P2.1 Forces And Their Effects

P2.1.1 Resultant Forces
1. Whenever two objects interact, the forces they exert on each other are equal and opposite.
  1. A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This single force is called the resultant force.
    1. A resultant force acting on an object may cause a change in its state of rest or motion.
      1. If the resultant force acting on a stationary object is zero, the object will remain stationary and if it is not not zero, the object will accelerate in the direction of the resultant force.
        If the resultant force acting on a moving object is zero, the object will continue to move at the same speed and in the same direction and if it is not zero, the object will accelerate in the direction of the resultant force.
P2.1.2 Forces And Motion
1. The acceleration of an object is determined by the resultant force acting on the object and the mass of the object. a=F/m or F=m×a where F is the resultant force in newtons, N, m is the mass in kilograms, kg and a is the acceleration in metres per second squared, m/s2.
  1. The gradient of a distance–time graph represents speed.
    1. To calculate the gradient of the line on a graph, divide the change in the y axis by the change in the x axis.
      1. The velocity of an object is its speed in a given direction.
        The gradient of a velocity–time graph represents acceleration. a=v−ut where a is the acceleration in metres per second squared, m/s2, v is the final velocity in metres per second, m/s, u is the initial velocity in metres per second, m/s and t is the time taken in seconds, s.
        The distance travelled is represented by the area under the line on the velocity-time graph.
P2.1.3 Forces And Braking
1. When a vehicle travels at a steady speed the resistive forces balance the driving force. Most of the resistive forces are caused by air resistance. The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance.
  1. The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver's reaction time (thinking distance) and the distance it travels under the braking force (braking distance).
    1. A driver's reaction time can be affected by tiredness, drugs and alcohol.
      1. When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increase. A vehicle's braking distance can be affected by adverse road and weather conditions and poor condition of the vehicle.
P2.1.4 Forces And Terminal Velocity
1. The faster an object moves through a fluid the greater the frictional force that acts on it.
  1. An object falling through a fluid will initially accelerate due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity (steady speed).
    1. W=m×g where W is the weight in newtons, N, m is the mass in kilograms, kg and g is the gravitational field strength in newtons per kilogram, N/kg.
P2.1.5 Forces And Elasticity
1. A force acting on an object may cause a change in shape of the object. A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.
  1. For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.
    1. The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded: F=k×e where F is the force in newtons, N, k is the spring constant in newtons per metre, N/m and e is the extension in metres, m.

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P2.1 Forces And Their Effects

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