Pressure

Lesson 1

Fluids Demonstration What happened to the shape of the water as it was moved to different containers? Is a liquid the only substance that can take the shape of its container? In your groups discuss what else behaves in this way. Do these substance apply any force?

Both Liquids and Gases are Fluids Any substance that can flow and take the shape of the container that holds it Both Liquids and Gases are Fluids

Pressure of Fluids 1 All fluids apply something we call pressure and is related to weight 2 Does air have weight? Let’s find out!

Balloon Demonstration What is happening to the meter stick as the air is let out of the balloon? Why is this happening? Will the same happen if the balloons were filled with water?

What is Air Pressure? It is caused by the weight of all the air in the atmosphere pressing down on Earth Air pressure is greater near the ground than it is higher in the sky. Why is that? Hint: Think back to our weather unit

Air Pressure Imagine a pile of books stacked one on top of the other. The weight or pressure on the bottom book would be much greater than the pressure on the top one. It’s the same with air molecules; those molecules on the bottom have greater pressure on them than those above.

Definition of Atmospheric Pressure The ratio of the weight of all the air above you to your surface area is atmospheric pressure

Definition of Underwater Pressure It is a combination of the weight of the water on top of the object and the weight of the air on top of the water. As the object moves deeper underwater the amount of water increases above it (Figure 4 pg. 898)

The deeper the water the greater the pressure because the weight of the water is greater near the bottom.

Lesson 2

Lesson Objectives How do force and area affect pressure?

How do we calculate Pressure? Pressure is the FORCE over a given AREA Pressure is measured in Pascals (Pa)

Force and Area Pressure decreases when the area over which a force is applied increases

High pressure and low pressure The shoes have flat soles, so they spread the person’s weight over a large surface area. These boots exert a low pressure on the ground. In contrast, the soles of the high-heeled boots have a smaller surface area and so exert a higher pressure. These shoes are likely to sink into soft ground. Photo credit: © 2008 Jupiterimages Corporation

Which of these objects rely on low pressure, and which on high pressure? How do they achieve this? Snow shoes Hypodermic syringe Sharp knife Camel’s feet Comfy chair

Calculating pressure f p a force pressure = area Pressure is the force per unit area and is calculated using this formula: f p a pressure = area force This can also be represented in a formula triangle. Pressure is measured in newtons per square metre (N/m2), which are also called pascals (Pa).

Pressure Calculations A force of 20 N acts over an area of 4 m2. Calculate the pressure. Pressure = Force ÷ Area 20 N ÷ 4 m2 = 5 N/m2 Sometimes you will see another unit being used. This is called the pascal and it has the symbol Pa.

Pressure Calculations A hammer supplies a force of 700 N. The hammer head has an area of 7.1 x 10-4 m2. What is the pressure? P = F/A P = (700 N/7.1 x 10-4 m2) P = 9.86 x 105 N/m2 P = 9.86 x 105 Pa

Applying Pressure Calculations Ice Road Truckers Activity

Teacher Instructions Watch the opening two minutes of https://www.youtube.com/watch?v=-mj0zKeLTR4 to set the scene. Print and distribute slides 31-36 Groups will swap posters and evaluate the other group on whether they did the calculations correctly and if they got the correct answer

Objective: To calculate pressure and apply to a real life situation.

A dangerous Job

But Science and Math help to make it possible!

How would you work out how much pressure is being put on the ice?

The trucks tyres have an area 1000cm2 in contact with the ice Its weight (the force) is 400000 N. Pressure = Force / Area 400000 / 1000 = 400 N/cm2 The ice road can withstand pressure of up to 500N/cm2.

The ice road can withstand pressure of up to 500N/cm2. So the truck will not break ice!

Group task You are the latest recruit for the ice road truckers. You must select a vehicle that will drive on the ice roads. It must not fall through the ice!

How to achieve in this task Work out the amount of pressure that each truck puts on the ice. Select which truck will be able to do the job. Present your findings in the form of a poster.

The ice road can withstand pressure of up to 500N/cm2. Which trucks will break the ice? Which truck will be suitable?

Truck Name: Do not say Snow to me Truck Name: Do not say Snow to me! Weight: 500,000 N Area of Tyres: 800cm2

Truck Name: Ice Ice Baby Weight: 400,000 N Area of Tyres: 500cm2

Truck Name: It’s Snow Joke Weight: 600,000 N Area of Tyres: 1000cm2

Truck Name: Ice to Meet You Weight: 400,000 N Area of Tyres: 900cm2

Peer Marking Swap posters with another group. Award it a level based on the outcomes of the lesson. Recall the equation for calculating pressure (level 4) Use the equation to calculate pressure (level 5) Provide scientific evidence to back up a decision made (level 6)

Plenary: Hold up the correct card What is the equation for pressure? (a) Pressure = force/mass (b) Pressure = area/force (c) Pressure = force/area (d) Pressure = mass/area

Plenary: Hold up the correct card An elephants weight is 12000N and the area of its feet is 400cm2 .How much pressure does the elephant exert on the ground? (a) 1000N/cm2 (b) 8000N/cm2 (c) 12400 N/cm2 (d) 30N/cm2