Speed and Motion
Speed or Velocity?
It is very important to understand the difference between these terms because Physicists are very fussy about the precise definition of words.
Speed is the rate of movement. Speed = distance / time
Velocity is not just the rate of movement, but the rate of movement in a certain direction. Velocity = displacement / time. The diagram below shows displacement. See how it can have positive and negative values.
Acceleration is when the speed of an object is increasing. Like when you put up your speed when on the motorway. It does not go straight to 130km/h, but must speed up to reach it. The equation for acceleration is.
Speed - Time Graph
From the start to point A the train is travelling at a constant speed of 10ms-1
A and B it is accelerating because the line
goes up, on a distance time graph this would appear as a curve. It accelerates at:
a = (v-u)/t
a = (30 - 10)/100
a = 0.2ms-2
From B to C it is at a constant speed of 30ms-1
To find the distance you calculate the area under the line using simple laws of geometry.
The main difference with a velocity / time graph is that when the line goes down it may mean the object is decelerating or it is moving in the opposite direction, so it is possible to have a negative velocity.
Stopping distance is the distance a vehicle travels from the point the driver should stop, to when he or she
actually becomes stationary. It combines thinking distance and deceleration distance.
Here it is represented in a speed/time graph
Factors which can increase stopping distance are: