Centre of mass of velocity (q61)

[jack1234]

Hi, for the following problem
http://tinyurl.com/249gz8
the answer is d

What i have done is
Centre of mass of velocity
=(m1*v1+m2*v2)/(m1+m2)

but I am not sure how does it leads to the result d...
can anybody help on this?

 

[Doc Al]

It might be easier to view this conceptually. What if both masses had a speed of v1? Then the speed of the center of mass would be v1, right? But some of the total mass (m2) is moving faster, so what must happen to the speed of the center of mass?

And it works the other way as well. What if both masses had a speed of v2. Then the speed of the center of mass would be v2? But some of the total mass (m1) is moving slower, so what must happen to the speed of the center of mass?

 

[jack1234]

I see, thanks for the help :)