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A rope, attached to a weight, goes up through a pulley at the ceiling and back down to a worker. The worker holds the rope at the same height as the connection point between the rope and weight. The distance from the connection point to the ceiling is 40 ft. Suppose the worker stands directly next to the weight (i.e., a total rope length of 80 ft) and begins to walk away at a constant rate of 3 ft/s. How fast is the weight rising when the worker has walked:

Respuesta :

Complete question is;

A rope, attached to a weight, goes up through a pulley at the ceiling and back down to a worker. The worker holds the rope at the same height as the connection point between the rope and weight. The distance from the connection point to the ceiling is 40 ft. Suppose the worker stands directly next to the weight (i.e., a total rope length of 80 ft) and begins to walk away at a constant rate of 3 ft/s. How fast is the weight rising when the worker has walked:

A) 10 feet

B) 30 feet

Answer:

A) 0.728 ft/s

B) 1.8 ft/s

Explanation:

Let the the position of the worker in ft be denoted by s.

Since he begins to walk away at a constant rate of 3 ft/s, then;

ds/dt = 3 ft/s

Now, the rope will form a triangle, with width "s" and the height 40. Since distance from the connection point to the ceiling = 40 ft

Using pythagoras theorem, we can find the length of the rope on this side of the pulley.

Hence, the length of rope on this side of the pulley = √(s² + 40²)

Meanwhile, on the other side the length will be;

(80) - √(s² + 40²)

Also, height of the weight will be;

h = 40 - ((80) - √(s² + 80²))

h = √(s² + 80²) - 40

Differentiating this, we have;

dh/dt = (ds/dt) × (s/√(s² + 40²))

From earlier, we saw that ds/dt = 3 ft/s

Thus;

dh/dt = 3s/√(s² + 40²)

A) when he has walked 10 ft, it means that s = 10. Thus;

dh/dt = (3 × 10)/√(10² + 40²)

dh/dt = 0.728 ft/s

B) when he has walked 30 ft, it means that s = 30. Thus;

dh/dt = (30 × 3)/√(30² + 40²)

dh/dt = 1.8 ft/s

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