Pressure

Which would you rather? Would you rather have your foot trodden on by an elephant or by someone wearing stiletto heels?

What is pressure? Pressure = Force /area N/m2 N m2 The same force can have a different effect if it acts over a different area. A small force can have a big effect if it is concentrated on a small area. The relationship between force and area is called pressure The unit for pressure is Newtons per square meter (N/M2) or Pascals (Pa) Pressure = Force /area N/m2 N m2

What is an Inverse relationship? There is an Inverse relationship between area and pressure This means the bigger the area over which you spread a force, the smaller the pressure

your pressure in Pascal's You can calculate your pressure in Pascal's By multiplying your pressure value so far by 1000

Can you explain….? At outdoor events, car parks are sometimes in muddy fields. The organisers may put boards down for the cars to drive along. Explain why?

Can you give some examples of things that reduce the pressure being exerted? Snow shoes Snowboards Camels feet

Can you give some examples of devices that use high pressure? Scissors Drawing pins Jet wash Axe Knife

Air Pressure or atmospheric pressure The Earth’s atmosphere is pressing down on our bodies all the time. At sea level the atmosphere pushes down on us with a force of 100 000N. The surface area of the human body on average is about 1M2. What is the pressure of the air on your body when you are at sea level?

What about the pressure? What is the pressure of the air on your body when you are at sea level? Why do are bodies not get crushed by the atmosphere above and around us? What is the pressure on your body if you were in space? Explain your answer. What do astronauts have to wear in space so they can deal with the different pressure. If you were underwater, would the pressure be bigger or smaller? Explain your answer. Why do you ears sometimes hurt if you dive into the deep end of a swimming pool? Why are watches used by swimmers and divers ‘pressurised to three atmospheres’?

Crashed can demo

Magic Glass? Can you help?

Water pressure. Can you explain……?

Gases and pressure The particles in a gas are far apart. Under high pressure the particles can be pushed together This means that under pressure the gas can change shape and volume. The gases are COMPRESSIBLE

Pneumatic? This means something is filled with or uses compressed air to run Examples: Car tyres Aerosol cans Air under pressure acts as a propellant for the liquid Why can aerosol spray cans explode when on fire?

Air in a syringe Make a prediction about how far (if at all) the plunger will move when it is pushed. Record the distance the plunger moves. How accurate was your prediction? Write a conclusion: Describe how the particles are arranged in the air Include diagrams of the air particles before and after the plunger is pushed

Liquids and pressure The particles in a liquid are closer together. Under high pressure the particles cannot be pushed together This means that under pressure the liquid can change shape BUT can’t change volume so it pushes out on its container. The liquid is INCOMPRESSIBLE

Hydraulics? This means liquid pressure Many machines use hydraulics to work Examples: Car brakes Hydraulic jack to lift heavy loads (cars to change tyres) Robots in factories

Water in a syringe Make a prediction about how far (if at all) the plunger will move when it is pushed. Record the distance the plunger moves. How accurate was your prediction? Write a conclusion: Describe how the particles are arranged in the water Include diagrams of the water particles before and after the plunger is pushed

A simple hydraulic system Syringes same size: What happens to the second plunger when the first one is pushed? Syringes different size: Predict: When the plunger is the small syringe is pushed what will happen to the plunger in the large syringe?

A simple hydraulic system Plot a line graph of the results. (remember to have a sensible scale, label the axis, distance of small plunger on x axis, distance of large plunger on y axis) What is the relationship between the distance moved by the small plunger and the distance moved by the large plunger? Was your prediction correct? Write a conclusion How does the surface area of the plungers affect the distance moved by each one? Evaluation: How accurate were the results and where do you think we were inaccurate?

Why is a hydraulic system so useful?

Why is a hydraulic system so useful? The pressure in a hydraulic system is the same throughout. The force on the large plunger is bigger because the pressure is acting over a bigger area. A small force can be used to create a bigger force in a hydraulic system.

Homework

Homework

Differences in pressure. Air always moves from an area of high pressure to an area of low pressure. Can you explain the following. Why a syringe fills with water. How a liquid moves up a straw. Why a balloon completely deflates if you let it go before you have tied it Why the two pieces of paper move together when you blow in between them.