Anthroxy
Question

Alex i sitting on a motorcycle, the wheel goes makes 95 turns per minute. The distance between center of the wheel to the wheels end is 0,25m (which gives us the radius) but is affected by a breaking force of 28N, how big is the work of friktion during the 45min? and what is the effect during this time?

My attempt to a solution

Since we have the radius of the wheel we can find out the wheels perimeter combined with the amount turns per minute and the time.

Perimiter of a cirlcle is = π2r
so, π2(0,25) = 1,57m now it does 1,57m ninetyfive times per minute thus 95 * 1,57 = 149,15m which is the distance covered per minute so 149,15 * 45 = 6711,75m which is the distance covered over 45min

S = 6711,75m

since we now have the distance we can get the velocity using v = d/t (and i converted 45min in to seconds) therefore v = 6711,75/2700 hence we get v = 2,48m/s

But this is where i get stuck... =/
boit
Work done is given by Force x Distance. How far does the motorcycle travel in forty five minutes? Distance is given by Time x Speed. By the way, you got speed quite early on in this exercise. It was given by 2Πr x 95. That's its speed per minute. Not necessary to convert to seconds if all we wanted was distance.
Anthroxy
QUOTE (boit+Dec 2 2010, 06:41 PM)
Work done is given by Force x Distance. How far does the motorcycle travel in forty five minutes? Distance is given by Time x Speed. By the way, you got speed quite early on in this exercise. It was given by 2Πr x 95. That's its speed per minute. Not necessary to convert to seconds if all we wanted was distance.

Yeah i just noticed that the velocity wasn't that relevant. But the problem lies at deciding the amount of friction during the 45min because when i have that i should just be able to add the friction force with the breaking force then use W = F * s to get the work and then use P = W/t
boit
Force = 28N and distance travelled is 6715. The work done goes into producing heat and the motorcycle is moving slower in the process. You didn't state in the OP that you needed to calculate power. It is a paltry 69.6J anyway.
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