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Collisions of partcles

khanpatel321

How do you know the particles will move off in the same direction after collision?

Reply 1

Zacken

Original post by khanpatel321

How do you know the particles will move off in the same direction after collision?

You're taking right as positive, so since your

v_2

is positive, this means the second ball moves to the right and since your

v_1

is negative (

u

is positive by definition) then the first ball moves to the left after the collision.

Reply 2

khanpatel321

Original post by Zacken

You're taking right as positive, so since your

v_2

is positive, this means the second ball moves to the right and since your

v_1

is negative (

u

is positive by definition) then the first ball moves to the left after the collision.

Both balls are moving to the right after collision according to the mark scheme above?

Reply 3

Zacken

Original post by khanpatel321

Both balls are moving to the right after collision according to the mark scheme above?

No, they've put the arrow facing to the right but the velocity is negative, so it opposes the direction of the arrow. I can say I am moving downwards

-1

metres to mean I'm moving upwards 1 metre. So it says the first vall is moving to the right at a negative velocity means that it is actually moving to the left.

Reply 4

khanpatel321

Original post by Zacken

No, they've put the arrow facing to the right but the velocity is negative, so it opposes the direction of the arrow. I can say I am moving downwards

-1

metres to mean I'm moving upwards 1 metre. So it says the first vall is moving to the right at a negative velocity means that it is actually moving to the left.

Oh ok now I get it. Thank you

Reply 5

atsruser

Original post by khanpatel321

Oh ok now I get it. Thank you

Until you have solved the problem, you won't know which the particles are going to move after the collision, in general, so you have to assume a direction. The signs then tell you if your assumed directions were correct. It makes no difference if you point your assumed vectors to the left or right; the signs will tell you all that you need to know.

One point: it is always a good idea to label 1D vector problems with a big clear +ve arrow, before you start work. This stops you becoming confused about what sign to give any particular vector - if it points in the same direction as that arrow, it's +ve, if not it's -ve. It makes no difference where you point the arrow "left" or "right". (Note that the people who wrote the mark scheme *didn't* do this - that's sloppy)

This becomes more important when you have two connected particles, both undergoing 1D motion, such as in pulley problems. You should then assign a +ve direction arrow to *each* particle, prior to drawing a free-body diagram, and label vectors as +ve or -ve relative to the arrow for that particle. (And again, it makes no difference which way these arrows point, as long as you stick to the convention that they indicate)