CHAPTER 3: FORCES AND PRESSURE

PASCAL’S PRINCIPLE

Recall

• Pressure act on a solid object:

Pressure = Normal force

area

P = F/A

• Pressure at any point in liquid acts in all directions. Also increases with depth and affected by gravitational force

ghP

What is this??

Do you believe or not that I can raisean Elephant?

YES!!! We can raise up an elephant.

Pascal’s principle

Stated that an external pressure applied to an enclosed fluid transmitted

equally to every part of the fluid, as well as to the enclosed wall

ACTIVITY

I want eat egg..

Cannot !!! It is a raw egg..

What happened if wrap your palm around the raw egg and try to crush it ??

What happened if we change the raw egg to the cooked egg?Isn’t the same answer?

ExplanationWhen we wrap palm around a raw

egg and press it, it will not crush.

This is because inside the egg is liquid. When we wrap and press the egg, the pressure is transmitted throughout the liquid.

Pressure acts in all directions

DEMONSTRATION TIME

As the pressure is transmitted equally,

P1 = P2

Output Force = Output Piston area

Input Force Input Piston area

Therefore,

1

212

A

AFF

2

2

1

1

A

F

A

F

1

2

1

2

A

A

F

F

Because the volume of fluid pushed down on the left side equals the volume of fluid that is lifted up on the right side, the following formula is also true.

V1 = V2

by substitution, A1 D1 = A2 D2

A = cross sectional areaD = the distance moved

Applications of Pascal’s principle

Hydraulic brakeHydraulic jack

Hydraulic jack

Hydraulic brake

Problem Solving

Example 1:

In hydraulic brake, a force of 80 N is applied to a piston with area of 4 cm².a. What is the pressure transmitted throughout

the liquid?

b. If the piston at the wheel cylinder has an area of 8 cm², what is the force exerted on it?

Solution

(a) P = F/A

= 80 N/4 cm²

= 20 N/ cm²

b) F = P x A

= 20 N/cm² x 8 cm²

= 160 N

Example 2:

The figure shows a 10 N weight balancing a X N weight placed on a bigger syringe. What is the value of X ?

Solution:

10 N / 1.5 cm² = X N / 4.5 cm²

Therefore X = (10 / 1.5) x 4.5

= 30 N

2

2

1

1

A

F

A

F

PASCAL’S PRINCIPLE

Pressure

Enclosed fluid

-Hydraulic brake

-Hydraulic jack

Is transmitted equally in every part of a liquid in an Hydraulic System

Two connected piston

Related to

P1 =P2

With formula

ExampleDescribe that