Posted on by Max Schmarzo

Physics (Potential Energy and Kinetic Energy)

Potential energy is the possible energy an object has based on its position.

Formula

Potential Energy (PE) = Mass * Gravity * Height

How can we use this formula to better understand movements? Lets look at athletes jumping on and off boxes…

If we are looking at two athletes standing on the same height box we will immediately know that the Height (box height) and Gravity variables of the equation will be the same, but if the weight (Mass) of the two athletes are different then should immediately know that the heavier one will have a higher PE.

A similar example can be used to demonstrate the differences in PE between two athletes of equal mass on different box heights. Again, because gravity is the same and so is weight (mass), we can immediately determine that the athlete on the higher box will have greater potential energy.

 

Same athlete on the taller box will have a higher PE (larger height)
Same athlete on smaller box will have a lower PE (smaller height)

Kinetic Energy

 

Kinetic Energy (KE) is amount of energy an object has due to motion.

Kinetic Energy (KE) = 1/2Mass * Velocity ^ 2

By looking at the above formula we can see that the velocity of the object heavily influences the total KE the object will have.

Comparing KE and PE

 

Now look at the PE and KE equations side by side

KE = 1/2Mass * Velocity ^ 2

PE = Mass * Gravity * Height

If we look at PE, the only thing we can really change to influence PE is the height at which the object or athlete is positioned. The Mass (weight of the athlete) can’t be changed and neither can gravity. However, we can always place the athlete on a taller or smaller box from which they jump off of.

Now looking at KE, what part of the PE formula would influence the velocity aspect of the KE formula? HEIGHT.

Utilizing PE and KE

 

The higher the object is positioned, the further the object has to fall and the greater the velocity the object will be upon contact. In other words, the higher the PE the greater the potential an object has to achieve a higher KE. The key word is potential. Just because PE is really high doesn’t mean the subsequent action will result in an extremely high KE, but this is only the case if one of the aspects of the KE equation is reduced (typically height).

For example, if you have an athlete jump from a 6 foot box to a 4 foot box the athlete will reduce their potential energy by reducing the height by 2 feet, but you will only have this reduction in PE by a subsequent development of KE (falling from 6 to 4 feet). Even though the potential energy from 6 to 4 feet is reduced, the athlete will still have a potential energy.

 

Landing on a box will reduce the KE on impact by preserving some of the PE (Athlete still has PE while on the box)
At the top of the jump the athlete’s PE will be at its highest and KE will be at its lowest

 

 

 

 

Example

 

Athlete’s Potential Energy at 6 feet:

Mass (200lbs) * Height (6feet) *Gravity 9.8m/s

Athlete’s Potential Energy at 4 feet:

Mass (200lbs) * Height (4feet) *Gravity 9.8m/s

The athlete had to use kinetic energy to move from 6 feet  to 4 feet, which is what allowed the reduction in PE to occur.

Once you understand the concepts of KE and PE you can design training programs to manipulate aspects of their equations to either increase or reduce their magnitudes.

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