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## Physics library

### Course: Physics library>Unit 5

Explore the concept of mechanical advantage with simple machines like levers. Understand how input force can be multiplied to output a greater force, albeit over a shorter distance, using the law of conservation of energy. Discover the relationship between force, distance, and work, and get introduced to the concept of moments. Created by Sal Khan.

## Want to join the conversation?

• Isn't this a geometry mistake? I mean, when the swing moves down, its first position and the final position do not make a right triangle but rather an isosceles one. You could not use trigonometric equations there. Or am I somehow wrong?
• you are right, for larger values of the angle theta this would not work. but in this method we assume that theta is very (in fact infinitely) small. Thus the error disappears. This method is called "virtual work", idk if sal mentions it. you could do this with the correct geometry and sould get the same result for F.
• I am having a hard time explaining to a friend that mechanical advantage doesn't mean you are doing less work. Just spreading the work (energy used) over a longer distance. In your video above you do not emphasize the law of conservation of energy.
Do you have a video that emphasizes that.
• OK, I have watched the video. Say I am riding my bike (lots of mechanical advantage) from point A to point B at 10 mph. I walk the same distance at 3 mph. I should be using the same amount of energy plus the energy it takes to pedal the mass of the bike. If there is less friction with the bike I would think it would still take more energy. ?
• So, I see that at , Sal uses Newtons both as a unit of force and a unit of weight. Will this work for other units? e.g. "pounds of force" or "tonnes of force"
If so, can this theoretically be done with any weight unit?
• yes it dosent matter what unit of weight you use, as long as that wait is being applied to the whole object.
(1 vote)
• Could one say that we are determining a ratio of work equivalency?
ex. 10N*1m = 1N*10m
We arrange Forces and distances to match: Work in = Work Out
... setting up newton *meter or joule congruence..
• Guys please help I am lost. Perhaps it is sound silly, but I don't understand why the weight STARTS to move. When it is moving and the work is done it is all clear - the energy and work conservation law. But why does the lever multiply the force when the system is stationary? I mean why does the machine START to move? Sorry I don't know how to explain better...
• I believe that you're asking why and how the lever multiplies the force even when the system is stationary. It is because of the following principle of levers: for two forces F1 and F2 on opposite sides of the lever, where the distances from each force to the fulcrum of the lever are D1 and D2 respectively, the lever balances when the product of F1 and D1 is equal to the product of F2 and D2. Once the lever is balanced, it is in a stable state, and remains in such a state until it is disturbed.
(1 vote)
• so if im correct if the object is in the same place no matter of how it got there the work is the same
• Yes, at least in this video and in most other physics situations, because the force you are trying to work against is gravity, which is a conservative force. As long as the force is conservative, then the only thing that matters is the initial and final position of the object. The most important non-conservative force is friction. You can learn more about this concept by watching this KA video:
• Does Khan Academy offers some practice exercises on this topic (besides the videos)? If not, can someone tell me where to find them?
• when the 10N object goes up, how do we know it goes straight up and not going in a track that is curved to the left?
• It doesn't matter whether it goes straight up or in a curve-y track.