What is her speed relative to the ice surface?

AI Thread Summary
To determine the girl's speed relative to the ice surface, the discussion highlights the need to consider both her movement on the plank and the plank's reaction. The girl walks at 1.37 m/s relative to the plank, which is initially at rest on a frictionless surface. The plank's mass is 170 kg, and the girl's mass is 44 kg, leading to a calculation of the plank's acceleration due to the force exerted by the girl. The correct approach involves using conservation of momentum rather than directly calculating force as speed times mass. Ultimately, the girl's speed relative to the ice surface can be derived by accounting for both her speed on the plank and the plank's movement.
Ry122
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A 44 kg girl is standing on a plank that has a mass of 170 kg. The plank, originally at rest, is free to slide on a frozen lake, which is a flat, frictionless supporting surface. The girl begins to walk along the plank at a constant speed of 1.37 m/s relative to the plank.
What is her speed relative to the ice surface?

My attempt:
The horizontal force the girl exerts on the plank is f=1.37(44)=60.28N
so the speed of the plank relative to the ice should be 60.28=170a a=.354ms
and her speed relative to the ice should be 1.37-.354=1.016
but this is incorrect, what am i doing wrong?
 
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Ry122 said:
The horizontal force the girl exerts on the plank is f=1.37(44)=60.28N

Here's at least one error in your thinking: How can a force be a speed times a mass?
 


where do i begin then?
 


would momentum be involved?
 

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