Question About Jumping

Classical Mechanics Level 2

Suppose you are standing still and upright, and you decide you want to jump into the air. Why do you have to bend your legs first?

To take advantage of the change in gravitational potential energy To hyper-oxygenate your bloodstream in the denser air beneath, in preparation for physical exertion To store energy in your legs like springs So that work can be done on your equivalent center of mass as you move upward again

2 solutions

Michael Mendrin
Dec 2, 2016

Otherwise it's like trying to use a slingshot to propel a rock without pulling back the elastic string. Work needs to be done in increasing the kinetic energy of the rock (or human) from $0$ to something else. Force $\times$ Distance = Kinetic Energy, note the "distance" part of this equation.

Of course your right with the distance part, however I would use another example than the slingshot as that would favour the answer that, like springs, the legs store energy

Kai Ott - 4 years, 5 months ago

I wonder if 1 reason it's believed muscles store energy like elastic is the school model used to show 2 muscles in opposition (usually the biceps and the triceps) with 2 hinged sticks and 2 elastic bands.

Sven Wraight - 4 years, 3 months ago

Bending your legs first before jumping is like putting the cannon shell at the bottom of the barrel instead of leaving it at the top of the barrel. The cannon shell needs to travel the distance of the cannon barrel as the powder charge goes off in order to gain speed.

Michael Mendrin - 4 years, 3 months ago

Josh Silverman Staff
Feb 8, 2017

Fundamentally, a muscle contracts by sliding fibers of a material called actin using molecular motors known as myosin . This movement actually costs energy in the form of spending a molecule of ATP according to $\ce{ATP -> ADP + P}$ (the body can't make much use of "gains" in potential energy).

To get off the ground, our body needs to generate upward velocity, e.g. work must be done. By bending down, we make a distance through which our muscles can accelerate our center of mass back up to standing height, thereby doing work on the body. Without this distance, there'd be no mechanism to accelerate upwards.

Question About Jumping

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