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question about momentum

This question seems really easy but for some reason I can't work it out. The mark scheme and examiners report are no help because it's a multiple choice question.

question is attached or here.

thanks

question.png28.3KB

Reply 1

Joinedup

I think the simplest way to get the marks is by recognising that impulse and change in momentum are the same thing.

http://www.physicsclassroom.com/class/momentum/Lesson-1/Momentum-and-Impulse-Connection

impulse is Force x Time

so you can read off the change in momentum from the graph and the time taken and get the force from that (making sure to convert to SI units first)

Reply 2

klosovic

Original post by Joinedup

(making sure to convert to SI units first)

That's why I couldn't get it, I didn't realize time was in milliseconds and not seconds! What a silly error!

Reply 3

Qcomber

FΔt = mv
F=(0.4--0.6)/(4x10^-3)
F=1/0.004
F=250N

Reply 4

derpz

Original post by klosovic

First thing to always do is to check the axis and make sure the units are correct, here the time is in milliseconds so we mulitplay by x10^-3 for any calculation.

Now, Newton's second law states that Force is directly proportional to the rate of change of momentum. THe y-axis is momentum and the change is 1 (difference between 0.4 and -0.6).

We divide this by the time taken for the change in momentum to occur which is 2x10^-3 which is equal to 250N.

I think the main bit to remember here is the axis units and also that force = rate of change of momentum.