What Is Acceleration — Physics (Grade 9)

Define and explain What Is Acceleration in your own words
Use key terms such as acceleration accurately
Apply what you have learned to new examples and questions
Avoid the common mistakes learners make with this topic

This lesson focuses on What Is Acceleration: working out how quickly velocity changes using a = (v − u) ÷ t.

Definition: What Is Acceleration

Working out how quickly velocity changes using a = (v − u) ÷ t.

Key ideas

Graphs turn journeys into pictures

On a distance–time graph, the gradient (slope) equals the speed — steeper means faster, flat means stationary. On a velocity–time graph the gradient gives acceleration, and the area under the line gives the distance travelled. One graph can replace a whole table of data.

Acceleration measures changing velocity

Acceleration = (final velocity − initial velocity) ÷ time. A car going from 0 to 20 m/s in 5 s accelerates at 4 m/s². Because velocity includes direction, turning a corner at constant speed still counts as acceleration.

Key term — acceleration: The rate of change of velocity, in m/s². Positive means speeding up, negative means slowing down.

Worked example: What Is Acceleration

A van accelerates from 10 m/s to 22 m/s in 6 s. Find the acceleration.

(22 − 10) ÷ 6 = 12 ÷ 6 = 2 m/s².

Answer: (22 − 10) ÷ 6 = 12 ÷ 6 = 2 m/s².

Common mistakes

Reading the gradient of a velocity–time graph as speed The gradient of a velocity–time graph is acceleration; speed is read straight off the vertical axis.
Mixing up distance and displacement Distance is the total path travelled; displacement is the straight line from start to end — they match only on a straight one-way trip.

Practice

How is distance found from a velocity–time graph?

Think of the shape under the line.

It equals the area under the graph, between the line and the time axis.

A train rounds a bend at a constant 40 m/s. Is it accelerating? Explain.

Velocity includes direction.

Yes — its direction is changing, so its velocity is changing, which means it is accelerating even though its speed is constant.

A runner covers 100 m in 12.5 s. Calculate the average speed.

Divide distance by time.

100 ÷ 12.5 = 8 m/s.

A car travels at a steady 25 m/s for 40 s. How far does it go?

Rearrange: distance = speed × time.

25 × 40 = 1000 m.

Quick check

What Is Acceleration — quick check

Which of these best defines "acceleration"?

The rate of change of velocity, in m/s². Positive means speeding up, negative means slowing down.

On a distance–time graph, what does a flat horizontal section mean?

The object is stationary — distance is not changing as time passes.

Key takeaways

What Is Acceleration: working out how quickly velocity changes using a = (v − u) ÷ t.
Graphs turn journeys into pictures: On a distance–time graph, the gradient (slope) equals the speed — steeper means faster, flat means stationary.
velocity: Speed in a stated direction — a vector, so 30 m/s north and 30 m/s south are different velocities.
Watch out for: reading the gradient of a velocity–time graph as speed

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