P2 Motion

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  • Created by: Phoebe
  • Created on: 28-12-12 10:19

1.1 Distance-time graphs

The distance-time graph for any object that is:

  • stationary is a horizontal line
  • moving at a constant speed is a straight line that slopes upwards

The gradient of a distance-time graph for an object represents the object's speed

The steeper the line on a distance-time graph, the greater the speed it represents

Speed (m/s) = distance travelled (m)                                                                            time taken (seconds)

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1.2 Velocity and acceleration

Velocity is a speed in a given direction:

  • an object moving steadily round in a circle has a constant speed. Its direction of motion changes continuously as it goes round so its velocity is not constant
  • two moving objects can have the same speed but different velocities, e.g. two cars travelling at 30 m/s, one north and one south

Acceleration is the change of velocity per second. The unit of acceleration is the metre per second squared (m/s²)

Acceleration= change in velocity (m/s)              a= v-u                                                                        time taken for the change (s)               t

Where a=acceleration, v= the final velocity, u=the initial velocity, t=time taken

Deceleration is the change of velocity per second when an object slows down

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1.3 More about velocity-time graphs

If the line on a velocity-time graph is horizontal, the acceleration is ZERO, therefore the object is travelling at a steady speed

The gradient of the line on a velocity-time graph represents acceleration. The steeper the gradient, the greater the acceleration

The area under the line on a velocity-time graph represents the distance travelled in a given time. The bigger the area, the greater the distance travelled

If the gradient of the line is negative, the object is decelerating

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1.4 Using graphs

Distance-time graphs:

  • the gradient of the line= the height of the triangle                                                               the base of the triangle
  • gradient= the object's speed
  • for a moving object with a changing speed, the distance time graph is not a straight line

Velocity-time graphs:

  • gradient = acceleration (m/s²)
  • acceleration= change of velocity (m/s)                                                                    time taken (seconds)
  • The distance travelled by an object is given by the area under the line of its velocity-time graph
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