Step by step
The question
Question: the speed-time graph joins (0, 20) (4, 20) (8, 0), find the acceleration of each part.
speed-time graph: (0, 20) (4, 20) (8, 0)
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.
acceleration = change in speedtime
Part A is flat: the speed stays the same.
The acceleration is 0.
A: acceleration = 04 = 0 m/s²
Part B: the speed goes down by 20 in 4 s.
-20/4 is −5.
Negative: it is slowing down, a deceleration.
B: acceleration = −204 = −5 m/s²
Careful!
A slope going down gives a negative acceleration.
The object is slowing down.
Careful: going down = slowing down: negative
3 Check
Let's check: acceleration times time gives back each change in speed.
So it is right!
−5 × 4 = −20
Let's look back.
First: read the graph.
Then: acceleration of each part.
Last: check.
Answer
Careful: going down = slowing down: negative