# Magnitude of average force

A woman with a mass of 44 kg runs at a speed of 8 m/s and jumps onto a giant 30 kg skateboard initially at rest. What is the combined speed of the woman and the skateboard?

44 kg / 8 m/s=5.5
30 kg / 8m/s=3.75

5.5+3.75=9.25m/s

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What do you know about mechanics that will help? Any equations?

The Bob

Think momentum.

i tried the linear momentum equation..mass * velocity and the answer was still incorrect

But what did you do to get this incorrect answer?

The Bob

mass x velocity

44kg x 8m/s= 352

30kg x 8m/s=240

352-240=112m/s

didn't seem right but i thought i would try it anyways

Right, now I see what you're doing. Have you heard of something called 'conservation of momentum'? Basically, the momentum before a 'change' must be equal to the combined momentum afterwards. You've, therefore, wrongly assumed that the skateboard has velocity of 8ms-1.

So... try imagining the woman and the skateboard as one mass after they have 'combined'.

The Bob

i tried the linear momentum equation..mass * velocity and the answer was still incorrect
Conservation of momentum. The total momentum before she jumps on the board=total momentum after she jumps on the board.