AQA additional physics GCSE - Speeding up and slowing down P2 2

AQA additional physics GCSE - Speeding up and slowing down P2 2


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Forces measured in Newtons N

Objects exert equal and opposite forces on each other. If object A exerts a force on object B, Object B exerts an equal force in the opposite direction on object A.

Sometimes called 'action and reaction'

If a car hits a barrier it exerts a force on the barrier. The barrier exerts a force on the car in equal sxe in the opposite direction.

Or if you place a book on a table the book will act vertically downwards on the table. The table will exrt an equal and opposite reaction force upwards. 

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Most objects have more than one force acting on them. The resultant force is a force which is the result of two or more forces acting together, or a motion which is the result of two or more motions combined.

When the resultant force is zero:

  • if the object is at rest it will stay at rest.
  • If the object is moving is will carry on moving at the same speed in the same direction.

When the resultant force is not zero:

  • If the object is at rest it will accelerate in the direction of the resultant force.
  • If the object is moving in the same direction as the resultant force it will accelerate in that direction.
  • If the object is moving in the opposite direction to the force it wil decelerate.
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Acceleration is a change in velocity. An object can accelerate by changing its direction even if it is going at a constant speed. So a resultant force is needed to make an object change direction.

To work out the resultant force use:

resultant force N = mass kg x acceleration m/s 2

The greater the resultant force the greater the acceleration.

The bigger the mass the biger force needed to accelerate.

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The driving forces are equal and opposite to the frictional forces.

The faster the speed of a vehicle the bigger the deceleration needed to bring it to rest. So the bigger the braking force needed.

The stopping distance = thinking distance + braking distance

The thinkig distance is the distance travelled by the driver before reacting.The thinking distance is increased if the driver is tired or under the influence of alcohol or drugs.

The braking distance is the distancr the vehicles travel under th ebaking force. The braking distance can be incrased by poorly maintained roads, bad weather conditionsand the condition of the car.

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If an objects falls freely, the resulant force acting on it is the force of gravity.

We call the force of gravity 'weight' and the acceleration 'the acceleration due to gravity'.

The equation:

resultant force = mass x acceleration


weight N = mass kg x acceleration due to gravity n/kg

When an object falls through a fluid e.g. air, the fluid exerts frictional forces e.g air ressistance on it, resisting its motion. The faster the object falls , the bigger the frictional force becomes. Eventually it will be equal to the weight of the object. The resultant force is now zero so the body stops accelerating and moves at a constant velocity calles 'terminal velocity'. Shown on velocity time graphs.

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