Chapter 7

Pressure

At the end of this chapter,you should be able to Define pressure in terms of force

and area and recall the relationship: Pressure = Force/Area

Apply the relationship between pressure, force and area to new situations or to solve related problems.

Quiz Time Why do camels have large flat feet? Why do eskimos wear snow shoes?

6.1 Pressure

Pressure is defined as

Formula

force acting per unit area.

P =AF

where P = PressureF = Force (N)A = Area where the force is acting on (m2)

6.1 Pressure

The S.I. unit of pressure is

or

Newton per square metre (N m-

2) Pascal (Pa).

Quiz Time An elephant weighs 40 000 N

while a lady wearing high heels weighs 400 N. Who will exert a greater pressure on the ground if both were to stand on one leg?

Given:

Area of a elephant foot = 0.1 m2

Area of a heel = 0.0001 m2

Quiz Time

Pressure exerted by elephant

= Force / Area

= 40 000 / 0.1

= 400 000 Pa

Pressure exerted by lady

= Force / Area

= 400 / 0.0001

= 4 000 000 Pa

Quiz Time

Pressure exerted by elephant

= 400 000 Pa

Pressure exerted by lady

= 4 000 000 Pa

The lady’s heel exerts a higher pressure than the elephant foot.

So who hurts more when they step on you?

The elephant’s foot or the lady’s heel?

Quiz When a balloon is pressed against a

single nail, it will burst. However, when the balloon is pressed against a bed of nails with the same amount of force, it will NOT. WHY??

Quiz Applying the same amount of force, a

single nail having a smaller area of contact will exert a higher pressure to the balloon as compared to the pressure exerted by a bed of nails.

On the bed of nails, the force is distributed over all the nails, hence the area of contact is larger which result in a lower pressure acting on the balloon. Hence the balloon does not pop easily on it.

6.1 Pressure

In conclusion,

To increase pressure, we can either

Increase the force applied.

Decrease the area of contact.

Example 1

The atmospheric pressure is 100 kPa. What is the force exerted by the atmosphere on a rectangular surface that measures 0.5 m by 0.4 m?Pressure P = F A F = P × A = (100×103) ×(0.5×0.4)

= 20 000 N

Example 2

The tyres of a car are in contact with the ground over a total area of 3.0 10-2 m2. The total weight of the car is 6300 N. Calculate the pressure exerted by the tyres on the ground.Pressure = F A = 6300 3.0×10-2

= 2.1 × 105 Pa

Example 3

A standing man has a total contact area between his feet and the ground of 400 cm2. He is 60 kg. Calculate the pressure exerted on the ground by the man. Weight of the man W = mg = 60 × 10 = 600 NArea A = 400 cm2

= 400 × 10-4

= 4 × 10-2 m2

Example 3

Pressure = W A = __600___ 4 × 10-2

= 15 000 Pa