Answer on Mechanics | Relativity Question for Jessica
a = V/t.
Using the Newton's Second Law:
F - Fres = ma,
where Fres is the force of
resistance (Fres = 400 N),
F - force of engine. So, F = Fres + ma.
power can be found from the equations for total engine's work:
A = F*S = P*t,
P = F*S/t.
S can be found from the equation of displacement:
So, P = (Fres + m*V/t)*(V/t*t^2)/(2t) = (Fres + m*V/t)*V/2 =
(400+1500*18/12)*18/2 = 23850 W.
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It is negative, the sign here means direction. We have F-Fres=ma, so F and Fres have opposite directions, F is collinear with direction of the car motion, Fres is opposite to it. The magnitude of Fres is positive, that's right, but when construction equations we keep in mind that it's directed opposite to the motion of the car.
But the friction force is supposed to be negative