M1 help

A bullet of mass 0.05kg is traveling horizontally at 450m/s when it hits a vertical wooden target.

The speed of the bullet is reduced to 200m/s, 0.002s after the bullet hits the target.

i) find the change in momentum of the bullet: I= mv - mu = (0.05)(200)-(0.05)(450)= -12.5

ii) Find the resistive force exerted by the target on the bullet. I=Fs, F=I/s = -12.5/0.002 = -6250Ns

however, the mark scheme says:

I = –F x 0.002 = –12.5
F = 6250 N

Why does 'F' have a minus sign in the equation? Thanks :smile:

Reply 1

Kerch

8

As force (and momentum) are vector quantities, they have a direction. In this case, you don't really need to think of them as a vector because everything takes place along a line, but you do need to think about the sign of the force and momentum.

It's purely a matter of convention, so if their force has a negative sign when yours has a positive sign, that's fine so long as you defined your direction of a positive force as being in the opposite direction to theirs.

Part i) could also be interpreted as asking about the change in magnitude of the momentum, which would be independent of the sign.

For part ii), it's a bit clearer because they call the force "resistive" which implies a positive force is in the direction opposite to the motion of the bullet. Hence the answer should really be positive.

Reply 2

upthegunners

OP

10

Original post by Kerch

As force (and momentum) are vector quantities, they have a direction. In this case, you don't really need to think of them as a vector because everything takes place along a line, but you do need to think about the sign of the force and momentum.

It's purely a matter of convention, so if their force has a negative sign when yours has a positive sign, that's fine so long as you defined your direction of a positive force as being in the opposite direction to theirs.

Part i) could also be interpreted as asking about the change in magnitude of the momentum, which would be independent of the sign.

For part ii), it's a bit clearer because they call the force "resistive" which implies a positive force is in the direction opposite to the motion of the bullet. Hence the answer should really be positive.