Energy capacity to do work (frames of references)

In summary, the conversation discusses the concept of energy and its relationship to mass and frame of reference. It is mentioned that kinetic energy is not an intrinsic property of an object and can change depending on the frame of reference. The idea of equating kinetic energy to mass is also explored, with the conclusion that they are not equivalent. It is also noted that specifying the frame of reference is necessary when discussing an object's kinetic energy.
  • #1
326
1
I have a problem understanding how we can say that an object has energy of x joules because energy depends on the frame of reference right ? I mean let say a bird is moving with velocity of 5 m/s if I were a stationary man I would observe that it has a kinetic energy but if I were moving with 5 m/s then it would have zero energy, so how can energy be a meassure of the capacity of an object to do work and why is it considered to be the same thing as mass as being a property of the object itself if it depends on the frame of reference?
 
Physics news on Phys.org
  • #2
If you and the bird are both moving at 5 m/s with respect to the ground, in the same direction, how can the bird do any work on you?
 
  • #3
If you take into account all the objects affected by a change in energy, then the total energy for that closed system remains constant, regardless of the (inertial) frame of reference. Ignoring losses to heat, potential energy (electrical, chemical, gravitational) can be converted into kinetic energy. Momentum of the system will also be conserved (the center of mass remains moving at constant velocity or zero).

For example, imagine a rocket in outer space. The chemical energy of the fuel is used to accelerate the rocket and the exhaust fuel. The total increase in kinetic energy of rocket and fuel are equal to the chemical energy of the fuel converted (ignoring losses to heat), and are the same regardless of the frame of reference.

A similar analogy could be made for a car accelerating on the earth, as long as you consider the car and the Earth as the closed system where both car and Earth experience an energy change when the car's engine performs work (chemical to kinetic energy convertion).
 
  • #4
well it can't so what I am asking is that whether energy like mass is a property of the object itself rather than affected by the one who measures it or does it mean the capacity to do work on a certain object and changes by the change of the motion of the object? if so then how could it be equated to mass?
also is it insufficient to say an object has kinetic energy of some value without saying with which references it was taken from? I mean the bird had different kinetic energies in the two scenarios which one is to be considered?
 
  • #5
madah12 said:
well it can't so what I am asking is that whether energy like mass is a property of the object itself rather than affected by the one who measures it or does it mean the capacity to do work on a certain object and changes by the change of the motion of the object?
Kinetic energy depends on the frame of reference; it's not an intrinsic property of an object.
if so then how could it be equated to mass?
It isn't.
also is it insufficient to say an object has kinetic energy of some value without saying with which references it was taken from?
Right. To say an object has a certain KE is meaningless if the frame of reference is not specified. (Of course, often it's clear from the context what the frame of reference is.)
I mean the bird had different kinetic energies in the two scenarios which one is to be considered?
You choose the frame of reference most useful for your purpose. The physics doesn't change.
 
  • #6
Kinetic energy isn't equated to mass? KE = 1/2mv^2 seems to equate to mass by 1/2 the relative velocity squared is how to me...It is equated to mass by accounting for the velocity, at the same relative velocity two equal masses have equal kinetic energy...thus (mass = KE*X) mass is always proportional to KE, the variable proportionality(X) being (the relative velocity squared) to (2)...I would say
 
  • #7
Zula110100100 said:
Kinetic energy isn't equated to mass? KE = 1/2mv^2 seems to equate to mass by 1/2 the relative velocity squared is how to me...
KE doesn't 'equate' to mass, but to 1/2mv^2. Big difference! Obviously it depends on the reference frame.
 
  • #8
Definition of EQUATE
transitive verb
1
a : to make equal : equalize b : to make such an allowance or correction in as will reduce to a common standard or obtain a correct result
2
: to treat, represent, or regard as equal, equivalent, or comparable <equates disagreement with disloyalty>

It is made equal, equalized, by a variable proportionality we can correct KE to give the result of mass by dividing it by 1/2v^2.. to use an example, At the same relative speed, we have the same mass and the same KE, if you have relative velocity compared to me, and I don't know your mass, but I could measure you KE, I could equate that to you mass by allowing for your velocityTo be equatable just means to be proportional...imho
 
  • #9
Zula110100100 said:
It is made equal, equalized, by a variable proportionality we can correct KE to give the result of mass by dividing it by 1/2v^2.. to use an example, At the same relative speed, we have the same mass and the same KE, if you have relative velocity compared to me, and I don't know your mass, but I could measure you KE, I could equate that to you mass by allowing for your velocity
Did you read the thread to see what the OP might have meant by his statement?

To be equatable just means to be proportional...imho
Reread the definition that you quoted.
 
  • #10
madah12 said:
well it can't so what I am asking is that whether energy like mass is a property of the object itself rather than affected by the one who measures it or does it mean the capacity to do work on a certain object and changes by the change of the motion of the object? if so then how could it be equated to mass?
also is it insufficient to say an object has kinetic energy of some value without saying with which references it was taken from? I mean the bird had different kinetic energies in the two scenarios which one is to be considered?
Seems to me he is asking How can in be equated to mass if it is relative to your frame of reference... And the answer is it can be because it is dependant on only two quantities, mass and velocity, since velocity is relative to the reference frame, KE will be proportionately relative. i.e. the relative KE and the relative v change at the same rate if you were to compare in two different reference frames. that being the case, the mass will be the same if you divide relative KE by 1/2relative velocity^2 even though they are both relative to the frame of reference
change at the same rate is a bad wording, they change at a porportional rate...hah

but it probably is my vocabulary that is lacking...it maybe cannot be equated, but it can be related?
 

1. What is energy capacity to do work?

Energy capacity to do work refers to the ability of a system or object to perform work, which is the transfer of energy from one form to another. It is a fundamental concept in physics and is measured in units of joules (J).

2. What are the different forms of energy?

There are many forms of energy, including kinetic energy (energy of motion), potential energy (stored energy), thermal energy (heat), chemical energy, electrical energy, and nuclear energy. Each form of energy can be converted into another form.

3. How is energy related to frames of reference?

Frames of reference refer to different perspectives or points of view from which we can observe and measure the same physical phenomenon. Energy is a relative quantity, meaning it depends on the frame of reference from which it is observed. For example, the kinetic energy of an object will appear different depending on whether you are observing it from a stationary or moving frame of reference.

4. Can energy be created or destroyed?

According to the law of conservation of energy, energy cannot be created or destroyed, only transferred from one form to another. This means that the total amount of energy in a closed system remains constant.

5. How is energy capacity to do work calculated?

The energy capacity to do work is calculated using the formula E = Fd, where E is energy, F is the force applied, and d is the distance over which the force is applied. This formula is based on the work-energy theorem, which states that the work done on an object is equal to the change in its kinetic energy.

Suggested for: Energy capacity to do work (frames of references)

Replies
4
Views
837
Replies
2
Views
842
Replies
2
Views
739
Replies
23
Views
962
Replies
16
Views
996
Replies
2
Views
924
Back
Top