New velocity between two spacecraft moving in opposing directions

[Capncanada]

Homework Statement

A 60-tonne (1 t = 1000 kg) spacecraft moving in the + x-direction at 0.70 m/s docks with a 65-tonne craft moving in the -x-direction at 0.64 m/s

Find the velocity of the joined spacecraft

Homework Equations

Conservationg of kinetic energy?

KE_i=KE_f

The Attempt at a Solution

I don't know if this is silly, but I tried solving for the velocity using kinetic energies of the separate space ships equalling the joined space ships:

1/2mv_1^2+1/2mv_2^2=1/2mv_3^2
1/2(60000kg)(0.70m/s)^2+1/2(65000kg)(0.64m/s)^2=(125000)v_3^2
14700+13312=62500v_3^2
v_3^2=0.4482
v_3=0.6695m/s

 

[lightgrav]

Which direction do you think that velocity would be?

no, they "lock them together" before they can bounce apart.
what's conserved here is total momentum (vector).

 

[Capncanada]

Okay, so I can use P=mv for each respective part?

 

[Capncanada]

The velocity would be in the direction the heavier ship is going

 

[lightgrav]

the heavier ship isn't going as fast ... just add them up, being careful with +/- signs.

 

[Capncanada]

Okay so the lighter ship is going faster but the larger ship is going slower, I don't understand what equation I am supposed to use to find the velocity of the combined*

 

[lightgrav]

you compute each one's momentum, then you add them.

Total momentum is the same after connecting as it was before connecting.

 

[Capncanada]

So I'll be subtracting the second ships momentum* since it's moving in the opposite direction, correct?