Why is there no initial momentum in the y direction when two objects collide?

[godkills]

Can someone tell me why there is no initial momentum when these 2 objects collide in the y direction?

At first these objects were going in the x direction this picture shows what happens when the objects collided.

Apple Orange
=======> <========

that is what it would look like when the objects going in x direction

 

[cepheid]

Can someone tell me why there is no initial momentum when these 2 objects collide in the y direction?

Your wording is a bit confusing here. Do you mean that there is no initial momentum in the y-direction, or that there is no initial momentum, period?

If the former, the answer is simple -- there is no initial momentum in the y-direction because, initially, both objects are traveling solely in the x-direction.

 

[gneill]

If both objects have velocity components only along the x-axis, then there are no velocity components in the y-direction (or rather, their y-components are zero), hence no momentum in the y-direction.

 

[godkills]

Oh sorry, What I meant was no initial momentum. When you do momentum for y direction.

Such that the equation would be 0 = final momentum.

I have the problem if you want me to type it.

Astronauts on opposite ends of a spaceship are comparing lunches. One has an apple, the other has an orange. They decide to trade. Astronaut 1 tosses the 0.130kg apple towards astronaut 2 with a speed of 1.11m/s. The 0.160kg orange is tossed from astroanut 2 to astronaut 1 with a speed of 1.21m/s. Unfortunately, the fruits collide, sending the orange off with a speed of 1.16m/s at an angle of 42 degrees with respect to its original direction of motion. Find the final speed and direction of the apple, assuming an elastic coolision and give the apple's direction relative to its original direction of motion.

Well that is not the question but I am doing the part 2 of it where this is the question.

In the apple-orange collision in Example 9-7 suppose the final velocity of the orange is 1.03 m/s in the negative y direction. What are trhe final speed and direction of the apple in this case?

Well I find the velocities in y direction and x direction.

Yet I don't seem to know why for y direction 0 = momentum of apple - momentum of orange to find the velocity in y direction of apple




Sorry If this turns out to be not a quick question but don't really have to solve just need explanation why is initial 0

 

[godkills]

Oh another question why do i have to subtract orange momentum from apples?

I mean if there is no initial momentum before the colllision shouldn't initial momentum is equal to the final? which would mean something in this form 0 + 0 = 0 + 0?

so that it is plus? instead of minus?

 

[cepheid]

Sorry If this turns out to be not a quick question but don't really have to solve just need explanation why is initial 0

It's just what both gneill and I said before. Before the collision, the objects aren't moving in the y-direction, therefore, neither of them has any momentum in that direction.

 

[godkills]

Yes, It turned out I was a little confused with the pictures. I have another question that was stated above your post.

 

[cepheid]

It should go something like this:

sum of momenta before = sum of momenta after.

Applying this separately in the x and y-directions you get:

y-direction ( (1) for apple, (2) for orange, i for initial, f for final):

0 = p⁽¹⁾_f,y + p⁽²⁾_f,y

p⁽¹⁾_f,y = -p⁽²⁾_f,y

x-direction:

p⁽¹⁾_i,x + p⁽²⁾_i,x= p⁽¹⁾_f,x + p⁽²⁾_f,x

 

[godkills]

Yup that's what I did. I should really start explaining what happens in the problems before solving problems.

Anyway, Thanks a lot!