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 diagramImage result for free body diagram gcse
  • 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:

Image result for force extension 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 Image result for moment diagrams gcse
  • When the applied force > transmitted force, the distance is increased.Image result for lever diagrams gcse
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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

      Image result for pressure and area diagram gcse

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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. Image result for pressure in depth diagram gcse

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.Image result for displacement of waterThe size of upthrust = weight of water displaced
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