Work done by weight of a block?

[karkatvantas]

Ok the problem is from ExamKrackers 1001 physics, #361:

There's an image of a 4 kg block suspended from a rope with a tention T pointing upwards. The question is:

The block is lowered by a rope as shown. The tension T in the rope is 35 N. If the block is lowered 10 m, how much work is done by the weight of the block?

A. 50 J
B. 175 J
C. 350 J
D. 400 J​

They say the answer is 400 J (4 kg x 10 m/s x 10 m). My question is, why?

The way I was looking at it is: the net force on the block is 5 N.

W = Fd

W = 5N x 10 m = 50 J, right?

Or are they saying that gravity does 400 J on the block, but the rope does -350 J on the block?

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[movax]

Ok the problem is from ExamKrackers 1001 physics, #361:

There's an image of a 4 kg block suspended from a rope with a tention T pointing upwards. The question is:

The block is lowered by a rope as shown. The tension T in the rope is 35 N. If the block is lowered 10 m, how much work is done by the weight of the block?

A. 50 J
B. 175 J
C. 350 J
D. 400 J​

So yeah, net force on the block is 5N, W = FdcosTheta. But we want to know how much work the weight does. So F = 4 kg * 10m/s^2 = 40N, d = 10m, W = 400J.

I think your answer would be correct if we were looking for some kind of net work done, maybe?

 

[90210]

There are two forces acting on the block:

- Tension (pulling upwards)
- Gravity or Weight (pulling downwards)

The block is lowered 10 meters. As it's lowered, both of the forces above do some work:

Tension = 35N (upwards direction). It does (-35N) x (10 meters) = -350 J of work
Gravity = 40N (downwards direction). It does (40N) x (10 meters) = 400 J of work

Note: I called the upwards direction negative.

The work done BY gravity (weight) is 400 J.
The work done BY tension is -350 J.
The net work down ON the block is 50 J (a positive value).

 

[BestDoctorEver]

Ok the problem is from ExamKrackers 1001 physics, #361:

There's an image of a 4 kg block suspended from a rope with a tention T pointing upwards. The question is:

The block is lowered by a rope as shown. The tension T in the rope is 35 N. If the block is lowered 10 m, how much work is done by the weight of the block?

A. 50 J
B. 175 J
C. 350 J
D. 400 J
​

They say the answer is 400 J (4 kg x 10 m/s x 10 m). My question is, why?

The way I was looking at it is: the net force on the block is 5 N.

W = Fd

W = 5N x 10 m = 50 J, right?

Or are they saying that gravity does 400 J on the block, but the rope does -350 J on the block?

They are only asking for the work done by the weight, which is the same as asking for the work done by gravity (not the work done the block).
W = mgdcos0 = 4 x 10 x 10 = 400 J