Velocity-time graphs
3 marks · HardDescribe, with reference to a velocity–time graph, how you would recognise when an object is decelerating and determine how to calculate the deceleration using the graph.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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Must show: reference to downward line or negative gradient, state method for calculating gradient, comment on value being negative/meaning deceleration
Also accept: Deceleration: line slopes downwards, calculate using (final velocity - initial velocity)/time, negative answer is deceleration
Common mistakes: Mixing up up and down slopes; Omitting explanation of negative value; Using speed not velocity
Show mark scheme and worked solution
Marking points
B1 · 1 mark · States that deceleration is shown by a downward/negative-sloped line on a velocity–time graph.
M1 · 1 mark · Mentions or describes calculating gradient: (final velocity – initial velocity) / (time taken).
B2 · 1 mark · Explains correctly that a negative gradient (or answer) indicates deceleration.
Worked solution
On a velocity–time graph, deceleration is shown when the line slopes downwards (from left to right).
To calculate deceleration, take the gradient: (final velocity – initial velocity) divided by time. If the answer is negative, this means the object is slowing down (decelerating).