Physics, Physics!!!!!!!!!?

Question

A car with a mass of 1.50 x 10^3 kg starts from rest and accelerates to a speed of 18 m/s in 12 s. Assume that the force of resistance remains constant at 400 N during this time. What is the average power developed by the car's engine??



How long does it take a 19 kW steam engine to do 6.8 x 10^7 J of work??

Answer 1

The engine did work which was divided between giving the car kinetic energy and overcoming the resistance.

The engine gives the car kinetic energy of
KE = (1/2)*m*v^2

It also overcame the 400 N resistance (probably friction of various types). The work the engine did in overcoming the resistance was
Wr = 400N*d
where d is the distance traveled while getting up to 18 m/s. That distance can be found from the equation
x = (Vo + Vf)*t / 2
where Vo = 0, Vf = 18 m/s, and t = 12 s.

Therefore the total amount of work was
Wt = KE + Wr

And power is related to work by
W = P*t

For 2nd question, you will also need to know that
1 watt = 1 joule/sec

Answer 2

first find acceleration of car, a=(18-0)/12

NEXT use F=ma, u have value of m and a so can find F

resistive force is constant, so forward driving force= F + 400
so power developed is P= (F+400) x 9

for 2nd qn, Power= Work Done/time
so t = work done/power u can find time taken t