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# Mechanics help watch

1. I'm doing my Mechanics Momentum Assessed Homework. When, in a momentum question, the velocity of each particle is given in a vector form, how do I convert that into speed?

The question I'm stuck on is:
Two particles, A and B, are moving on a smooth horizontal plane when they collide. The mass of A is 6 kg and the mass of B is m kg. Before collision, the velocity of A is (2, 4) ms-1 and the velocity of B is (3, -2) ms-1. After the collision, the velocity of A is (1, 3) ms-1 and the velocity of B is (7, b) ms-1.

I have to find each of the missing values.

I've attempted this question in two ways so far:
1) resolve momentum horizontally and working backwards, however I got stuck when I reached the value m = (-12) / (1-7b)
2) calculating the resultant of the two velocities, but that would mean that I am not considering the fact they are travelling sideways

Please can someone help, I'm confusing myself and my notes aren't helpful at all!
2. Hey Molly,

All you need to do here is apply the conservation of momentum equation :

And go from there. You do not need to convert these vectors into speed for this equation, just stick with the format that they give you as they want you to answer it in the same way. If you get stuck or it still isn't clear, let me know

edit: You are (usually) not supposed to do any type of resolving in impulse questions! Don't over-complicate it. It might help to split the label the vectors into 'i' and 'j' components.
3. (Original post by mollyjoy1998)
I'm doing my Mechanics Momentum Assessed Homework. When, in a momentum question, the velocity of each particle is given in a vector form, how do I convert that into speed?

The question I'm stuck on is:
Two particles, A and B, are moving on a smooth horizontal plane when they collide. The mass of A is 6 kg and the mass of B is m kg. Before collision, the velocity of A is (2, 4) ms-1 and the velocity of B is (3, -2) ms-1. After the collision, the velocity of A is (1, 3) ms-1 and the velocity of B is (7, b) ms-1.

I have to find each of the missing values.

I've attempted this question in two ways so far:
1) resolve momentum horizontally and working backwards, however I got stuck when I reached the value m = (-12) / (1-7b)
2) calculating the resultant of the two velocities, but that would mean that I am not considering the fact they are travelling sideways

Please can someone help, I'm confusing myself and my notes aren't helpful at all!
To convert to speed, you would find the magnitude of the velocity using Pythagoras' theorem. For example, for A (2i + 4j), the speed (the resultant) = . However, for this question you should not convert to speed; speed has no direction. Momentum is mass x velocity, and has a direction.

The above poster seems to know what they're talking about. We will get you through this.

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Updated: March 28, 2016
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