# Forces

• Created by: Glendia
• Created on: 21-08-17 10:15

## Introduction To Forces

• Forces are vector quantities- they include magnitude and direction.
• Forces are either:
• Contact Forces- the objects are actually touching, e.g. friction, tension, upthrust and drag
• Non-contact Forces- the objects are not touching, e.g. Gravitational, electrostatic and magnetic force
• These can be shown on a free body diagram
• Resultant Force: the total force acting on an object, as a result of many forces.
• These can be shown on vector diagrams
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## Forces and Elasticity

An object changes shape when more than one force is applied onto it.

• It was elastically deformed, if it returen to original shape.
• It is inelastically deformed, if it does not return to original shape.

The extention/compression of the object is directly proportional to the force applied until it reaches the limit of proportionality. This is shown in the force-extension/compression graph:

The work done to stretch/compress object is equal to the elastic potental energy in object.

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## Moments, Levers and Gears

A moment  is when a force causes an object to rotate aroud a pivot point.

• If the object is balanced:
• the total clockwise moment is equal to the total anti-clockwise moment.
• F1 x d1 = F2 x d2
• When the applied force > transmitted force, the distance is increased.
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## Pressure in a Fluid

A fluid can be liquid or gas.

• When particles in a fluid collide with a surface (partical or container surface), they exert force.
• Pressure is force per unit area.
• Small area = large pressure -> force created is larger
• Large area = small pressure -> force created is smaller

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## Pressure at Depth

The lower the height means the greater the pressure.

• This is because there are more particles above you, so the force is greater.
• This applies to atmospheric pressure as well as pressure in a liquid.

The pressure at a particular point in a column of liquid is greater when:

• The higher the column and the denser the liquid
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## Upthrust

Upthrust is created when the pressure at the bottom surface of an object > the pressure at the top surface of an object.

• This happens in a liquid because:
• there are more particles underneath object than above.
• Therefore the resultant force is upwards.
• An object floats when: force = upthrust, and sinks when: force > upthrust
• An object less dense than the liquid will:
• displace a volume of liquid greater than its weight, so it will rise
• the less dense the object, the more will remain above surface.
• An object denser than the liquid, cannot displace enough liquid equal to its own weight, so it sinks.The size of upthrust = weight of water displaced
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