Unit 9L Pressure and Moments Lesson 2 Pressure

Objective: to understand that the pressure due to a force varies with the area of contact,

to be able to carry out calculations for pressure.

Targets: 1-10

What is Pressure?

• Pressure is, the application of continuous force by one body on another.

• In other words pressure is caused by one object putting a force on another.

• When you push against a wall you are applying pressure to it.

Air and Water Pressure• The atmosphere is constantly pushing down on us.• This constant push creates pressure and we call it

air pressure.• Think about what happens when you fly in an

airplane . What happens to your ears? Why?• The same occurs under the water as well.• Observe Water Pressure Demo

– When we dive under the water the weight of the water is a force pushing on our body.

– What happens as we dive deeper?– How does this pose a problem for divers and submarines.

Pressure Formula• The formula for determining

pressure is:Pressure = Force/Surface Area

• The unit for pressure is the Pascal and the symbol is Pa.

• For the following questions find the pressure given the following forces and surface areas.

• When you increase the surface area what do you do to the pressure.

• When you increase the force what do you do to the pressure?

F= 15 N

S/A= 5 m

P=

F= 15 N

S/A= 100m

P=

F= 15 N

S/A= 3 m

P=

F= 3 N

S/A= 100 m

P=

What Does Pressure Depend On • Pressure depends on surface area and force.• For example compare the surface area of the heels of

these two shoes and the forces. Which would hurt more being stepped on by A or B?

A B

10N 10N

What Does Pressure Depend On • Compare the surface area of the soles of these two

shoes and the forces which they are exerted. Which would hurt more being stepped on by A or B?

A B

10N 15N

Inverse Relationships

• When we increase the surface area we decrease the pressure.

• This is called an inverse relationship.• Now you know how this inverse relationship

works how would you explain:– Why snowboards work?– Why sharp knives cut better than dull ones.– Imagine you are the only person around and you have

to save someone who had fallen through thin ice how would you do it?

Work to Do

• Read pages 126-127

• Complete questions 1-6 page 127

• Homework:– Complete Homework Sheet 10.1 and 10.2

Revision