If you stand vertically over a rigid surface and try to jump without bending your knees, you will find that it cannot be done. You need to crouch a little before jumping.

Why is that? Give a complete satisfactory explanation of it!!

Hint:-

Not required. It's extremely simple. Think simple, Think physics.

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## Comments

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TopNewestbecause i dont have jetpacks under my shoes, and my muscles cannot stretch , they can only contract, like springs, so i contract my thigh muscles on the back side to bend my leg and then suddenly contract the thigh muscles on the front side so that my body has a non zero velocity when it reaches back to its equillibirum position, and hence i jump

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You need an external force to jump. The normal reaction will provide for this force but normally it is equal to the weight so we need to crouch ; which actually is the act of pushing the ground beneath ; which inturn increases the contact force between the jumper and the ground ; thus producing a normal force greater than weight of the body. This extra force gives us the required initiative ...

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We actually can jump without bending our knees if we have a strong ankle like Bruce Lee or Jackie Chan

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Newton's third law .. You have to apply a force on the ground so the opposite force lets you body go up and thats called jumping

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Here's another attempt:

Imagine an unstretched spring lying vertically on the ground.Will it ever "jump"? Can it ever? The answer is in the negative. This is because for the spring to jump, the ground has to exert a force greater than the weight of the spring. This can be achieved by compressing the spring by some distance, which, when released exerts a force on the ground. The ground, by Newton's Third Law, exerts the same force on the spring in the upward direction. And thus, the spring lifts.

The same thing happens when we bend our knees prior to a jump. We are effectively compressing some of the muscles in our legs, which, upon relaxation, exert force on the ground...and similar reasoning as above follows.

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Well said!!! The most perfect explanation i've seen until now for this problem!!!

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For jumping normal should be greater than gravity so acceleration of centre of mass is vertically upward which is not possible when you stand as you will gain a very small velocity

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Well your ankles alone just ain't strong enough to lift you body higher than the ball of your feet.. You need more power.. That's where the thighs come in an to utilize your thighs in a jump you'll need to bend at the knees.

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Newton Baba's First Law - Every body continues to stay in a state of rest or motion until and unless an external force is applied on it.Thus we wont be able to jump until an external force is applied on us.But to exert this external force some amount of work has to be done.This work can be done using the potential energy stored in our muscles which act as springs when we bend(Work-Energy Theorem)and thus we are able to jump.But its not necessary that we SHOULD bend our knees to perform a simple jump.We can also stand on our toes without bending our knees and perform a simple jump.Try it!!!

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Newton's third law ! Every action has equal and opposite reaction. So for jumping we must do the action of bending ,which contracts the muscles on Legs , so the extra potential energy has to be used up which we use it for jumping

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By crouching we store potential energy which provides the necessary force for the jump

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Yeah our muscles can only contract

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Our muscles can also stretch......not only contract

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Yeah. this is very much similar to the fact what when you are sitting on a chair with your knees at 90 degrees to the ground, In that state - Just by using your legs, you cannot get up .

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i can do it.

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Ok so im thinking: 1core or upper body muscles. 2 feet. There is muscle contraction and stretching in motion enabling the "Jump" while" knee" is locked.

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To accumulate K.E and P.E

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Every action has an equal and opposite reaction. The earth pushes you up with the same force you push down on it. The force applied on the ground without bending your knees is not strong enough to lift your body weight.

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So that reaction and weight will act as a couple to lift us easily for a jump.

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When we bent our knees to jump, then we actually applying a force on the ground by which we get a equal and opposite reaction from the ground that we already read in NEWTON'S THIRD LAW. By the help of which we can jump.

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This is more of bio-physics (or something like that). It depends upon the ability of the ankle to bend/stretch,and its ability to store potential energy. So, we can jump without bending our knees.

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as every force has equal and opposite reaction, when you exert force on the ground, you move in the opposite direction. As the mass of earth is very large it don't accelerate, but you do in the opposite direction. SIMPLE!

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You need impulse and gracity

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\that is so incorrect if i stand on a clif and try to jump with knees flat it is totally possible take that as an answer boom

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Because we need impuls to do it

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Because no external force is acted opposing gravity.

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We can do it! Just bend your feet with your toes a pivot point. Try to make it as fast as possible that we have more kinetic energy, and energy will be converted to potential energy, which makes your position higher. Not that high though, but would you count as jumping?

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To jump certain energy has to build up prior the act. This is done by bending knee which stores P.E. and when we jump this is coverted to motion .

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A force is required to start motion since ur body will be at rest and be attracted by gravity

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Comment deleted Jan 15, 2015

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Work has to be done by muscles in the body which is not possible without bending

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