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why we should use p 's initial direction?'

why after collision we should follow the p's direction? is that just because p is heavier? so we do not have to concern about their initial velocity' s different magnitude to determine which direction we should follow after collision? IMG_20200416_164343-compressed.jpg.jpeg

image-a64223fb-1db9-48a2-927c-7255e23cf6307524425318253068180-compressed.jpg.jpeg

Reply 1

Notnek

Original post by Hedwigeeeee

Look at your second diagram and think about the speeds of the two particles - imagine the situation in real life. Is it possible for P to change direction but have a slower speed than Q?

Reply 2

Hedwigeeeee

Original post by Sir Cumference

Look at your second diagram and think about the speeds of the two particles - imagine the situation in real life. Is it possible for P to change direction but have a slower speed than Q?

but it there anything about their mass? so u mean their direction is not related to their mass but their velocity?

Reply 3

Hedwigeeeee

Original post by Sir Cumference

Look at your second diagram and think about the speeds of the two particles - imagine the situation in real life. Is it possible for P to change direction but have a slower speed than Q?

but 1 and 1.5 are their final velocity... I think should we care about their initial velocity in order to work out their final direction?sorry I think I still cannot understand why if q has faster final v than p then we should follow its direction...

Reply 4

Notnek

Original post by Hedwigeeeee

No I'm saying that the motion you drew in your second diagram is impossible. If P and Q are travelling in the same direction with P in front of Q then if Q has a faster speed it will hit P. But P and Q have just collided so how is it possible for Q to be faster than P? If you're still unsure, try to ignore all the maths and just focus on the real life situation to hopefully realise that it is impossible.

Reply 5

Hedwigeeeee

Original post by Sir Cumference

the velocity I labeled is final velocity. not before collision. I want to ask is their direction nothing about their masses?

(edited 4 years ago)

Reply 6

Notnek

Original post by Hedwigeeeee

the velocity I labeled is final velocity. not before collision. I want to ask is their direction nothing about their masses?

I know. And I'm saying that the final velocities you've drawn are impossible - do you understand why? The masses are irrelevant to this.

Maybe you're trying to ask me a separate question but first you need to tell me you understand why the second diagram you drew is impossible.

Reply 7

Hedwigeeeee

Original post by Sir Cumference

the initial v wasn't given. We only know they are at opposite directions and their v after collision. sorry my graph is a bit unclear

Reply 8

Notnek

Original post by Hedwigeeeee

the initial v wasn't given. We only know they are at opposite directions and their v after collision. sorry my graph is a bit unclear

If two balls hit each other and then move in the same direction, it is not possible for the ball at the back to be faster than the ball at the front - this doesn't make any sense. You don't need A Level knowledge to see this, just think about the real life situation. Maybe pretend your fists are the two balls and try it yourself.