# Physics - Friction

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What is friction?
It tries to stop objects sliding past each other
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Good Points of Friction
IT ALLOWS THINGS TO START AND STOP
Slows a plane down when it's landing
Allows tyres to grip a road
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IT SLOWS YOU DOWN
Wasted energy (bike gears and brakes heat up)
Limits top speed -Air resistance
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What Is Friction Caused By?
It's caused by the bumps on an object
and acts between two objects that rub together
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What is resistance?
Air and water resistance (or DRAG) push against objects that are moving and slows them down.
(To speed things up they need to be streamlined)
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How can we reduce friction?
Use oil and lubricants in machinery
Polish a surface
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What is the difference between MASS and WEIGHT?
MASS is the amount of stuff in an object (DENSITY)
WEIGHT is caused by the pull of gravity (you are weightless in space)
weight=mass x gravitational field strength
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How Do Objects Lift?
Flying involves forces.
If the lift force acting upwards on an object exceeds its weight then the object accelerates into the air.

Hot air in a hot air balloon is less dense than cold air and hot air rises, so the balloon floats up.
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How Do Objects Float?
There are two forces acting on the object. Its WEIGHT and the UPTHRUST, force of the water pushing up.
If the weight is equal to or less than the upthrust, it floats (bouyant).
If the weight is greater than the upthrust, it sinks
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How do we work out DENSITY?
Density is the mass per unit volume of any object.
Density = Mass ÷ Volume

The volume of objects can be worked out by multiplying height by length by width.

Density of Water Measurement = 1g/cm3
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## Other cards in this set

### Card 2

#### Front

IT ALLOWS THINGS TO START AND STOP
Slows a plane down when it's landing
Allows tyres to grip a road

#### Back

Good Points of Friction

### Card 3

#### Front

IT SLOWS YOU DOWN
Wasted energy (bike gears and brakes heat up)
Limits top speed -Air resistance

### Card 4

#### Front

It's caused by the bumps on an object
and acts between two objects that rub together

### Card 5

#### Front

Air and water resistance (or DRAG) push against objects that are moving and slows them down.
(To speed things up they need to be streamlined)