# Need help with collision and conservation of momentum problem

• Richardparker800
In summary: It was supposed to say 3v/2m not v/m.In summary, the block of mass 2m after the collision is moving at a speed of 2v.
Richardparker800

## 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.

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!

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!

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 ^^

scottdave said:
3v/2. I misunderstood the numerical value part.

## 1. What is the conservation of momentum?

The conservation of momentum is a fundamental principle in physics that states that the total momentum of a closed system remains constant over time, unless an external force is applied. This means that the total momentum before a collision or interaction is equal to the total momentum after the collision or interaction.

## 2. How do you calculate momentum?

Momentum is calculated by multiplying an object's mass by its velocity. It is represented by the formula p = mv, where p is momentum, m is mass, and v is velocity. The SI unit for momentum is kilogram-meters per second (kg*m/s).

## 3. What is an inelastic collision?

An inelastic collision is a type of collision where kinetic energy is not conserved. This means that after the collision, the total kinetic energy of the objects involved is less than the initial total kinetic energy. This often happens when objects stick together or when there is friction involved.

## 4. How do you solve a collision and conservation of momentum problem?

To solve a collision and conservation of momentum problem, you need to apply the conservation of momentum principle. This involves setting the total initial momentum equal to the total final momentum and solving for the unknown variables. It is also important to use the correct units and to account for any external forces that may be present.

## 5. What is the difference between elastic and inelastic collisions?

In an elastic collision, kinetic energy is conserved, meaning the total kinetic energy before the collision is equal to the total kinetic energy after the collision. In an inelastic collision, kinetic energy is not conserved, as some energy is lost to other forms such as heat or sound. Additionally, in an elastic collision, the objects involved bounce off each other, while in an inelastic collision, the objects may stick together or deform.

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