Conservation of Energy: Kinetic & Reference Frames

In the frame where they are initially moving at -2 m/s the initial PE is 0 J and the initial KE is -4 J. After release one mass will move at -1 m/s and the other will move at 0 m/s. So the final KE is -3 J (-2.5 J in one mass and 1.5 J in the other) and the final PE is 0 J. Energy is not conserved. In the frame where they are initially moving at 0 m/s the initial PE is 0 J and the initial KE is 0 J. After release one mass will move at 0 m/s and the other will move at 2 m/s. So the final
  • #1

Zubair Ahmad

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If velocity is reference frame dependent then kinetic energy should also be.
Considering this we will have different energies in different frames.
Doesn't it violate conservation of energy?
 
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  • #2
No it doesn't. Being frame dependent and conserved are different things. Conserved means it is constant in time, but it can be different constants in different frames.
 
  • #3
So it means conservation of energy is limited to a single frame
 
  • #4
No, it means that energy is constant in every frame, but in principle this constant may have different (but constant in time) values in different frames.
 
  • #5
So we can have a zero energy frame also?
 
  • #6
Zubair Ahmad said:
So we can have a zero energy frame also?

When a mass m is at rest in your reference frame, what is its kinetic energy?

Zz.
 
  • #8
Zubair Ahmad said:
Zero!

Haven't you just answered your question?

Zz.
 
  • #9
That I know but what about the original question..
How to explain conservation?
 
  • #10
Zubair Ahmad said:
So we can have a zero energy frame also?
For a system of particles there is no zero kinetic energy frame as there is a zero momentum frame. That should be obvious, as kinetic energy is non negative.
 
  • #11
Zubair Ahmad said:
That I know but what about the original question..
How to explain conservation?

Again, as has been stated by other responses, there is no issue with energy conservation.

If you are in one reference frame, and the object is moving at speed v, the object has KE = ½ mv2.

On the other hand, an observer moving with the object will measure zero KE.

There is no issue with conservation of energy here. Each observer is in a different frame, as has already been mentioned.

If you are in one reference frame and wish to go to another reference frame, you have to BOOST yourself to that frame, and thus, require external energy input. This is now no longer an isolated system and energy should not be conserved for the original system.

Zz.
 
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  • #12
Zubair Ahmad said:
If velocity is reference frame dependent then kinetic energy should also be.
Considering this we will have different energies in different frames.
Doesn't it violate conservation of energy?
The fact a quantity doesn't vary changing the frame of reference is not called "conservation" in physics, it's (usually) called "invariance". They are both important, but completely different concepts.

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  • #13
Zubair Ahmad said:
If velocity is reference frame dependent then kinetic energy should also be.
Considering this we will have different energies in different frames.
Yes, this is correct.

Zubair Ahmad said:
Doesn't it violate conservation of energy?
No, it does not. Let’s consider a specific toy example. Suppose we have two 1 kg masses joined by a compressed spring containing 1 J of elastic energy.

In the frame where they are initially at rest the initial KE is 0 J and the initial PE is 1 J. If the spring is released then one mass will move at 1 m/s and the other mass will at -1 m/s. So the final KE is 1 J (evenly divided between the masses) and the final PE is 0 J. Energy is conserved.

In the frame where they are initially moving at 2 m/s the initial PE is 1 J and the initial KE is 4 J. After release one mass will move at 3 m/s and the other will move at 1 m/s. So the final KE is 5 J (4.5 J in one mass and 0.5 J in the other) and the final PE is 0 J. Energy is also conserved.
 

1. What is the principle of conservation of energy?

The principle of conservation of energy states that energy cannot be created or destroyed, but can only be transformed from one form to another. This means that the total amount of energy in a closed system remains constant over time.

2. What is kinetic energy?

Kinetic energy is the energy an object possesses due to its motion. It is dependent on the mass and velocity of the object, with the equation KE = 1/2 * m * v^2, where m is the mass and v is the velocity.

3. How does kinetic energy relate to reference frames?

Kinetic energy is a relative quantity that depends on the observer's frame of reference. In different reference frames, the same object may have different kinetic energies. However, the total energy of the system remains constant in all reference frames.

4. What are some examples of conservation of energy in everyday life?

Some examples of conservation of energy in everyday life include: a swinging pendulum, a bouncing ball, a rolling car, and a falling object. In all of these cases, energy is transformed from potential to kinetic and back to potential again, but the total energy remains constant.

5. How does conservation of energy apply to renewable energy sources?

Conservation of energy applies to renewable energy sources such as solar, wind, and hydro power. These sources harness the energy of the sun, wind, and water, respectively, and convert it into usable forms of energy. This is possible because energy cannot be created or destroyed, but only transformed.

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