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## Homework Statement

Two people are skating towards each other. The person on the left has a

**mass of 80kg**and is skating at

**2 m/s**. The person on the right is skating towards the person on the left at

**1.5 m/s**and has a

**mass of 60kg**

At what velocity do they collide and in what direction do they move off in?

## Homework Equations

Momentum = Mass x Velocity

## The Attempt at a Solution

I multiplied the first skater's mass by their speed and then multiplied the second skater's mass by their speed and added them together to get a combined momentum.

Skater One: 80kg x 2m/s = 160 kg m/s.

Skater Two: 60kg x 1.5m/s = 90 kg m/s.

**Total combined momentum of 250 kg m/s**

The revision guide suggests:

1) Choose which direction is positive? What does this mean?

2) For some reason, they have worked it out as (80kg x 2m/s) + (60kg x -1.5m/s). Why is it minus? I don't understand why one of the speeds is then negative?

Thanks in advance.