Chapter 3 Forces And Pressure.docx

CHAPTER 3: FORCES AND PRESSURE 3.1 UNDERSTANDING PRESSURE 1. Pressure is defined as force acting per unit area

P = pressure (unit Pascal/Pa) F = Force (unit Newton/N) A = Area (unit m2) 2. Application of pressure in daily life a) Area of sharp edge of a knife blade is extremely small. Thus produced the high pressure enable the knife to cut through material b) The large tyres of a tractor have a large area of contact with the ground. Thus reduced the pressure on the surface of the soil prevents the tractor from sinking to the ground EXERCISE 3.1

1

Diagram below shows a wooden block with a dimension of 10 cm × 40 cm × 60 cm being placed on a floor on a floor. The mass of the block is 0.2 kg.

60 cm 10 cm

40 cm What is the minimum pressure that can be exerted by the block on the floor?

2

Diagram below shows the mattress compress more when the boy is standing on it, compared when he is lying.

This is due to the difference in A Force B Weight C Pressure 3

Diagram below shows four different postures of a person doing some exercises. Which posture exerts the maximum pressure on the floor? A

B

C

D

3.2 UNDERSTANDING PRESSURE IN LIQUIDS 1. Pressure in liquid depend of depth, density of liquid and gravitational force. It does not depend on the surface of area. The SI unit for pressure is Pascal(Pa)

3. When the depth increase, the pressure of liquid also increase. If the density of the liquid increase, the pressure of the liquid also increase

Experiment pressure in liquid

4. Application of pressure in liquid a) Construction of dam - The wall of dam is thicker at the bottom than at the top. It is because the deeper the water the higher the pressure. Thicker dam can withstand the high pressure.

EXERCISE 3.2

1

Diagram below shows three containers filled with water to the same level. The pressure caused by the water at points X, Y and Z are Px, Py dan Pz respectively.

X

Y

Z

Which comparison is correct? A Px = Py = Pz B Px < Py < Pz C Px > Py > Pz 2

Which of the following pressure, P in liquid against depth, h, graph is correct? A

0

h

0

h

B P

C P

0

h

D P

0

3

h

Diagram below shows a cross section of swimming pool which has different depth. Water Air

2m 4 m 5.5 m

6m

What is the maximum pressure experienced at the bottom of the swimming pool? [ Density of water = 1000 kg m-3 ] A 2.0 × 104 Pa B 4.0 × 104 Pa C 5.5 × 104 Pa D 6.0 × 104 Pa 4

Liquid pressure increases with the depth of the liquid. When the depth of the liquid is greater, the liquid pressure is A higher B lower C unchanged