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Cos final momentum - initial momentum = change in momentum =impulse. So it will always be in the direction of the final velocity right?

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- Thread starter tubworld
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- #1

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Cos final momentum - initial momentum = change in momentum =impulse. So it will always be in the direction of the final velocity right?

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Yup, you are correct.

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thanx alot!

- #4

andrevdh

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HallsofIvy

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Your statement "the impulse is in the final direction?" is, in general,

IF you are dealing with motion on a straight line and IF the impulse is enough to reverse the direction of motion then, yes, the impulse

Suppose you have an object moving along a line and you give it "hit" that slows it down to 1/2 its speed. In that case, the "final speed" is still in the same direction and

If you work in two or three directions, it's much more complicated. Imagine a pool ball bouncing off a cushion. The impulse is perpendicular to the cushion but neither the initial nor final speeds are in that direction.

The best thing to say is "impulse is change in momentum: subtract the two momenta." Since momentum is mass*velocity and mass has no direction, as far as the direction is concerned, subtract the initial velocity from the final velocity.

In the example you gave: inital velocity -ve, final velocity +ve, ve-(-ve)= +2ve. The impulse is in the + direction, the same as the final velocity. In the example I gave, initial velocity is ve, final velocity is (1/2)ve, impulse is (1/2)ve- ve= (-1/2)ve, opposite to the final velocity.

For the pool table example, take initial velocity vector to be (vx, -vy), final velocity (vx,+vy). The impulse is the difference of those vectors: (vx, vy)- (vx,-vy)= (vx-vx, vy+vy)= (0, vy), not in the direction of either initial or final velocity.

- #6

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Oh! Thanx for the explanation! I really UNDERSTAND now from your examples given! Appreciated! :)

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