# P2 Revision Notes:Speeding Up & Slowing Down

Notes on P2 Speeding Up and Slowing Down

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• Created on: 03-04-12 09:27

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REVISION NOTES: PHYSICS 2
UNIT 2: SPEEDING UP & SLOWING DOWN
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
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.
Force and Acceleration
A resultant force always causes acceleration, remembering that negative acceleration is deceleration.
Without acceleration present, the resultant force must be zero. Resultant force, mass and
acceleration are all related in the following equation:
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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REVISION NOTES: PHYSICS 2
UNIT 2: SPEEDING UP & SLOWING DOWN
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.…read more