Lose weight (mass) or train? - effect of losing weight (mass) on power

[Byrne]

Lose weight (mass) or train? -- effect of losing weight (mass) on power

I'm having troubles wrapping my head around this one (I've provided the example from my class notes below):

Q: Is it better to lose 5 kg of body mass (assume no change in muscle strength) or to train harder and gain 15 W of power?

Given:
75 kg rider producing 250 W of power

ANSWER:

75 kg / 70 kg = 1.071
In terms of Watts: 250 W x 1.071 = 268 W, ~ 18 W increase

So, depending on the initial weight, the athlete might see as large a benefit from losing weight as training harder.

My question -- how does a decrease in the mass of the rider increase his power producing ability by 18 W? Intuitively, I would think that a rider with lower mass will experience less resistive forces when riding (i.e., gravity when on an incline, kinetic friction) and so can sustain the same velocity as a heavier rider without applying as much force and, therefore, less power.

From a physics standpoint, how does a mass decrease result in an increase in Power as this example suggests (taken from my class notes)?

Thanks!

 

[LowlyPion]

If it was strictly moving at constant velocity and his air resistance profile doesn't change, it might not.

But when you take into account changes of height - like a hill - where effort is going into increasing gravitational potential, then he would benefit from the lower mass, as that would be less work. Or any starts and stops where he would be accelerating.

The lecturer was merely trying to get you to think about the consequences of getting heavier. The harder training and weight gain would need to develop an additional amount of power to be worth it I think is the point.