Revision Notes: P2

Just some brief notes about AQA P2, highlighting important key terms.

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  • Created by: Lauren
  • Created on: 10-03-12 19:38

Equal and Opposite Forces

Equal and Opposite Forces

We measure forces in newtons, N. Objects always exert equal and opposite forces on each other. For example, if object A exerted a force upon object B, object B would exert an opposite force of the same power on object A. These are often referred to as action and reaction forces. Examples include:

  • when a car hits a barrier it exerts a powerful force on the barrier, but the barrier exerts a force in the opposite direction of an equal amount on the car
  • if you were to lay a book on a table, it exerts a force vertically down on the table, but the table exerts an equal and opposite force on the book
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Resultant Force

Resultant Force

Because most objects tend to have multiple forces acting on them, the resultant force is the single force that would have the same effect on the object as all the other forces together. When the resultant force is zero, it means the object will remain stationary if already stationary, or if moving it will carry on moving at a constant speed.

When the resultant force is not equal to zero, it means that a stationary object will be accelerated in the direction of the resultant force; or if the object is moving in the same direction as the resultant force is will dramatically accelerate; or if the object is moving in the opposite direction to the resultant force is will decelerate.


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Force and Acceleration

Resultant Force=mass x acceleration A resultant force always causes acceleration, remembering that negative acceleration is deceleration. Without acceleration present, the resultant force must be zero

The greater the resultant force, the greater the acceleration. The larger the mass of an object, the bigger the force needed to give it a particular acceleration.

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Road Travel

Road Travel

A vehicle travelling at a steady speed has a resultant force of zero. This means the driving forces are equal and opposite to the friction forces. The faster the speed of the vehicle, the bigger the deceleration needed to bring it to rest in a particular distance – i.e. the bigger the breaking force needed. The stopping distance of a vehicle is the distance it travels during the driver’s reaction time (thinking distance) plus the distance it travels under the breaking force (breaking distance). The thinking distance is increased when the driver is under the influence of drugs or alcohol.


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Falling in Air

Falling in Air

When an object falls freely, the resultant force acting on it is gravity. It will make the object accelerate around 10m/s² close to the earth. We call this force of gravity “weight” and the acceleration “the acceleration due to gravity.” Therefore the above equation becomes:

weight (N) = mass (kg) x acceleration due to gravity (m/s²)

If the object is on the Earth, not falling, we use:

weight (N) = mass (kg) x gravitational field strength (N/kg)

When an object falls through a fluid (i.e. a liquid or a gas, e.g. air), the fluid exerts opposite forces on the falling object reducing its motion, for example air resistance. The faster the object falls, the greater the frictional force. Eventually, this would be equal to the weight of the object – this resultant force is now zero, so the object will stop accelerating and begin moving at a steady velocity – called the terminal velocity.


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fab...resit on wednesday and i'm ready for it!!!!! :D thank you **


thanks i got resit on wednesday and im ready thanks


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