1. The problem statement, all variables and given/known data A tractor moves, at constant velocity, on a flat surface with known friction coefficient μ (0.24), ten objects each of which has known mass m (12 kg). It does so for a known distance d (500 m), and it takes known time t (150s) to do so. Calculate the Power of the tractor. 2. Relevant equations v = s/t P = W/t W = F*distance Kinetic energy = (0.5)*m*v2 F = m*a Friction force = Normal force * μ 3. The attempt at a solution I managed to get to the solution proposed by the workbook, but I had to do random guesswork, and I'm not convinced/still have some doubts. The way it's supposed to be calculated, apparently, is like this: Friction Force = m*g*μ Here m = 10*12, so: Friction Force = 10*12*9.81*0.24 = 282.5 N I'm kinda lost at this passage, because apparently the Friction Force equals the force of the tractor, at least in module...? If that were the case, then Force of the tractor is: F_tractor = 282.5 N Then W = F*distance W = 282.5 * 500 = 141 kJ P = 141K / 150 = 0.94 kW This is the correct result. However, before finding what seems to be the correct way to proceed, I previously attempted to proceed like this: Since velocity is constant, and it takes 150 seconds to do the 500 meters, then v = s/t, so v = 500/150 = 3.3 m/s Then I tried using the formula of kinetic energy, since Work = Kinetic energy (or at least it seemed to be that way in previous problems). Is this a mistake? Because I feel like it is (if anything because the numbers are waaay too small), but I can't understand why. K = (0.5) * 120 * (3.32) = 653.4 J Then I thought that the calculated energy was the result of all the forces that act on the mass(es), so I wrote down W = F*s W = K 653.4 = F_all * 500 => F_all = 1.3 N Then I thought: F_all = Friction force (negative) + Tractor force (positive) => Tractor force = 1.3 + 282.5 = 283.8 N The numbers are so close that mere approximation can still make the 0.94 kW result come out in the end, but a net force of 1.3 N is nothing like what I'd expect from real-life experience, so... Anyway, I don't understand why in this context we can't say that Work = Kinetic energy = 0.5 * mass * velocity2. Is it because velocity is constant, so there is no acceleration? If not, then why doesn't that (Work = Kinetic energy) apply here (whereas I'm sure it applies in other situations; I've solved more than one problem by putting together K = 0.5*m*v2, W = F*distance, and W = K_end - K_start) ? There's definitely something wrong here, but I don't know what. Also, am I to understand that when there is no acceleration, the force needed to move a body (at constant velocity) is equal to the Friction force? I understand the concept that "when the sum of all the forces applied on a body is null, the body moves at constant velocity", which would confirm that the force of the tractor = -Friction force here...but what I don't understand is, how does the tractor move in the first place? Shouldn't there be a starting force that sets things in motion, gives that starting speed of 3.3 m/s, and THEN the tractor force, by equalizing the Friction force, makes sure that velocity remains constant? The questions in bold are the most important ones (the only ones really). Thanks in advance.