Collision of cars - calculate mass

[BobbyTee]

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?

 

[Doc Al]

Show what you did. What's your equation for conservation of momentum?

 

[BobbyTee]

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.

 

[Doc Al]

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"

 

[BobbyTee]

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?

 

[Doc Al]

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.

 

[BobbyTee]

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 = -?

 

[Doc Al]

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.)

 

[BobbyTee]

Thanks a lot, it worked.