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PHYA4: Physics, Impulse Question, just me or ambiguous?

Figure 1: Force vs Time graph. So the area represents the impulse which is 10Ns. I have not shown it here to save space.

Now, the thing is, is it just me or is this question really unclear.

How do you know which direction the footballer is kicking the ball?
This is what I did.
Ft = m(v - u).
10 = 0.42(v - 10).

But, in the mark scheme it is.
Ft = m(v - u)
10 = 0.42(v - (-10)).

The question doesn't say whether u is in the opposite or same direction.
It says it returns to the same horizontal line it followed.
Then it doesn't matter which direction the ball is going at as long as it is still along the same horizontal line? So "u" could be either positive or negative?

Just me or is it far too ambiguous?

I know the examiner is trying to imply that the ball is returning to the same path, but it would have been more clear if it stated "Path" instead of "horizontal line". Because, it could be interpreted incorrectly?

Or, it could just be my poor English and that it wasn't actually ambiguous?

Sorry for my poor English.

Reply 1

uberteknik

Original post by Sayonara

Figure 1: Force vs Time graph. So the area represents the impulse which is 10Ns. I have not shown it here to save space.

The sign tells you the relationship between the direction of the initial and final velocity.

Since the ball is moving in the opposite direction to the boot (explicitly clear in the question), you need to arbitrarily assign a +ve value to one and -ve value to the other. As long as you are clear which one (ball or boot) was assigned +ve and maintain that throughout, then the sign of the final answer will tell you which direction the answer is.

i.e. ft = m(v - (-u)) = m(v+u) gives the same answer as -ft = m((-v) - u) = -m(v + u)