Page 143 - Physics

Pressure

On the other hand, if two wooden blocks Other units of pressure are atmosphere,
of different surface areas are driven into torr, bar and mmHg.
a soft surface using force of the same
magnitude, the block with a smaller 1 atmosphere (atm) = 760 mmHg.
surface area will penetrate deeper into 1 atm = 1 × 10 N/m = 1 bar
2
5
the soft surface than the one with a larger
surface area, as illustrated in Figure 7.2. 1 torr = 1 mmHg = 133.3 Pa.
Large surface area Small
surface Acti vity 7.1
Soft surface area Aim: To demonstrate the
Small
surface concept of pressure.
area
Materials: Two buckets, one having

Figure 7.2: Driving a wooden block into a thin handle (A) and
the soft surface the other having a thick
handle (B) and water
These situations show that when a force
acts on an object, the result can be strongly Procedure
affected by the surface area over which it 1. Fill each bucket with an equal
acts. The relationship between force and volume of water.
the surface area over which it acts can 2. Use one hand to lift the buckets
be described by the concept of pressure. containing water, one at a time.
When a force is applied in a direction 3. Record your observations.
perpendicular to the surface of the object,
the object experiences pressure. Question

Which of the buckets gave you more
Thus, pressure is defi ned as the force pain when lifting? Why?
acting normally or perpendicularly on
a unit surface area of an object. That is, If you lift a bucket of water with a thin
handle, you will experience some pain.
Normal force(F) F
Pressure(P)! ! . However, if the handle is made thicker,
Area(A) A the pain will be much less. This is because,
F
P ! for a thicker handle, the area over which
A the force is applied is large and so, the
For a given amount of force, the pressure exerted is small.
smaller the area of application the
larger the pressure exerted. The SI unit Task 7.1
of pressure is newton per square metre,
(N/m ). This unit is usually referred to Discuss the variation of pressure with
2
as the pascal, Pa, where 1 Pa = 1 N/m . area in various contexts.
2

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