A  500N/5cm2= 100Pa
 If the piston on the other side was 50cm2 then the 100Pa
of pressure acting on it would produce a force of 5000N.
 Because the piston is 10 times larger in area it will
produce 10 times as much force.
What is Work?
 Work occurs when you are exerting a force on an
object that causes it to move.
 The equation for work is:
Work= Force x distance
 Or W= F x d
 The units for work are joules (or J)
 A joule is the amount of work it takes to move a 1N
weight 1m.
Pascal’s Law and Mechanical
 The area of the small piston is 1 unit. If you push down on
the piston with a force of 10 N, you will generate a pressure
in the fluid of 10 N per unit of area. The large piston has an
area of 9 units. According to Pascal’s law the pressure on
every unit of area of the large piston with be 10 N.
__10 N__
unit area
= 90N
x 9 unit areas
 By exerting 10 N of effort force you could cause the
large piston to exert 90 N of force on the load.
 Therefore, the hydraulic lift provides a mechanical
 The mechanical advantage of hydraulics can be written
MA = Load Force = 90 N = 9
Effort Force 10 N
 If the MA is 9. Then the force on the larger piston will
be 9 times greater than the force on the smaller piston.
 However you will have to move the small piston 9
times further than the large one. This is because the
total work done must be equal in both pistons.
 Work (effort) = Work (load)
 This can be shown in the equation:
Work = Force x Distance or