Answers for Problem Set 7

Chapter 7

24.

(a) W_helicopter = 1.1 m g d = 1.2 x 10⁴ J

(b) W_gravity = - m g d = - 1.1 x 10⁴ J

(c) KE = 0.1 m g d = 1 x 10³ J

(d) v = sqrt ( 0.2 g d ) = 5.4 m / s

27.

2.5 blocks giving 25 J

29.

(a) There should be 12 blocks. My closest estimate was 11 3/4.

(b) W = [ 1 a / x ]₁³ = 6 J

31.

W = [ x² ]₂⁴ + [ 3 y ]₃^-3 = - 6 J

40.

(a) W_g = m g (delta L) = 0.29 joule
(b) W_k = - (1/2) k (delta L)² = - 1.8 joule
(c) v = sqrt { (delta L) * [ - 2 m g + k (delta L) ] / m } = 3.47 m/s
(d) delta L = ( -1 / k) { - m g - sqrt [ m ( m g ² + 4 k v² ) ] } = 23 cm

50.

P₂ = 9 P₁ = 90 hp