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AQA AS Physics Question

How do you do this multiple choice question? I thought that when velocity is uniform, the resultant force is 0. In the Velocity-time graph, the acceleration=0 just towards the end of the line so why is the answer C and not A?
Thanks in advance ImageUploadedByStudent Room1495483271.910025.jpg

ImageUploadedByStudent Room1495483271.910025.jpg

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Reply 1

rexs2000

relate this to F=ma where acceleration is (change in velocity)/time
Try it

Reply 2

sgsb0y

I think I got it,
a=v/t

F=ma

Therefore F=m(v/t)

Therefore F is directly proportional to v/t
So Ft is directly proportional to v/t

Ft=v/t

Thus F= v/t^2

So the graph increases as velocity increases but since time is squared, the line increases more towards the time (x axis).

Thanks for helping!

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Reply 3

A Slice of Pi

Actually, you were right the first time. The answer is A, for the reason you gave in the original post. Refer to the updated v1.1 mark scheme for the specimen paper on the AQA website.

Aside: There are actually many possible F-t graphs, since the equation of the original v-t graph is not given, but all will share certain characteristics. For example, all will cross the time axis at the end points (as you pointed out in the original post). The given v-t graph resembles a portion of a cubic function such as the function v = -t^2(t-2) on a restricted domain. If you are a mathematics student, you may like to investigate the derivative of this function and sketch it. The result is a quadratic function that crosses the time axis at the end points, as predicted. We can also get a graph that looks similar to that given by "sticking" two quadratic functions together. In this case, the graph of the derivative will more closely resemble A.