Need help with collision and conservation of momentum problem

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Homework Help Overview

The problem involves two blocks colliding on a frictionless surface, where one block has a mass of m and moves to the right with speed v, while the other block has a mass of 2m and moves to the left with speed 3v. After the collision, the first block moves at a speed of 2v, and the task is to find the speed of the second block after the collision.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss the conservation of momentum principle and the need to define a positive direction for momentum calculations. There are inquiries about the direction of the blocks' movements and how to calculate the momentum before and after the collision. Some participants express uncertainty about their calculations, particularly regarding the direction and numerical value of the resulting speed.

Discussion Status

Multiple participants are engaging with the problem, offering guidance on how to approach the momentum calculations. There is a recognition of the need for clarity in defining directions and the momentum of each block. Some participants are sharing their attempts and expressing concerns about potential errors in their reasoning.

Contextual Notes

There is mention of a requirement for a numerical value as the answer, which has led to some confusion among participants regarding the calculations. Additionally, the original poster's attempts have not been fully detailed, which may affect the discussion's progression.

Richardparker800
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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.
 
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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.
 
Hi Richardparker800,

Welcome to Physics Forums!

Please show us the details of at one of your attempts.
 
scottdave said:
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 want to get it wrong..
 
gneill said:
Hi Richardparker800,

Welcome to Physics Forums!

Please show us the details of at one of your attempts.
Richardparker800 said:
The answer keeps coming out to be zero... but I don't want to get it wrong..
scottdave said:
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 ^^
 
Richardparker800 said:
I did it! Thank you so much ^^
So what was your answer?
 
scottdave said:
So what was your answer?
3v/2. I misunderstood the numerical value part.
 

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