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## Physics library

### Course: Physics library>Unit 5

More on mechanical advantage, levers and moments. Created by Sal Khan.

## Want to join the conversation?

• hey guys,just out of curiosity,can we apply this mechanical advantage stuff to multiply the forces in machines like gears and pulleys etc??
• That's exactly what these machines are designed to do:
Make one's life easyer. i.e. you can do things with less force.
But remember: you are paying for this advantage: you have to apply the force over a larger distance, because the work has to stay the same. Look at the video at
• I paused the video and since 7*35/5=7*7=49, can he lift a 49 N weight if he is exerting 7 N of force and is 35 meters away from the fulcrum while the weight will be placed 5 meters from the fulcrum?
• technically, he can't, because it would only be in equilibrium with 49N, but any force above that would be used to lift the weight. However, this still is constrained by conservation of energy, as he must move his side of the lever down about 5 times as much than the distance he lifts the weight.
• What happens if you add to much force?
For example, from on, what would happen to a 42N weight?
• The weight would accelerate upwards until gravity brings it back down. It would "jump", as you can try with any lever in real life.
• Is this concept having any relation with Newton's third law of motion?
• Not in the way you are thinking. Yes, when you push down on the lever, the other end goes up, but Newtons 3rd law is stating that when you push down on the lever, you are being pushed up with the same force you apply.
• anyone else think that the force be drawn perpendicular to the lever and not directly down? It may be an ok as an approximation, but my impression was Moment = (distance * force perpendicular)
• Just wondering...does mechanical advantage affect energy?
(1 vote)
• Does anybody know where I can find mechanical energy? I need to know how to determine mechanical energy at a certain point above earth's surface, and it's a different formula than the traditional Wm=Wp+Wk, as it deals with the gravitational field around earth. I thought that this may be where I would find answers, but it's not. Please help.
• How do you incorporate the mass of the lever in the calculation since it too plays a role. My mind tells me that it plays a larger role as the fulcrum is moved further from the center. Thanks
• You would need to know the dimensions of the lever and it's density. Then you could calculate the centre of mass of each half, assume that the entire mass was being acted on through this point and include it in the calculations that way. Of course, if it is a very thick lever it gets even more complex since the movement of the lever would also change the centre of mass of each part of the lever to a greater degree depending on the thickness. For these reasons we often assume the lever to be without mass, or perfectly symmetrical and very thin unless it is a real world problem that needs a more realistic solution.
• Does the basic F*d=F*d equation work for other classes of levers? That is, when the fulcrum is not between the force and the load?
• Okay, i don't know if I am fully qualified to answer this question, but I believe what you are referring is a system similar to a wheelbarrow. I believe it will act in a very similar way. The way to visualize this is by looking at the distance the weight on the wheel barrow is moving up and compare to the distance the handle you are holding on is going up. So a weight of 3N one meter away from the wheel, being lifted .1m and handles that are 3m long would require an upwards force greater than 1N to lift it, and 1N to maintain equilibrium to a height of .3m. You increased the distance the force had to travel by 3 times so the force is cut by one third and the amount of work is conserved. The key is F*D=F*D work in = work out. Someone please corroborate this or correct me if I am wrong.
(1 vote)
• In general ho w would you find the mechanical advantage if you knew the weight of the load, and the lengths from both ends to the fulcrum?