# Need help with collision and conservation of momentum problem

## Homework Statement

Two blocks move along a horizontal frictionless surface and collide head-on with each other. The mass of the first block, moving to the right with speed v is m, and the mass of the second block moving towards the left with speed 3v is 2m. After the collision, the block of mass m is moving at a speed 2v. Find the speed u of the block of mass 2m after the collision. The answer shall be a numerical value.

## Homework Equations

Newton's second law and kinetic energy equation.

## The Attempt at a Solution

I have tried my best to use the second law, but of no avail, considering the the answer should come out to be a numerical value.

scottdave
Homework Helper
The sum of momentums (vector quantities) after the collision will be equal to what the sum of momentums was before the collision.
You need to define one direction as positive (for example, right).
Initially, one block has positive momentum, and the other is negative. Get the sum of momentums.
They give you information for the momentum of one of the blocks after the collision. You didn't specify direction - can you figure out which direction it is going?
Once you know the momentum (with direction) of this block, you should be able to find the momentum of the other block.

gneill
Mentor
Hi Richardparker800,

Welcome to Physics Forums!

The sum of momentums (vector quantities) after the collision will be equal to what the sum of momentums was before the collision.
You need to define one direction as positive (for example, right).
Initially, one block has positive momentum, and the other is negative. Get the sum of momentums.
They give you information for the momentum of one of the blocks after the collision. You didn't specify direction - can you figure out which direction it is going?
Once you know the momentum (with direction) of this block, you should be able to find the momentum of the other block.
The answer keeps coming out to be zero... but I don't wanna get it wrong..

Hi Richardparker800,

Welcome to Physics Forums!

The answer keeps coming out to be zero... but I don't wanna get it wrong..
The sum of momentums (vector quantities) after the collision will be equal to what the sum of momentums was before the collision.
You need to define one direction as positive (for example, right).
Initially, one block has positive momentum, and the other is negative. Get the sum of momentums.
They give you information for the momentum of one of the blocks after the collision. You didn't specify direction - can you figure out which direction it is going?
Once you know the momentum (with direction) of this block, you should be able to find the momentum of the other block.
I get the fact that the first block will move to the left..but what now?

I did it! Thank you so much ^^

scottdave