Why isn't Kinetic Energy always equal to Potential Energy?

In summary: This gives the equation for kinetic energy as a function of momentum: ##E=\int p\ dv+\int v\ dp##.In summary, K=U. This is because the force is -dU/dx, not dU/dx. Furthermore, the potential is only defined up to an arbitrary constant. If you account for these things, what you get is just the work-energy theorem.
  • #1
phasacs
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K= ∫mvdv = ∫m dx/dt dv = ∫m dx/dv dv/dt dv = ∫m dv/dt dx = ∫Fdx = U
=> K=U, why isn't this true? If it is, wouldn't that mean that Kinetic Energy is always equal to Potential Energy?
 
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  • #2
The force is -dU/dx, not dU/dx. Furthermore, the potential is only defined up to an arbitrary constant. If you account for these things, what you get is just the work-energy theorem.
 
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  • #3
What you found is that the change in KE is equal to the negative of the change in PE. Or the total change is zero. This is "conservation of "mechanical energy".
The fact that changes are equal in magnitude do not grand equality of the actual quantities.

Every time I pay taxes, the amount i give is equal to the amount they receive. But my bank balance is not even close to the one of the internal revenue agency.:)
 
  • #4
phasacs said:
K= ∫mvdv = ∫m dx/dt dv = ∫m dx/dv dv/dt dv = ∫m dv/dt dx = ∫Fdx = U
=> K=U, why isn't this true? If it is, wouldn't that mean that Kinetic Energy is always equal to Potential Energy?
Kinetic energy is not integral sum of momentum.. Then how u did this?
 
  • #5
Orodruin said:
The force is -dU/dx, not dU/dx. Furthermore, the potential is only defined up to an arbitrary constant. If you account for these things, what you get is just the work-energy theorem.
How he found kinetic energy in terms of integral sum of momentum?? Can you explain sir?
 
  • #6
STAR GIRL said:
How he found kinetic energy in terms of integral sum of momentum??
Isn't it the "change" in KE as nasu said earlier?
kinetic energy E=(1/2)*mv2
You can see dE/dv=mv.
So, dE=mv*dv.
 
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  • #7
cnh1995 said:
Isn't it the "change" in KE as nasu said earlier?
kinetic energy E=(1/2)*mv2
You can see dE/dv=mv.
So, dE=mv*dv.
Oh I see
 
  • #8
phasacs said:
K= ∫mvdv = ∫m dx/dt dv = ∫m dx/dv dv/dt dv = ∫m dv/dt dx = ∫Fdx = U
=> K=U, why isn't this true? If it is, wouldn't that mean that Kinetic Energy is always equal to Potential Energy?

Note that you have made an assumption that the net force (m dv/dt= F) is a conservative force ( ∫Fdx = [-] U ising @Orodruin 's correction in obtaining a potential energy), which isn't necessarily true... for example, if there is friction that is doing nonzero work.
And as @nasu mentioned, you have really calculated the change-in-K (on the left) and the change-in-[minus]U (on the right), assuming the net force is conservative.

From a more abstract viewpoint [which generalizes to special relativity], it is better to start off with ##\Delta K\equiv\int v\ dp##.
For non-relativistic physics, you will then see that (since v and p are linearly related) ##\int v\ dp=\int p\ dv##.
 
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Related to Why isn't Kinetic Energy always equal to Potential Energy?

1. Why isn't Kinetic Energy always equal to Potential Energy?

Kinetic energy and potential energy are two different forms of energy that are often found in a system. Kinetic energy is associated with the motion of an object, while potential energy is associated with the position or configuration of an object. These two forms of energy are not always equal because they depend on different factors and can change independently of each other.

2. What factors affect the difference between Kinetic Energy and Potential Energy?

The difference between kinetic energy and potential energy can be affected by various factors such as the mass of an object, its velocity, its height, and the force acting on it. For example, an object with a larger mass will have more potential energy due to its greater gravitational pull, while an object with a higher velocity will have more kinetic energy due to its greater speed.

3. Can Kinetic Energy and Potential Energy be equal in some cases?

Yes, there are some cases where kinetic energy and potential energy can be equal. For example, if an object is at the highest point of its trajectory, it will have no kinetic energy but only potential energy. Similarly, if an object is at rest on the ground, it will have no potential energy but only kinetic energy. In these cases, the total energy of the system (kinetic energy + potential energy) remains constant.

4. How does the Law of Conservation of Energy explain the difference between Kinetic Energy and Potential Energy?

The Law of Conservation of Energy states that energy cannot be created or destroyed, it can only be transferred or converted from one form to another. This law explains that the total energy of a system remains constant, but the forms of energy within the system can change. In the case of kinetic and potential energy, the sum of these two energies remains the same, but they can change independently of each other.

5. What are some real-life examples of the difference between Kinetic Energy and Potential Energy?

There are many real-life examples that demonstrate the difference between kinetic energy and potential energy. Some common examples include a rollercoaster at the top of a hill (high potential energy), a car driving down a hill (potential energy being converted to kinetic energy), a pendulum at its highest point (potential energy), and a swinging pendulum at its lowest point (kinetic energy). Understanding the difference between these two forms of energy is important in many fields of science, such as physics and engineering.

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