# Derivation of Common Formula

• Haftred
In summary, the formula for average kinetic energy, K = 0.5mv^2, is not actually for average kinetic energy but is the definition of kinetic energy. It was derived from the concept of work, which is equal to the change in kinetic energy between two points. This was noticed by early physicists and given a name as it is a conserved quantity in many dynamics problems.

#### Haftred

Can someone please explain the derivation of the common formula for average kinetic energy: K = 0.5mv^2?

Derivation of KE?

a = v^2 - v0^2 / 2d

&Sigma;Work = &Sigma;F * d = mad = 0.5m(v^2 - v0^2) = ke

First, K = 0.5mv^2 is not "average" kinetic energy- it is kinetic energy- in fact it is the definition of kinetic energy- there is no "derivation". At some time, back at the beginning of "physics" someone notice that, in many dynamics problems that quantity was conserved- so they gave it a name!

First, K = 0.5mv^2 is not "average" kinetic energy- it is kinetic energy- in fact it is the definition of kinetic energy- there is no "derivation". At some time, back at the beginning of "physics" someone notice that, in many dynamics problems that quantity was conserved- so they gave it a name!

Quantities, mv^2, 35mv^2, are also conserved!

Originally posted by PrudensOptimus

&Sigma;Work = &Sigma;F * d = mad = 0.5m(v^2 - v0^2) = ke

So ummmm ... This is relative kinetic energy not average kinetic energy?

That is, relative to the kinetic energy of the object when v0 = 0

Interesting derivation. Thanks.

energy is the ability to do work.

work is force times distance.

kinetic energy is that energy a body has due to its motion.

so let's see:

$$W=\int_A^B \mathbf{F}\cdot d\mathbf{s}$$

but by Newton
$$\mathbf{F}=m\mathbf{a}$$
so
$$W=\int_A^Bm\mathbf{a}\cdot d\mathbf{s} = m\int_{t_{A}}^{t_{B}}\mathbf{a}\cdot\mathbf{v}dt\\ = \frac{m}{2}\int_{t_{A}}^{t_{B}}\frac{d(v^2)}{dt}\! dt = \frac{1}{2}mv_B^2 - \frac{1}{2}mv_A^2$$

so you see that the work done is equal to the change in kinetic energy between the two points, and this is a derivation of the formula for that kinetic energy.

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