Conservation of Momentum in a Girl and Plank System

[ngorecki]

conservation of momentum??

Homework Statement

A 45.0 kg girl is standing on a 156 kg plank. The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless surface. The girl begins to walk along the plank at a constant velocity of 1.51 m/s relative to the plank.

(a) What is her velocity relative to the ice surface?
(b) What is the velocity of the plank relative to the ice surface?

variable:
girl=45kg
plank=156kg
Vi=0m/s
Vf=1.51m/s

Homework Equations

mavai + mbvbi = mavaf +mbvbf

The Attempt at a Solution

since Vi=0m/s left side of consercation of momentum equation =0
0 = 45(Vf) + 156(1.51)
0 = 45(Vf) + 235.56
-235.56 = 45(Vf)
-5.23 = Vf

this answer is not possible because the absolute value of part a and b should add up to 1.51 while 5.23 is already much bigger

 

[ngorecki]

sorry for putting the blank template 2X below...didnt see that before posting

 

[haruspex]

[
A 45.0 kg girl is standing on a 156 kg plank. The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless surface. The girl begins to walk along the plank at a constant velocity of 1.51 m/s relative to the plank.

(a) What is her velocity relative to the ice surface?
(b) What is the velocity of the plank relative to the ice surface?

variable:
girl=45kg
plank=156kg
Vi=0m/s
Vf=1.51m/s

What is your Vf here? How about putting in some variables for speeds of girl and plank relative to the ice?

 

[ngorecki]

What is your Vf here? How about putting in some variables for speeds of girl and plank relative to the ice?

Vf = 1.51 m/s

and the question is asking what the speeds are... Which I am struggling to solve. How do you suggest I go about soving for the speeds?

 

[haruspex]

Vf = 1.51 m/s

No, I mean what does it represent?

and the question is asking what the speeds are... Which I am struggling to solve. How do you suggest I go about soving for the speeds?

So put in variables to represent them.

 

[ngorecki]

Vf represents the total velocity at the end. The velocity relative to the ice for the girl and the plank added together should equal the Vf (1.51)

 

[haruspex]

Vf represents the total velocity at the end. The velocity relative to the ice for the girl and the plank added together should equal the Vf (1.51)

That isn't what you are given: "The girl begins to walk along the plank at a constant velocity of 1.51 m/s relative to the plank."
Define unknowns for the various velocities relative to the ice and see what equations you can write using them.