Absolutely central elastic collision (or translate lying)

[Gustavs1337]

Homework Statement

2 balls, their size is the same but mass is different, they are going towards a collision. I do not know what is the word for it in english but its like this m₁= 1,2m₂. Speed of the balls are v₁=12m/s and v₂= 15m/s.Calculate the speed of each ball after absolutely central elastic collision.

Homework Equations

p=mv
p=p^,

The Attempt at a Solution

I could not even think of a solution since I missed out some classes.

 

[Doc Al]

What's conserved in an elastic collision?

 

[BvU]

I don't see the connection between the relevant equations and the attempt at solution ?

 

[Gustavs1337]

What's conserved in an elastic collision?

I don't see the connection between the relevant equations and the attempt at solution ?

yes all i can write is p1= 1,2m2 x 12 = 14,4m2 but i can't get past this part, the p= p^, is just a reminder for me. And i think that its kinetic energy that's being conserved.

 

[Doc Al]

the p= p, is just a reminder for me.

It should remind you that momentum is conserved.

And i think that its kinetic energy that's being conserved.

Right.

So combine those two conservation laws.

 

[Gustavs1337]

Its going to be hard since english is not my born language. so i couldn't figure it out so i looked on the internet and this is what i got K = 1/2m(p/m)²= p²/2m

 

[Doc Al]

Its going to be hard since english is not my born language. so i couldn't figure it out so i looked on the internet and this is what i got K = 1/2m(p/m)²= p²/2m

That's just expressing the kinetic energy in terms of momentum instead of velocity. That won't help you.

You need to write equations for:

• conservation of energy
• conservation of momentum

I assume you have a textbook. I suspect it gives examples of this sort of collision.

 

[Gustavs1337]

As I said before I missed out on some classes, they were doing the similar but easier tasks and I did not ask for their notes so that is my fault. Conservation of momentum is m₁xdv₁=-m₂xdv₂ as it says. And Energy i don't know that, or maybe its because of the differences in language.

 

[Doc Al]

Conservation of momentum is m1xdv1=-m2xdv2 as it says.

There are four velocities involved: The initial and final velocities of each object.

And Energy i don't know that, or maybe its because of the differences in language.

I don't think language has much to do with it.

Again, check your textbook.

And read this: Wiki: Elastic Collisions in one dimension

 

[Gustavs1337]

There are four velocities involved: The initial and final velocities of each object.

so you mean i should do it like this ?

or this

? And about that energy, i could not find anything but E_k= E_k^'. There is also this ∑p^→ = constant but i doubt it, also E_k= mv²/2 but also i doubt that. Only equation that i think would be right is the first one. :/. I have something to add
m1v1 + m2v2 =[m1+m2]v3 mayb e this

 

[Doc Al]

Both conservation equations are given in that Wiki link. They even solve them for you!

 

[Gustavs1337]

Both conservation equations are given in that Wiki link. They even solve them for you!

so basically

this is the momentum equation as it says and this

is energy equation.
and i get

by combining them ?

 

[Doc Al]

That's correct. (But you'd better understand how to solve those equations.)

 

[Gustavs1337]

You want me to put numbers in those equations or understand how i got the combinations, i have a hard time understanding what comes next.

 

[haruspex]

You want me to put numbers in those equations or understand how i got the combinations, i have a hard time understanding what comes next.

Yes, just plug the numbers in.
By the way, the correct translation is probably "head-on elastic collision", i.e. the mass centres, direction of relative motion and point of impact are all in a straight line.