pressure and moments

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9L Pressure and moments

Pressure

Particle theory

Moments



Pressure



9L Pressure and force

Ever felt under pressure?

Pressure is related to force.

What do you know about forces?

Forces can speed things up Forces can slow things downForces can change the shape of thingsForces are measured in newtonsWeight is a force


9L Pressure and force

What makes the pressure greater? Try this thought experiment...

Take an elephant called Nellie (she’s a nice friendly elephant).

Stand her on a blow-up rubber ball (the rubber’s very thick).

She can balance on the ball (just)! Nice trick Nellie, well done!

Now give her some running spikes (I think red is probably her colour) and do the trick again.

Oh dear! I’m sure we have another ball somewhere…


9L Ideas about Nellie’s downfall

What do you think of these ideas about Nellie’s downfall?

So why did the ball break? Any ideas?

The extra weight of the running spikes

broke the ball?

Nellie weakened the ball the first time?

She nipped off for a snack in between and got heavier?

NoNo

No

pressure =force

area the force acts on


9L It’s down to pressure

Nellie’s weight did not change. So the force was the same in both experiments.

Let’s look at the area.

The pressure goes up because... ...the area goes down.

weight spread out over area of feet weight spread out over area of spikes– and the spikes are small!– and Nellie’s feet are big!

pressure =force



9L Playing with pressure

What will happen to the pressure when you change…… Nellie’s weight? … the area of her foot? … both?

pressure =force



9L The formula triangle

A formula triangle makes it easy to remember and use the formula:

force = pressure × area

pressure =force


force

areapressure =

forcearea =

pressure


9L So explain this then!

These pictures show things that depend on the relationship between force, pressure and area.

Explain how.


9L Pressure in liquids and gases

It’s not just solid objects that exert pressure. When you swim underwater, you can feel the pressure in your ears.

The deeper you go, the higher the water pressure. Why?

What about air pressure? Is the air pressure lower at sea level or up a mountain?


9L Small forces can crush cars!

This equation can crush cars with a force of just 10 N!

Where has all the extra force come from?

pressure =force


force = 10 N

area = 1 cm²

pressure in liquid = 10 N/cm²

The pressure is the same throughout the liquid.

pressure in liquid= 10N/cm² area = 6 m² or 60 000 cm²

force = 10 N/cm² × 60 000 cm² = 600 000 N!


9L Quiz

1. To work out pressure you need to know…

a) both the force and the area

b) only the force

c) only the area.

2. Weight is…

a) a measure of the amount of substance in a body

b) a force pulling masses towards the Earth

c) a long time.


9L Quiz

3. Snowshoes have a large area to…

a) reduce the force acting on the snow

b) reduce the temperature of the snow

c) reduce the pressure acting on the snow.

4. A sharp knife cuts better than a blunt one because…

a) the thinner blade concentrates the force over asmaller area

b) the thinner blade is lighter

c) the thinner blade can slide between the molecules more easily.


9L Quiz

5. A force of 25 N acts on an area of 10 cm2. The pressure is…

a) 2.5 N/cm2

b) 250 N/cm2

c) 0.4 N/cm2.

6. The liquid in the car crusher:

a) transmits the pressure

b) increases the area

c) lubricates the moving parts.


Particle theory



9L Remember particles?

solid particles…- are very close together in a fixed arrangement - have a small amount of energy- vibrate but do not move

liquid particles…- are close together but have no fixed arrangement - have more energy than solid particles- vibrate and can move about

gas particles…- are far apart and have no fixed arrangement- have a large amount of energy- move rapidly in all directions


9L Explaining gas pressure

Particles can help us see why gases exert a pressure.

Every time a gas particle hits the container, it exerts a force. Millions of these small collisions create pressure.


9L Pressure’s rising

How do particles explain changes in gas pressure?

Explain why the pressure is higher when there are more gas particles.

Explain why the pressure is higher when the plunger is pushed in and there are the same number of particles.

Explain why the gas starts to become a liquid at high pressure.


9L Applying the model

Explain what is happening to the particles in each of these situations.

You can hear a liquid sloshing around in a can of camping gas, but when you use it only gas comes out.

An aerosol can explodes if you put

it on a fire.

Someone lights a scented candle in the lounge.

Within a minute or two you can smell it in the hallway.

In a hydraulic system, the pressure is the same throughout the liquid.


Moments



9L Levers

What’s a lever?

It’s a tool that helps you do a job more easily.A lever turns around a pivot.

Your force is the effort.

The force you are moving is the load.

Find the pivot, effort and load in each of these levers.

pivot

load

load

effort

pivot

load

effort

pivotload

effort

pivot

load

effort


9L Turning effect of a force

How does a lever make a job easier? It’s all about the turning effect of the force.

turning effect = force × distance

Why is this seesaw

balanced?

turning effect = 800N × 2m= 1600Nm

turning effect = 800N × 2m= 1600Nm




What would happen if we moved one sheep another metre away from the pivot?

turning effect = 800 N × 3 m

= 2400 N m

The turning effect created by the other sheep is still 1600 N m, so the seesaw is unbalanced.




Would this be easier with a long or short screwdriver?

What would happen if we replaced one of the sheep on the seesaw with an elephant?

How could you balance the sheep and the elephant?


9L Distance amplifiers

In your arm, levers don’t increase the effect of the force. Instead they magnify the distance. What does this mean?

Which force is bigger here, the effort or the load?

pressure and moments

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