since velocity is always based on a frame of reference, can I just change my frame of reference to have different amounts of kinetic energy? Where does that energy come from?



since velocity is always based on a frame of reference, can I just change my frame of reference to have different amounts of kinetic energy? Where does that energy come from?

In: Physics

If velocity, a necessary part of the formula for kinetic energy, is based on a frame of reference…then wouldn’t kinetic energy have to be based on that same frame of reference?

So if you change your frame of reference for velocity to get a different measurement, you have a proportionally equal amount of kinetic energy because it’s based on the same frame of reference.

You aren’t “creating kinetic energy”, you’re changing the frame of reference with regard to kinetic energy.

When you change frames of reference, you change the problem. you can’t compare the energy in two different problems in a meaningful way.

It comes from nowhere, that number for kinetic energy is misleading.

The only way you can extract energy from kinetic energy is by using two different objects moving at different speeds. The amount of energy extracted is a function of a few things, including the difference in velocities between those two objects.

Difference in velocities doesn’t change as your frame of reference changes. Two cars colliding at 10 m/s still collide at 10 m/s if you are watching the collision from nearby or an airplane. So the amount of energy that exists in practice stays constant.

Sure, but for that amount of kinetic energy to be meaningful, the frame of reference has to be something meaningful.

Kinetic and potential energy are just accounting tricks, to show what *could* happen if two objects interact.

Two racecars on a track are moving at 199mph and 200mph. If they bump each other, there isn’t much of an impact, because the kinetic energy between the two of them is very low. But if one hits the wall of the track, it’ll be a huge collision, because the kinetic energy difference between the car and the wall is huge.

So you, sitting in your chair, have ludicrous amounts of kinetic energy if you consider that you’re on the Earth, and the Earth is hurtling through space at thousands of miles per hour. But that’s only a meaningful number if you want to talk about what’d happen if you collided with an asteroid or something. For the purposes of your daily life, your kinetic energy is very low, because you’re just sitting in a chair. If you collide with something, it’ll only be a low-energy collision.

Yes and no.
While not usually elaborated as such, the kinetic energy of an object is usually assumed to be in reference to something. For example, when car is driving on a highway, the kinetic energy of the car implies in relation to a stationary object on earth. But if you consider your reference point to be the Sun, then the kinetic energy of the car is much higher, because the velocity is now not the 100 kph, but more like 1600 kph.
Or imagine two people in different cars driving parallel to each other throwing apples at each other. The kinetic energy of those apples is relatively small when hitting each other, but if you miss and hit an innocent pedestrian, he will be hit with an apple with kinetic energy much higher, due to combined velocity of the apple throw + velocity of the car from where the apple was thrown. Basically the resultant kinetic energy comes from the difference in relative velocities of two objects

> can I just change my frame of reference to have different amounts of kinetic energy?

“Just,” like it’s a trivial matter…

But yes, exactly.

> Where does that energy come from?

It comes from your change of reference!

If you and a brick wall are both standing still, there’s no kinetic energy between you. So change your frame of reference: strap yourself to a [rocket sled]( and fire it at the brick wall. Your frame of reference is changed alright.

A frame of reference isn’t just a relative position to another thing.

In relativity, energy is forced to be conserved, by requiring the total energy be invariant between frame of reference. Kinetic energy itself can change, but this change will change the mass of the object. Hence the famous Einstein equation: E^2 =m^2 c^4 +p^2 c^2

In Newton’s view of physics, there is absolute frame of reference, so there are no frame of reference problem. In Galileo’s physics, energy is merely a relative number, or in other word, the number itself is meaningless, only the rate of change matter (compare this situation to voltage vs voltage difference). So you can arbitrarily just add any fixed constant to energy, “conserved” just mean it rate of change is 0.