Problem 43P

If wind blows at 35 m/s over a house, what is the net force on the roof if its area is 240 m2 and is flat?

ANSWER:The initial velocity of the car is vi= 0 and final velocity is v =f30 m/s.The time difference to achieve the speed is, t = 10 s.So, the acceleration is, v v a = f i = 300 = 30 / 10 = 3 m/s . 2 t 10Mass of the car is 1000 kg.From Newton’s 2nd law, the acceleration is, F = ma = 1000 × 3 = 3000N.By using the equation of motion let’s calculate the distance travelled by the car in those 10seconds. 2 2 V final V initial 2 × a × s 2 30 0 = 2 × 3 × s s = 900 / (2 × 3) = 150 meters.Now, we have both force and displacement. So, the work done will be, W = Force × displacement = 3000 × 150 = 450000 jules.All this work done in 10 seconds. So, power will be, P = W / t = 450000 / 10 = 45000 watts = 45 K.W .CONCLUSION:The average power of the car is 45 KW.\n