Forces between objects, resultant forces,force and acceleration, On the road....

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Forces between objects..

  • Forces are measured in Newtons (N)
  • Objects always exert equal and opposite forces on each other. 
  • If object A exerts a force on object B, oject B exerts an equal and opposite force on object A. These are sometimes called 'action and reaction' forces.
  • A force can change the shape of an object or change its motion or state of rest.


When a car is being driven fowards there is a force from the tyre on the ground pushing backwards. There is an equal and opposite force from the ground on the tyre which pushes the car forwards.

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Resultant Force

  • most objects have more the one force acting on them. Th resultant force is the single force that would have the same effect on the object as all the original forces acting together.
  • When the resultant force of an object is zero:
  • if the object is at rest, it will stay at rest
  • if the object is moving, it will carry on moving at the same speed and in the same direction.
  • when the resultant force of an object is not zero, there will be an acceleration in the direction of the force.
  • This means that:
  • If the object is at rest, it will accelerate in the direction of the resultant force.
  • if the object is movung in the same direction as the resultant force, it will accelerate in the direction.
  • If the object is moving in the opposite direction to the resultant force, it will decelerate.
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Force and Acceleration

  • A resultant force always causes an acceleration . Remember that a deceleration is a negative acceleration. If there is no acceleration in a particular situation, the resultant force must be zero. 
  • 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.
  • We can find the resultant force on an object using the equation:
  • The greater the resultant force on a object, the greater its acceleration. The bigger the mass of an object, the bigger the force needed to give it a particular acceleration.
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On the Road...

  • If a vehicle is travelling at a steady speed, the resultant force on it is zero. The driving forces are equal and opposite to the frictional forces.
  • The faster the speed of a vehicle, the bigger the deceleration needed to stop it in a particular distance. so the bigger the braking 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 braking force (the braking distance).
  • The thinking distance is increased if the driver is tired or under the influence of alcohol or drugs.
  • The braking distance can be increased by:
  • poorly maintained roads or bad weather conditions
  • the condition of the car eg. worn tyres or worn brakes will increase the braking distance.
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