P9 Motion


9.1 Speed and distance-time graphs

  • Constant speed means the same amount of time to go to the next distance.
  • The gradient on a distance-time graph means speed. The steeper it is, the faster it is.
  • (average) speed, v (m/s) = distance, s (m)/time, t (s)
  • Long distance vehicles have tachographs which make sure drivers don't drive for too long.
  • If the object travels the furthest distance in the same time, it is the fastest.
  • A smaller gradient means it is going slower (for a distance-time graph).
  • A flat line means it is stationary (for a distance-time graph).
1 of 5

9.2 Velocity and Acceleration

  • Velocity is speed in a given direction. You can have the same speed but different velocities. An object moving around in a circle has a direction of motion changing continuously whilst it goes around: the velocity isn't constant even if the speed is constant.
  • Displacement is distance travelled in a given direction.
  • Acceleration is the change of velocity per second.
  • acceleration = change in velocity (m/s^2)/time, t (s) or
    a= v-u/t
    (v is in the final velocity and u is the initial velocity)
  • Deceleration is when the object slows down.
2 of 5

9.3 More about velocity time graphs

  • A motion sensor linked to a computer records how velocity changes and displays measurements as a graph.
  • In acceleration, velocity increases with time.
  • On a velocity-time graph, a straight line is constant acceleration.
  • On a velocity-time graph, a steeper gradient is faster.
  • On a velocity-time graph, the gradient represents acceleration.
  • Braking reduces velocity and velocity is constant until the brake is applied.
  • On a velocity-time graph, a flat line is constant velocity and a gradient of 0 means 0 acceleration. 
  • The area under the line on a velocity-time graph is displacement. To work out the area, the area of a triangle is 1/2bh and a square is lw. It could be made of compound shapes.
  • If the speed or velocity is in km/h, convert to metres.
3 of 5

9.4 Analysing motion graphs

  • On a distance-time graph, the speed of an object is the gradient of the line. If it is a straight line, draw a triangle underneath the line. The height is distance travelled and the base is the time. Work out the gradient by height/base.
  • For a moving object with changing speed (a curved line), draw a tangent at the point and then do height/base.
  • A straight line and constant gradient mean constant acceleration.
  • Equation: 
    acceleration = change in velocity/time taken
  • If time is not given and there is constant acceleration, use:
4 of 5

Required Equations

  • Distance travelled(m) = speed (m/s) x time (s)
    s = vt
  • Acceleration(m/s^2) = change in velocity(m/s)/time taken(s)
    a = Δv/t
  • Uniform Acceleration:
    Final velocity(m/s) - Initial velocity(m/s) = 2 x Acceleration(m/s^2) x Distance(m)
  • Resultant force(N) = Mass(kg) x Acceleration(m/s^2)
    F = ma
5 of 5


No comments have yet been made

Similar Physics resources:

See all Physics resources »See all Forces and Motion resources »