# Momentum physics using vectors.

1. Mar 22, 2013

### logan 1234343

1. The problem statement, all variables and given/known data
Block 1 (0.5 kg) travels with an initial velocity of [ 60i ] + [ 40j ] m/s and then collides with block 2 (0.5 kg) traveling with an initial velocity of [ -60i ] + [ -60j ] m/s.
After the collision, block 1 has a final momentum of [ 0i ] + [ 5j ] kg*m/s. Assume that no external forces are present and therefore the momentum for the system of blocks is conserved.

What is the total initial momentum of the system of blocks?
i + j kg*m/s

What is the change in momentum for block 1?
i + j kg*m/s

What is the change in momentum for block 2?
i + j kg*m/s

What is the final momentum of block 2?
i + j kg*m/s
--------------------------------------------------------------------------------
What are the initial kinetic energies of block 1, block 2, and the system of blocks?
Block 1: J
Block 2: J
System of blocks: J

What are the final kinetic energies of block 1, block 2, and the system of blocks?
Block 1: J
Block 2: J
System of blocks: J

What is the change in kinetic energy of the system of blocks due to the collision?
J

2. Relevant equations

p=mv

3. The attempt at a solution

I tried P1x= -10-40*.5
p1y=0
p2x=2.5-40*.5
where i got the final velocity from m1v1 +m2v2 + etc. =m1v1 +m2v2 + etc.

2. Mar 22, 2013

### Simon Bridge

If the initial momentum is $\vec{p}$ and the final is $\vec{q}$ then
$\vec{p}=\vec{p}_1+\vec{p}_2$ right?

If $\vec{u}$ and $\vec{v}$ are the initial and final velocities respectively, then:
$\vec{p}_1=m_1\vec{u}_1$ etc. but in your case $m_1=m_2=0.5\text{kg}=m$ so $\vec{p}=m(\vec{u}_1+\vec{u}_2)$

Vector addition using unit vectors works like this:

if $\vec{u}=u_x\vec{\imath}+u_y\vec{\jmath}$ and $\vec{v}=v_x\vec{\imath}+v_y\vec{\jmath}$;
then $\vec{u}+\vec{v}=(u_x+v_x)\vec{\imath}+(u_y+v_y)\vec{\jmath}$

Rewrite your calculations like this and they'll probably make more sense.

Last edited: Mar 22, 2013
3. Mar 22, 2013

### HallsofIvy

Staff Emeritus
Are these your answers or just a "template"? Do you mean for each that the answer is supposed to be of the form __ i+ __ j kg*m/s where you are to fill in the blanks? If so, why have you not made any attempt to solve them yourself? Do you know what "momentum" is?

--------------------------------------------------------------------------------
What are the initial kinetic energies of block 1, block 2, and the system of blocks?
Block 1: J
Block 2: J
System of blocks: J

What are the final kinetic energies of block 1, block 2, and the system of blocks?
Block 1: J
Block 2: J
System of blocks: J

What is the change in kinetic energy of the system of blocks due to the collision?
J

2. Relevant equations

p=mv

3. The attempt at a solution

I tried P1x= -10-40*.5
p1y=0
p2x=2.5-40*.5
where i got the final velocity from m1v1 +m2v2 + etc. =m1v1 +m2v2 + etc.[/QUOTE]
So what did you get? Why are you not showing us what you did and what result you got?

4. Mar 24, 2013

### logan 1234343

Yes this is a "template" for my class. I am to solve for each of the following. I have tried and simply do not understand. I missed this particular class due to illness and havent managed to understand the material. Least not this part.
My answers werent submitted because i figured they'd be irrelevant. The computer counted them as wrong. I showed how i got the answers.

5. Mar 24, 2013

### Simon Bridge

We don't understand your working as you have written it - we need to see how you are thinking about the problem more clearly in order to best help you. Did you follow the suggestions in post #2?