Question

A 42.0-kg projectile is fired at an angle of 30.0° above the horizontal with an initial speed of 134 m/s from the top of a cliff 134 m above level ground, where the ground is taken to be y = 0.

(a) What is the initial total mechanical energy of the projectile? (Give your answer to at least three significant figures.)

432000 Correct: Your answer is correct. J

(b) Suppose the projectile is traveling 95.0 m/s at its maximum height of y = 321 m. How much work has been done on the projectile by air friction?

-110351 Correct: Your answer is correct. J

(c) What is the speed of the projectile immediately before it hits the ground if air friction does one and a half times as much work on the projectile when it is going down as it did when it was going up?

I cannot figure out part c. I tried doing total W = 2.5 W = 2.5 (-110351) = -275878 J

Then I used W = KEf - KE0 and I got 183.6, but it's not correct.

Answer 1

I appreciate that you understand and did the initial part correct but in third part , it’s difference of total energy at final and initially position that is equivalent to work done by friction . Also , the sign associated with work done by friction is negative which needed to be taken care of .