# What is the direction of the velocity/ elastic collisions.

## Homework Statement

in the attached file

## Homework Equations

Momemtum = mass - velocity

## The Attempt at a Solution

I solved it E. Since object 2 has the larger mass, the effect is less and will move backwards a little bit.

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kuruman
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## Homework Statement

in the attached file

## Homework Equations

Momemtum = mass - velocity

## The Attempt at a Solution

I solved it E. Since object 2 has the larger mass, the effect is less and will move backwards a little bit.
This is a 2d collision that conserves momentum. Make this the basis of your explanation. Is E consistent with momentum conservation?

This is a 2d collision that conserves momentum. Make this the basis of your explanation. Is E consistent with momentum conservation?
I am not sure. But no It should have y and x components , I guess.

kuruman
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I am not sure. But no It should have y and x components , I guess.
You guess correctly. The total initial momentum has x and y components, therefore the total final momentum must have x and y components. You need to reconsider your answer.

I think it is C. Object 2 is having a y component and is pushed horizontally to the first quadrant. Object 1 is having an x component and is pushed vertically to the fourth quadrant. I am a little bit unsure since I don’t know how to right the momentum in the y.

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kuruman
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The total momentum is the vector sum of the momenta of the two balls. You don't have to write equations, but you need to be systematic. Not only you have to find the right answer, but also you have to exclude the others as incorrect. Fill in the blanks.

1. The total initial momentum is ____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.

2. Therefore the total final momentum must be
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.

3. The choice that is consistent with (2) is _________ .

4a. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.
4b. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.
4c. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.
4d. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.

The total momentum is the vector sum of the momenta of the two balls. You don't have to write equations, but you need to be systematic. Not only you have to find the right answer, but also you have to exclude the others as incorrect. Fill in the blanks.

1. The total initial momentum is ____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.

2. Therefore the total final momentum must be
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.

3. The choice that is consistent with (2) is ____C_____ .

4a. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.
4b. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.
4c. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.
4d. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.

That was hard, but helpful. Although I am still not sure. Can’t proceed before making sure I am correct, because basically they are all the same and the rest of the choices..

kuruman
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2021 Award
You are making progress. This exercise should be an eye-opener for you because it seems you have not grasped what momentum conservation means. It means that the total momentum vector must be the same before and after the collision. Two vectors are the same if and only if they have the same magnitude and point in the same direction. In (1) you say that the total momentum before the collision points to the left (negative) and up (positive). In (2) you say that the total momentum after the collision points to the right (positive) and down (negative). These two do not point in the same direction, therefore they are not the same before and after the collision, therefore your answers in (1) and (2) are inconsistent with momentum conservation. You need to reconsider. Hint: One of the answers, (1) or (2), is correct but the other is not. Which one?

You are making progress. This exercise should be an eye-opener for you because it seems you have not grasped what momentum conservation means. It means that the total momentum vector must be the same before and after the collision. Two vectors are the same if and only if they have the same magnitude and point in the same direction. In (1) you say that the total momentum before the collision points to the left (negative) and up (positive). In (2) you say that the total momentum after the collision points to the right (positive) and down (negative). These two do not point in the same direction, therefore they are not the same before and after the collision, therefore your answers in (1) and (2) are inconsistent with momentum conservation. You need to reconsider. Hint: One of the answers, (1) or (2), is correct but the other is not. Which one?
I think it is 2. Is it like vector addition?

kuruman
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Gold Member
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I think it is 2. Is it like vector addition?
(2) is what? Correct or incorrect? Yes, it is exactly like vector addition. As I indicated in post #6,
The total momentum is the vector sum of the momenta of the two balls.

(2) is what? Correct or incorrect? Yes, it is exactly like vector addition. As I indicated in post #6,
2 is the right one. Yes, sorry did not concentrate.

The total momentum is the vector sum of the momenta of the two balls. You don't have to write equations, but you need to be systematic. Not only you have to find the right answer, but also you have to exclude the others as incorrect. Fill in the blanks.

1. The total initial momentum is ____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.

2. Therefore the total final momentum must be
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.

3. The choice that is consistent with (2) is ____C_____ .

4a. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.
4b. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.
4c. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.
4d. Choice _____ is inconsistent because the total final momentum is supposedly
____________ (positive/negative/zero) in the x-direction and ____________ (positive/negative/zero) in the y-direction.
All blanks are the same as 2. Choices ABDE except C

kuruman