How Does a Skier's Energy Change Through Different Stages?

[katj]

"A skier (total mass, ski included) catches a ride on a life to the top of a mountain (Stage 1 to stage 2). Once at the top, the skier heads down the slope, reaches the base of the mountain (Stage 3), and coasts to a stop (Stage 4).

Stage 1: Work done on the skier by the chairlift.
Stage 2: Potential Energy of the skier at the mountaintop.
Stage 3: Kinetic energy of the skier at the bottom of the mountain.
Stage 4: Work done on the skier to come to a stop.

Us this scenario to describe the signs (positive or negative) of the energy in each of these stages of the scenario.

A. Positive, Positive, Positive, Positive
B. Positive, Positive, Positive, and Negative
C. Positive, Positive, Negative, Positive
D. Negative, Positive, Positive, Negative
E. Negative, Positive, Positive, Positive

I am thinking that the answer should E, but I am not sure. It takes energy to do work, so stages 1 and 4 would be negative (I guess?). Stages 2 and 3 describe energy that the skier HAS, so they would be positive. I think!

 

[Doc Al]

It takes energy to do work, so stages 1 and 4 would be negative (I guess?).

I don't understand your reasoning here. Hint: If the energy has increased, the work done was positive.

Stages 2 and 3 describe energy that the skier HAS, so they would be positive.

Makes sense.

 

[tiny-tim]

welcome to pf!

hi katj! welcome to pf!

work done is force times displacement (technically, force "dot" the displacement of the point of application of the force), so it'll be positive if the force is in the same direction as the displacement, and negative if it's in the opposite direction

and potential energy is defined as minus the work done (by a conservative force, such as gravity)

does that help? ​

 

[katj]

Let me try again! Does this make more sense?

Stage 1- positive because the force is in the same direction as displacement
Stage 2- positive because the skier would have lots of potential energy at the top
Stage 3- positive because the skier would have lots of kinetic energy at the bottom
Stage 4- negative because the displacement is forward and the force (of friction) is going back

PS- Thanks for the help!

 

[Doc Al]

Good!

 

[katj]

Yay! Thanks for all of the help! This makes sense now!