Two skater's momentum after collision

[teggenspiller]

Homework Statement

Daisy (50-kg mass) skates on ice at 4.0 m/s to greet her friend (70-kg mass), who is standing still, with open arms. As they collide, while holding each other, with what speed do they both move off together?

A. Zero
B. 1.7 m/s
C. 2.5 m/s
D. 5.0 m/s

Homework Equations

mi(vf-v1)=m2(vf-v1)
?

The Attempt at a Solution

50kg(vf-4m/s)=70kg(vf-0m/s)

I find that if I try to solve for vf, the other vf cancels it out, so I know i am not doing something right. Can anyone help?

 

[doublea500]

i believe you have your formula wrong

p1=p2.
M1V1i+M2V2i=Vf(M1+M2)

 

[chewytess]

Momentum must be conserved. For part a pi=pf, where pi=m1v1+m2v2. You know the collision is ineslatic so the 2 gals move off together so pf=vf(m1+m2) where vf is your unknown. Solve for vf. I get 1.66 m/s

 

[teggenspiller]

that isn't even one of the options.

 

[teggenspiller]

i am not given the velocity of the skater at anytime, i have no clue how to do this

 

[teggenspiller]

oh hey, I am sorry, you are right, i was looking at the wrong problem when i wrote that. 1.67 is correct

 

[chewytess]

Yes, but rounded up it is 1.7 m/s so b is the answer. Good luck!