Unit 1, lesson 3. About 12 minutes.

Speeding up and slowing down

Why does your tummy feel funny when a lift starts moving?

Play

Getting the experiment ready…

Explain

Things can go faster and faster.

They can also slow down.

Acceleration is how quickly speed changes.

If a car goes from 0 to 20 m/s in 5 seconds, it gains 4 m/s every second.

Slowing down counts too! Set the end speed lower than the start speed and look at the graph.

Acceleration is the rate of change of velocity. Its SI unit is the metre per second squared (m/s²), which means metres per second, per second.

When speed falls, the acceleration is negative. This is often called deceleration.

Worked example: a train goes from 10 m/s to 30 m/s in 8 s. So a = (30 − 10) / 8 = 2.5 m/s².

Speed at the start
Speed changes
Speed at the end

Want the full story?

The sim uses constant acceleration, so its speed graph is a straight line. Acceleration really means any change in velocity, which includes direction. So a car going round a bend at a steady speed is still accelerating.

The Formula

acceleration = change in speed ÷ time

a = \frac{\Delta v}{t} = \frac{v_2 - v_1}{t}

$a$: the acceleration, measured in metres per second squared (m/s²)
$v_1$: the speed at the start, measured in metres per second (m/s)
$v_2$: the speed at the end, measured in metres per second (m/s)
$t$: the time the change takes, measured in seconds (s)

Try the numbers

Start speed: 0 m/sEnd speed: 20 m/sTime: 5 s

Acceleration (negative means slowing down): 4 m/s²

Try it at home

You need a toy car, a large book, a smooth floor.

You need: a toy car, a large book, a smooth floor.

Let a toy car roll down a book.

Lean a book against something low to make a ramp.
Let the car roll from the top.
Watch how it moves near the top and near the bottom.
Make the ramp a little steeper and try again.

What's happening: the car keeps gaining speed all the way down the ramp. A steeper ramp gives more acceleration.

Physics is life

Your body

Your tummy feels it when you speed up.

Tiny organs in your inner ear sense acceleration. That is how you feel a lift start, even with your eyes closed.

Daily life

Cars speed up and slow down.

Good drivers accelerate gently. A sudden stop means a large deceleration, which throws things forward.

Nature

Falling things go faster and faster.

A falling apple gains nearly 10 m/s of speed every second, until air resistance matters.

Space

Rockets speed up as they go up.

Astronauts feel about three times their normal weight while their rocket accelerates.

Quick check

Question 1 of 3

What is acceleration?

Words to know

Acceleration: How quickly the velocity of something changes.

Acceleration: How quickly the velocity of something changes.
Deceleration: Slowing down. It is acceleration in the opposite direction to the motion.
Velocity: Speed in a particular direction.

Next up

Graphs that tell stories