Conservation of Momentum 2

[norcal]

Homework Statement

"Relative" is an important word. Block L of mass mL = 1.90 kg and block R of mass mR = 0.500 kg are held in place with a compressed spring between them. When the blocks are released, the spring sends them sliding across a frictionless floor. (The spring has negligible mass and falls to the floor after the blocks leave it.)

(a) If the spring gives block L a release speed of 1.80 m/s relative to the floor, how far does block R travel in the next 0.800 s?
correct check mark m
(b) If, instead, the spring gives block L a release speed of 1.80 m/s relative to the velocity that the spring gives block R, how far does block R travel in the next 0.800 s?

Homework Equations

m1v1=m2v2

v=d/t

The Attempt at a Solution

1.9*1.8=.5*velocityR
velocityR = 6.84 m/s

d=v*time
d=(6.84)(.8)
d=5.472m

Ok, so 5.472m is the correct answer for part A but I don't know how to get part B. Any suggestions?

 

[PhanthomJay]

Homework Statement

"Relative" is an important word. Block L of mass mL = 1.90 kg and block R of mass mR = 0.500 kg are held in place with a compressed spring between them. When the blocks are released, the spring sends them sliding across a frictionless floor. (The spring has negligible mass and falls to the floor after the blocks leave it.)

(a) If the spring gives block L a release speed of 1.80 m/s relative to the floor, how far does block R travel in the next 0.800 s?
correct check mark m
(b) If, instead, the spring gives block L a release speed of 1.80 m/s relative to the velocity that the spring gives block R, how far does block R travel in the next 0.800 s?

Homework Equations

m1v1=m2v2

v=d/t

The Attempt at a Solution

1.9*1.8=.5*velocityR
velocityR = 6.84 m/s

d=v*time
d=(6.84)(.8)
d=5.472m

Ok, so 5.472m is the correct answer for part A but I don't know how to get part B. Any suggestions?

You can use the same equation as in (a), realizing that if m1 moves left, m2 must move right. But you have 2 unknowns, v1 and v2, so you need a 2nd equation that relates v1 with v2. What is that relationship?