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## Physics library

### Course: Physics library>Unit 5

Lesson 3: Mechanical advantage

# Mechanical advantage (part 3)

Introduction to pulleys and wedges. Created by Sal Khan.

## Want to join the conversation?

• Isn't PE = mgh ? So isn't the PE of the block (10)(9.81)(5) = 490.5 J...not just 50J ?
• Sal does a funny thing in that video, where he says it is a "10 Newton box." See what he does there? He talks about the box in terms of the force of gravity acting on it, instead of talking about it in terms of its mass.

The force of gravity is equal to mg. So you are absolutely right that PE = mgh, but Sal specified mg all at once, instead of specifying m and then calculating mg.
• How does the rope move if it's attached to the ceiling?
• It doesn't. I think the lower pulley moves up when you pull on the rope.
• Is there a difference between an inclined plane and a wedge? In school they called the thing that Sal drew an inclined plane.
• From what I understand, an inclined plane is an abstract concept. It simply refers to a plane that has a slope relative to some reference. A wedge on the other hand is a simple machine--a physical construct that can provide mechanical advantage. So I would say that all wedges are inclined planes (they are tools constructed from a set of planes slanted relative to some center line), but not all inclined planes are used as wedges (though they are often described as having a "wedge shape").
• What is the name of that disc thing mentioned at ?
• A wheel and axle.
• So if the mechanical advantage of a machine is greater than 1 i.e the input force applied is less than the output force, does that mean that the machine is more efficient? I mean is it that we have to put less effort on that machine? Or is it the other way?
• you can think of a mechanical advantage as a kind of work transfer system (from force to distance)

say if you have a maximum 10N force with your hands but you want to life up an object with 20N gravitational force anyhow, it might be physically impossible to do that in any direct manner

but with a help of machines of MA=>2 like pullies or wedges, you can do it only with 10N (half of required force). but nothing is for free. you need to apply this force 2x distance (and thus time)

you can use the opposite machine with 0 < MA < 1 too. you have 40N of force with your hands to lift up the same weight above. but have not much patience to do that for a long distance or time. then you can use machines with MA=1/2. then you can do the same job of 20N with your 40N hands only for a 1/2 distance (and thus time)

in short, MA is way of balancing force and distance to do the same total work as W = Fd. but to be a bit frank and technical, i think the word of mechanical advantage is somehow narrow. cause it can't embrace the way of using less than 1 MA in the opposite direction (distance to force)
(1 vote)
• I guess this is a basic question, but why does the box have potential energy just by being a certain distance above the ground? Isn't the ground just an arbitrarily defined surface? Can you increase the potential energy by digging a hole in the ground beneath it? Would it have the same potential energy whether it's hanging by a thread or resting on a table? If you were to define the surface of the table as the ground, would the box have no potential energy? Is potential energy completely relative to whatever surface you define an object's height relative to?
• Yes, potential energy is always relative to some level that you arbitrarily define as zero.
We never really care about absolute potential energy, anyway. Usually what we care about is changes in potential energy.
• At , isn't Mr. Khan looking for "sheave"?
• yes. It is not a commonly used word though, so I do not blame him for not using it.
• if the weight move, that mean the weight accelerated , and there is net force applied , i mean the tension force must be greater than the weight in order to pull up the weight.

how the tension force equal to the weight ?
(1 vote)
• To get the block moving yes the tension needs grater that the block's weight but if the block is moving at a constant velocity then it is not accelerating so the net force has to be 0 so the weight has to be equal to the tension.