Average Power delivered to Engine

AI Thread Summary
To calculate the average power delivered by the engine of a 1000 kg car accelerating to 15 m/s in ten seconds, the average power formula is used: average power equals work divided by time. The total force acting on the car includes the force needed for acceleration and the opposing air resistance of 300 N. The calculated force is derived from the mass and acceleration, with the correct application of units being crucial. After determining the work done by the engine and factoring in the air resistance, the average power is found to be 13500 W. The discussion emphasizes the importance of correctly calculating force and understanding the impact of air resistance on power output.
davidatwayne
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Homework Statement


A 1000 kg car, starting from rest, accelerates uniformly to 15 m/s in ten seconds. Assume air resistance is constant at 300N. What is the average power delivered by the engine?

Answers
a. 13500 W
b.18000 W
c. 23000 W
d. 27000 W

Homework Equations


Average power = work/time
work = force(x) where x is magnitude of displacement.
force= mass x acceleration


The Attempt at a Solution



the force is equal to 1000 kg x 15 m/s = 15000

I don't know how to factor in air resistance, or arrive at a final answer. The answer is A...
 
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explain how you got the force of the car again...
 
force = mass times acceleration
1000 x 15= 15000
 
what is the units of the 15 value though
 
ok... i think i got it... after getting the correct force (and units) times the displacement of x, (1800 * 75)/10... gives the correct answer.

thanks.
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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