Non conservative force of a wave on a surfer

[thatgirlyouknow]

Homework Statement

A surfer is catching a wave. Suppose she starts at the top of the wave with a speed of 1.93 m/s and moves down the wave until her speed reaches 12.3 m/s. The drop in her vertical height is 2.95 m. If her mass is 72.3 kg, how much work is done by the (non-conservative) force of the wave?

Homework Equations

Wnc = Ef-E0
Wnc = .5mvf^2 + mghf - (.5mv0^2 + mgh0)

The Attempt at a Solution

Wnc = .5*72.3*12.3^2 + 72.3*-9.8*0 - (.5*72.3*1.93^2 + 72.3*-9.8*2.95)
.5*72.3*12.3^2 - (.5*72.3*1.93^2 + 72.3*-9.8*2.95)
5469.1335 - (134.66 + (-2090.19))
= 7424.66 J

This should become negative because the non conservative force is opposing displacement (since the surfer is moving down the wave). However, this answer is not correct. What should be changed?

 

[Astronuc]

Increasing velocity from 1.93 m/s to 12.3 m/s represents a change in kinetic energy of 5334.5 J

And the change in height of 2.95 m represents a change in grav. potential energy of only 2092 J.

So from where does the extra energy/work come?

 

[thatgirlyouknow]

The wave, I suppose... so do I take the total of those two and subtract the number I found?

 

[Astronuc]

Well the difference in change in kinetic energy and change in potential energy is 3242.5J and presumably that is work done by the wave.