Describing Motion (WJEC GCSE Physics: Combined Science)

Speed & Velocity

• Speed is a measure of how fast or slow an object is moving
• It is a scalar quantity
  • Because it only contains a magnitude (without a direction)

• The velocity of a moving object is similar to its speed, except it also describes the object’s direction
• Velocity is a vector quantity 
  • The velocity of an object contains both magnitude and direction
  • e.g. ‘15 m / s south’ or ‘250 mph on a bearing of 030°’

  Comparing Speed and Velocity

The cars in the diagram above have the same speed (a scalar quantity) but different velocities (a vector quantity). Fear not, they are in different lanes!

Acceleration

• Acceleration is defined as the rate of change of velocity
• In other words, it describes how much an object's velocity changes every second
• The acceleration of an object can be positive or negative, depending on whether the object is speeding up or slowing down
  • If an object is speeding up, its acceleration is positive
  • If an object is slowing down, its acceleration is negative (sometimes called deceleration)

Positive and Negative Acceleration

A rocket speeding up (accelerating) and a car slowing down (decelerating)

Calculating Speed

• For objects that are moving with a constant speed, use the equation below to calculate the speed:

• Where:
  • Speed is measured in metres per second (m/s)
  • Distance travelled is measured in metres (m)
  • Time taken is measured in seconds (s)

The Speed of Different Objects

A hiker might have a speed of 2.0 m/s, whereas a particularly excited bumble bee can have a speed of up to 4.5 m/s

Calculating Acceleration

• The equation below is used to calculate the average acceleration of an object:

• Where:
  • a = acceleration in metres per second squared (m/s²)
  • Δv = change in velocity in metres per second (m/s)
  • Δt = time taken in seconds (s)