Picture an express train setting off from a station on a long, straight track. It may take 300 s to reach a velocity of 300 km/h along the track. Its velocity has increased by 1 km/h each second, and so we say that its acceleration is 1 km/h per second.

These are not very convenient units, although they may help to make it clear what is happening when we talk about acceleration. To calculate an object’s acceleration, we need to know two things:

• its change in velocity (how much it speeds up)

• the time taken (how long it takes to speed up).

The acceleration of the object is defined as the change of an object’s velocity per unit time.

acceleration = change in velocity / time taken , A = Δv/Δt a=300 km/h−0 km/h300 s = 1 km/h per

Speeding Up & Slowing Down

• An object that speeds up is accelerating

• An object that slows down is decelerating

• 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)