PressureModify : Chaiporn Pattanajak

Banphue pittayasan SchoolUdonthani

Thailand

Is a pushing force

Pressure is when a push, squashes something

Walking pushes on the ground, if the ground is soft it gets squashed and

we leave footprints

If the ground is very soft we leave deep footprints

We can make less deep footprints by spreading out

our weight (push) with special shoes.

Which shoes do you think will push hardest on the

floor?

Sensible shoes?• The force of our body pushing

down will exert a pressure• The surface area of our shoes

in contact with the ground will affect the pressure

• These stiletto shoes will concentrate the force over a small area

• These shoes will give a larger pressure and would not be suitable for walking on soft ground

More sensible shoes

• These boots have a large surface area

• The weight of a person is spread out over the larger surface

• The pressure exerted by the person is smaller than if they were wearing stiletto shoes

• A small pressure means you will not sink into soft ground

Examples of where a large area is needed to reduce

pressure.Caterpillar tracks on diggers

and on tanks

Tyres on a tractor

Skis and snowboards

snowshoes

Examples of where a small area is needed to increase the pressure

• Nails

• Needles

• Kitchen knives

• Scissors

• Drawing pins

• Spade

Questions on pressure

1) What combination of force and area give a high pressure?

2) What combination of force and area give a low pressure?

3) What is the formula for calculating pressure?

4) What units are pressure given in?

Answers:

1) Large force and small area

2) Small force on large area

3) Pressure = force /area

4) Pascals (Pa)

Pressure Pressure is define as force per unit area, where the force F is understood to be the magnitude of force acting perpendicular to the surface area A.

: The SI unit of pressure is N/m2 .

This unit has the official name Pascal(Pa).

1 Pa = 1 N/m2

A

FP

Area

F

Example 10-2• Calculating pressure. The two feet of a 60-

kg person cover an area of 500 cm2. (a) Determine the pressure exerted by two the feet on the ground. (b) If the person stands on one foot, what will the pressure be under that foot?

• Approach. Assume the person is at rest. Then the ground pushed up on her with a force equal to her weight mg, and exerts a force mg on the ground where her feet (or foot) contract it.

• Because 1 cm2 = (10-2m)2 = 10-4m2

(a) The pressure on the ground exerted by the two feet is 23

2

2

/102.105.0

)/10()60(mNx

m

smxkg

A

mg

A

FP

(b) If the person stands on one foot, the force is stillequal to the person's weight, but the area will be half as so much, so the pressure will be twice as much:

= 2.4x103N/m2

Practice using the formula for pressure1. A box weighs 500N. It’s base

has an area of 5 m2. What pressure does it exert?

2. Another identical box is placed on top of the first box. What is the pressure now?

3. A snowboarder weighs 600N. The area of the snowboard is 0.5m2. What is the pressure on the snow?

Answers:1. 100 Pa

2. 200 Pa

3. 120 Pa