Speed-time graph: (0, 0) (3, 9) (7, 9) (9, 15) (12, 0), find the…

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The question

Question: the speed-time graph joins (0, 0) (3, 9) (7, 9) (9, 15) (12, 0), find the acceleration of each part.

1 Read the graph

Across: the time.

Up: the speed, in m/s.

Going up: speeding up.

Flat: a steady speed.

Going down: slowing down.

2 Acceleration of each part

Acceleration is the speed gained each second: the change in speed divided by the time.

On the graph, it is the slope.

Part A: the speed goes up by 9 in 3 s.

9/3 is 3.

Part B is flat: the speed stays the same.

The acceleration is 0.

Part C: the speed goes up by 6 in 2 s.

6/2 is 3.

Part D: the speed goes down by 15 in 3 s.

-15/3 is −5.

Negative: it is slowing down, a deceleration.

Careful!

A slope going down gives a negative acceleration.

The object is slowing down.

3 Check

Let's check: acceleration times time gives back each change in speed.

So it is right!

Let's look back.

First: read the graph.

Then: acceleration of each part.

Last: check.