What is the direction of the velocity/ elastic collisions.

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Homework Help Overview

The discussion revolves around a two-dimensional collision involving momentum conservation. Participants are analyzing the effects of mass on the direction of velocity after the collision, specifically questioning how the momentum vectors behave in both the x and y directions.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants explore the implications of momentum conservation in a two-dimensional context, questioning the consistency of their answers with the principles of momentum conservation. They discuss the need to consider both x and y components of momentum and express uncertainty about their interpretations.

Discussion Status

There is an ongoing examination of the momentum vectors before and after the collision, with some participants making progress in understanding the relationship between the vectors. Guidance has been provided regarding the need for consistency in the direction and magnitude of momentum vectors, prompting further reflection on their responses.

Contextual Notes

Participants are working within the constraints of a homework assignment that requires them to fill in specific blanks regarding momentum directions without writing full equations. There is a focus on systematic reasoning and the exclusion of incorrect choices.

YMMMA
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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.
 

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YMMMA said:

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?
 
kuruman said:
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.
 
YMMMA said:
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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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.
 
kuruman said:
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..
 
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?
 
kuruman said:
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?
 
  • #10
YMMMA said:
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,
kuruman said:
The total momentum is the vector sum of the momenta of the two balls.
 
  • #11
kuruman said:
(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.
 
  • #12
kuruman said:
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
 
  • #13
OK. I think you got it.
 
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  • #14
kuruman said:
OK. I think you got it.
Thank you for your help and time!:smile:
 

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