I have a bicycle weighing 8 kilos. An 10 kilometer ride averaging 20 km/h requires
z kWh's of energy.
How much more energy would I spend if I added 2 kilos to my backpack?
Or turning it around: If I used the same amount of energy, how much slower would I get?
UPDATE: I'm asking out of curiosity, not out of some homework assignment. It's a long time since I've had those :)
I have friends arguing that a 2 kg lighter bike (e.g. a racer instead of a cyclo cross) will give me serious advantages because of the decreased weight, even though I'm an amateur when it comes to bicycling. Me, on the other hand, can't really see why this would matter all that much until I get seriously more fit. Anyway, I'm not familiar with the physics needed to calculate this.
If it helps, let's assume a 5 degree rise from 0-5 km in 10 km/h and a 5 degree descent from 5-10 km in 40 km/h.