# Perfect inelastic collison

## Homework Statement

A 6 kg object moving with a speed of 9.9 m/s collides with a 19 kg object moving with a velocity of 7.2 m/s in a direction 24* from the initial direction of motion of the 6 kg object. What is speed of the two objects after the collision if they remain stuck together?

## Homework Equations

Px = m1v1icos$$\theta$$+m2v2icos$$\theta$$ = m1v1fcos$$\theta$$ + m2v2fcos$$\theta$$

Py = m1v1isin$$\theta$$+m2v2isin$$\theta$$ = m1v1fsin$$\theta$$ + m2v2fsin$$\theta$$

## The Attempt at a Solution

Edit I realized my original way was faulty in its logic.

So what I did was I began by finding the initial and final momenta components in both the x and y directions for the two particles. Being that they stick together their final velocity ought to be the same.

$$\Sigma$$Pxi = m1v1cos$$\theta$$ + m2v2cos$$\theta$$

=184.37302

$$\Sigma$$Pyi = m1v1sin$$\theta$$ + m2v2sin$$\theta$$

=55.641573

As I mentioned their final velocity should be equal since they are stuck together. Equating my initial momenta to my final momenta in the x direction I get the following:

Vf = $$\frac{(m1v1cos\theta + m2v2cos\theta}{(m1+m2)cos\theta}$$

=8.072856 m/s

When I checked it using the momenta of the y direction I come to a different value for the final velocity which doesn't make sense so I know I made a mistake somewhere... Thanks in advance.

Joe

Edit: I tried a new way, but

Last edited:

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Doc Al
Mentor
Looks good to me.

Thanks Doc Al, are you refering to the first way I solved the problem or refering to this edited version? The first way did not work, so thats why I tried a new method. Thanks for all your help in all of my posts.

Joe

vela
Staff Emeritus
Homework Helper
Actually, your first attempt was the correct solution. How do you know the answer you got is wrong?

Doc Al
Mentor
Thanks Doc Al, are you refering to the first way I solved the problem or refering to this edited version? The first way did not work, so thats why I tried a new method.
I was referring to your original post, which looked fine to me. (I haven't looked at your revision.)

It is an online homework module, and this is not the first question that I have been correct about and it kicked back my answer as incorrect...Thanks for your assistance.

Joe

vela
Staff Emeritus
Homework Helper
Thought so. Could be you're entering too many significant figures.

Thought so. Could be you're entering too many significant figures.
Do you know the direction (angle) after collision?

Doc Al
Mentor
Do you know the direction (angle) after collision?
Why would that matter?

Why would that matter?
I mean after collision, two objects stuck together. So velocities are the same but problem didn't mention direction is angle theta. I think angles are not the same before and after collision.

Doc Al
Mentor
I mean after collision, two objects stuck together. So velocities are the same but problem didn't mention direction is angle theta. I think angles are not the same before and after collision.
True, but the direction of the final velocity doesn't matter for this problem. All they ask for is the speed.