OCR A Level Physics

Revision Notes

3.1.1 Displacement, Velocity & Acceleration

Displacement, Speed, Velocity & Acceleration

Scalar quantities

  • Scalar quantities only have a magnitude (size)
    • Distance: the total length between two points
    • Speed: the total distance travelled per unit of time

Vector quantities

  • Vector quantities have both magnitude and direction
    • Displacement: the distance of an object from a fixed point in a specified direction
    • Velocity: the rate of change of displacement of an object
    • Acceleration: the rate of change of velocity of an object

Equations for Velocity & Acceleration

Equation Definitions, downloadable AS & A Level Physics revision notes

Equations linking displacement, velocity and acceleration

Worked Example

A car accelerates uniformly from rest to a speed of 150 km h–1 in 6.2 s.

Calculate the magnitude of the acceleration of the car in m s–2.

Acceleration Worked Example

Instantaneous Speed / Velocity

  • The instantaneous speed (or velocity) is the speed (or velocity) of an object at any given point in time
  • This could be for an object moving at a constant velocity or accelerating
    • An object accelerating is shown by a curved line on a displacement – time graph
    • An accelerating object will have a changing velocity
  • To find the instantaneous velocity on a displacement-time graph:
    • Draw a tangent at the required time
    • Calculate the gradient of that tangent

Instantaneous velocity on s-t graph, downloadable AS & A Level Physics revision notes

The instantaneous velocity is found by drawing a tangent on the displacement time graph

Average Speed / Velocity

  • The average speed (or velocity) is the total distance (or displacement) divided by the total time
  • To find the average velocity on a displacement-time graph, divide the total displacement (on the y-axis) by the total time (on the x-axis)
    • This method can be used for both a curved or a straight line on a displacement-time graph

Worked Example

A cyclist travels a distance of 20 m at a constant speed then decelerates to a traffic light 5 m ahead. The whole journey takes 3.5 s.

Calculate the average speed of the cyclist.

Step 1: Write the average speed equation

Average speed = total distance ÷ total time

Step 2: Calculate the total distance

Total distance = 20 + 5 = 25 m

Step 3: Calculate the average speed

Average speed = 25 ÷ 3.5  = 7.1 m s-1

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