Question

A heavy rope, 50 ft long, weighs 0.6 lb/ft and hangs over the edge of a building 120 ft high. Approximate the required work by a Riemann sum, then express the work as an integral and evaluate it.

Exercise (a)

How much work W is done in pulling the rope to the top of the building?

Exercise (b)

How much work W is done in pulling half the rope to the top of the building?

Answer 1

Exercise (a)

The work done in pulling the rope to the top of the building is 750 lb·ft

Exercise (b)

The work done in pulling half the rope to the top of the building is 562.5 lb·ft

Explanation:

Exercise (a)

The given parameters of the rope are;

The length of the rope = 50 ft.

The weight of the rope = 0.6 lb/ft.

The height of the building = 120 ft.

We have;

The work done in pulling a piece of the upper portion, ΔW₁ is given as follows;

ΔW₁ = 0.6Δx·x

The work done for the second half, ΔW₂, is given as follows;

ΔW₂ = 0.6Δx·x + 25×0.6 × 25 = 0.6Δx·x + 375

The total work done, W = W₁ + W₂ = 0.6Δx·x + 0.6Δx·x + 375

∴ We have;

W =
`2 * \int\limits^(25)_0 {0.6 \cdot x} \, dx + 375= 2 * \left[0.6 \cdot (x^2)/(2) \right]^(25)_0 + 375 = 750`

The work done in pulling the rope to the top of the building, W = 750 lb·ft

Exercise (b)

The work done in pulling half the rope is given by W₂ as follows;

`W_2 = \int\limits^(25)_0 {0.6 \cdot x} \, dx + 375= \left[0.6 \cdot (x^2)/(2) \right]^(25)_0 + 375 = 562.5`

The work done in pulling half the rope, W₂ = 562.5 lb·ft