Collision of cars - calculate mass

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Homework Help Overview

The problem involves a collision between two cars, a Civic and a Camry, where the Civic's mass is unknown. The Civic is moving East while the Camry is moving West, and after the collision, they move together in the West direction. The goal is to determine the mass of the Civic using the conservation of momentum principle.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the application of the conservation of momentum equation, with some expressing confusion about the setup and calculations. There are questions regarding the correct use of signs for direction in the momentum equation.

Discussion Status

Some participants have provided guidance on the importance of including direction in the calculations. There is an ongoing exploration of the correct application of the momentum formula, with varying results being shared. The discussion reflects a collaborative effort to clarify misunderstandings and refine the approach to the problem.

Contextual Notes

Participants mention the urgency of understanding the topic due to an upcoming test, indicating a time constraint that may affect the depth of exploration. There is also a reference to the nature of the collision being inelastic, which is relevant to the problem context.

BobbyTee
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Homework Statement


A Civic moving East at 21.48 m/s hits a 1386.25 kg Camry moving West at 32.07 m/s. After the collision, they stick together and move West at 8.38 m/s. What is the mass of the Civic?


Homework Equations


M1 x V1 + (M2+X)V2


The Attempt at a Solution


I feel like I'm doing something wrong, I didn't really understand my teacher.

But after plugging it all in, I got 1139 Kg.

Can anybody help me out?
 
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Show what you did. What's your equation for conservation of momentum?
 


Doc Al said:
Show what you did. What's your equation for conservation of momentum?

That's the whole problem, I don't know what I'm doing, which is why I came to these forums. I was expecting someone to lead me how to get the answer. Which equation to use.. etc.. I have a test tomorrow and I don't understand this.
 


The way we work on this forum is that you give it a shot, showing what you did and where you got stuck. And we then help you out. (Trust me, you'll get much more out if it that way.)

This is an example of an inelastic collision. Look it up in your text. You may find this helpful: https://www.physicsforums.com/showpost.php?p=2157983&postcount=5"
 
Last edited by a moderator:


Doc Al said:
The way we work on this forum is that you give it a shot, showing what you did and where you got stuck. And we then help you out. (Trust me, you'll get much more out if it that way.)

This is an example of an inelastic collision. Look it up in your text. You may find this helpful: https://www.physicsforums.com/showpost.php?p=2157983&postcount=5"

Ok so I used the M1xV1 + M2xV2 = (M1+M2)V Formula.

21.48x + (1386.25x32.07) = 8.38(1386.25+x)

Ended up with 2506.89 Kg this time. Is this right?
 
Last edited by a moderator:


BobbyTee said:
Ok so I used the M1xV1 + M2xV2 = (M1+M2)V Formula.
Good! That's the right formula.

21.48x + (1386.25x32.07) = 8.38(1386.25+x)
What you're doing wrong is not taking direction into account. Momentum and velocity are vectors, so direction counts. The way you include direction is by using an appropriate sign for the velocities. For example: Use + to represent East and - to represent West.
 


Doc Al said:
Good! That's the right formula.


What you're doing wrong is not taking direction into account. Momentum and velocity are vectors, so direction counts. The way you include direction is by using an appropriate sign for the velocities. For example: Use + to represent East and - to represent West.

Ok, did it again with the correct signs and I got 1099.81 Kg. Correct?

And what if the direction is north or south?

North = + and South = -?
 


BobbyTee said:
Ok, did it again with the correct signs and I got 1099.81 Kg. Correct?
Good!

And what if the direction is north or south?

North = + and South = -?
Exactly.

(It doesn't really matter which sign you use for what as long as you are consistent.)
 


Thanks a lot, it worked.
 

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