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## Physics library

### Course: Physics library>Unit 5

Introduction to simple machines, mechanical advantage and moments. Created by Sal Khan.

## Want to join the conversation?

• Well, how can the object move if the summ of the forces that act on them is equal to 0? That makes no sense, but in all these videos they "move", somehow....what did I miss? • Sure. Sal explains as much in the beginning of the video. He is only calculating a baseline force: a force necessary to keep any motion going, but not enough to move the system if resting. You therefor can either assume there was an impulse applied (remember, even the weight of a feather would move suffice), or just assume the object was already in motion.
• Why are the triangles around assumed to be right triangles. It seems to me as if the hypotenuse and a leg would both be equal to one, so why do we use the trig functions, as the triangle must be isosoles and does not always have a 45 degree angle. • 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.
• At and beyond.

The hypotenuses of the right-angled triangles indicate that length of each side of the swing has increased after rotating through theta. Clearly this is not possible in real life, and so how is it possible to solve this problem using these triangles?

Also, if you are to trace the path of a point at either end of the swing, isn't it a circular path, and not straight perpendicular lines? • Strange responses..
The diagram and explanation imply that it rotates, even though it 'looks' longer.

Also, yes it is a circular path. The fact that it is circular or not doesn't change the fact that by change in height he means change in Y, not total distance traveled over the curve. We could factor in X here just fine by using some trig, but the ME ratio would again come out to be 2, the force moves twice as much to the right, as the block moves to the left.
• 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. • 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? • At How can the lever move up if the object's weight is 10N(downwards) and the force is 10N (upwards) ? Isn't the system then in equilibrium and nothing moves? • 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..   