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March 15th, 2017, 11:20 PM   #1
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Why does kinetic energy increase as velocity squared?

The formula for kinetic energy is E=[1/2]mv^2 and the formula for momentum is P=mv. I ran into these in physics class long ago and was really bothered by the first formula. How can energy go up as the square of the velocity?
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March 16th, 2017, 12:04 AM   #2
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suppose we throw a ball vertically into the air with velocity $v_0$

At that point in time the ball has a certain amount of kinetic energy and as it rises against the force of gravity it does work against the gravitational field until it has exhausted it's supply of kinetic energy and stops at some height $h$.

We know that in the absence of friction the ball has converted it's kinetic energy into gravitational potential energy.

The potential energy gain is $m g h$

the balls stops moving at time

$t = \dfrac{v_0}{g}$

and the ball attains a height of

$h = v_0 t - \dfrac{g t^2}{2} = \dfrac{v_0^2}{g} - \dfrac{g v_0^2}{2 g^2} = \dfrac 1 g \left(v_0^2-\dfrac{v_0^2}{2}\right) = \dfrac{v_0^2}{2g}$

so the gravitational potential gained, and hence the kinetic energy started with is

$\Delta U = KE = m g \dfrac{v_0^2}{2g} = \dfrac 1 2 m v_0^2$

You can also do a units analysis.

Energy is work which is force times distance

Force is mass times acceleration

so using fundamental units, M, L, T, for mass, length, and time we have

$E \sim M \dfrac{L}{T^2} L = M \left(\dfrac{L^2}{T^2}\right) = M \left(\dfrac{L}{T}\right)^2$

$\dfrac{L}{T}$ corresponds to velocity so again we have

$E \sim M V^2$
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March 16th, 2017, 11:19 AM   #3
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romsek has given you a very good answer. But I thought I'd add my two cents anyway.

Energy is a very hard concept to pin down. There are a number of quantities in Physics that have that "property." Momentum, energy, force, and mass are some others that are difficult to pin down as well. So why do we define them the way we do? Simple: Over time we find that some definitions are more useful than others. The way we define energy has a nice property...it is conserved for closed systems.

So basically the definition for kinetic energy is as it is because it's more useful than any other way to do it. A lot of definitions in Physics are made that way.

-Dan
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March 16th, 2017, 04:06 PM   #4
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for uniform acceleration ...

$v_f^2 = v_0^2 + 2a \Delta x$

$v_f^2 - v_0^2 = 2a \Delta x$

$\dfrac{v_f^2}{2} - \dfrac{v_0^2}{2} = a \Delta x$

$m\bigg[\dfrac{v_f^2}{2} - \dfrac{v_0^2}{2}\bigg] = ma \Delta x$

$\dfrac{mv_f^2}{2} - \dfrac{mv_0^2}{2} = F_{net} \Delta x$

$\Delta KE = W_{net}$
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March 20th, 2017, 05:06 AM   #5
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Thank yo all for the calculation. but, I have another confusion:-

So Why does kinetic energy only increase quadratically and not linearly, with speed?
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March 20th, 2017, 06:20 AM   #6
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Originally Posted by JamSmith View Post
Thank yo all for the calculation. but, I have another confusion:-

So Why does kinetic energy only increase quadratically and not linearly, with speed?
It's just defined that way. Momentum varies linearly with speed and is a different thing to energy.
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March 22nd, 2017, 04:21 AM   #7
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Why does kinetic energy increase as velocity squared?

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Originally Posted by Benit13 View Post
It's just defined that way. Momentum varies linearly with speed and is a different thing to energy.
So what is the difference between Momentum & Kinetic energy? As you said, kinetic energy can be converted into other forms of energy. But, I have read somewhere that Kinetic energy in itself is not necessarily obtained only the total energy.
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March 22nd, 2017, 06:12 AM   #8
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Quote:
Originally Posted by JamSmith View Post
So what is the difference between Momentum & Kinetic energy? As you said, kinetic energy can be converted into other forms of energy. But, I have read somewhere that Kinetic energy in itself is not necessarily obtained only the total energy.
Some differences are

1) Momentum is a vector, kinetic energy is a scalar, like all forms of energy.

2) Momentum is always a conserved quantity Kinetic Energy is not. I think your reference from elsewhere is to total mechanical energy in a purely mechanical system which is a conserved quantity.

3) There is only one form of momentum and this cannot be converted into anything else, unlike energy which takes many different forms. It is these different forms that are the reason why Kinetic Energy alone is not conserved.
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