A person uses a block-and-tackle system as shown below (from Wikimedia Commons)

Question

A person uses a block-and-tackle system as shown below (from Wikimedia Commons) to lift a heavy load.

Can you solve this problem for me please?

Suppose the load has a weight of 1250 N (a) Suppose our person lifts this load two meters slowly (at constant velocity). What force must he exert on the rope to do so? (b) It seems like he's getting something for nothing - that he's able to lift a larger weight with a smaller force. But is he? Calculate the work done by the rope on the load, and calculate the work he does on the rope . (c) If he lifts this load 2m and then holds it there, clearly its change in kinetic energy is zero: it started at rest and ended at rest. However, the rope did positive work on the load; the work-energy theorem thus says that its kinetic energy should increase unless some other force did an equal amount of negative work on it What force was this? (d) Explain why, using the definition of workW-JF ds, that force does negative d) Explain why, using the definition of work W F·ds. that force does negative work

Explanation / Answer

ANSWER

Part a)

As W = 1250 N

Now, for moving load at constant speed,

Fnet = 0

Thus,

2T - 1250 = 0

T = 625 N

Look that the tension in the rope is 625 N. Therefore, the force exterted on the rope is 625 N.

Part b)

Work done by rope = 2 * 625 * 2

Work done = 2500 J

Work done by the rope is 2500 J.

Work done by person = work done by rope

Work done by person is 2500 J.

Part c)

This force was the gravity.

Part d)

Keep in mind that the gravity does negative work, as the force is oposite to the displacement.

Hence,

Work done = F*d*cos(180) = -ve

Regards!!!

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.