Kinetic Energy and relative momentum

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SUMMARY

Kinetic energy is a frame-dependent quantity, meaning it varies based on the observer's reference frame. In the discussion, it is established that Person A, who perceives themselves as stationary, has no kinetic energy, while Person B, moving close to the speed of light, possesses high kinetic energy from Person A's perspective. Conversely, Person B views themselves as stationary and Person A as moving, leading to the conclusion that both can perceive each other as having high kinetic energy simultaneously. This highlights the relativity of motion and energy in physics.

PREREQUISITES
  • Understanding of basic physics concepts, particularly kinetic energy
  • Familiarity with the principle of relativity in motion
  • Knowledge of reference frames in physics
  • Basic comprehension of energy transfer and its dependence on frame of reference
NEXT STEPS
  • Study the concept of kinetic energy in different reference frames
  • Explore the theory of relativity, focusing on time dilation and length contraction
  • Learn about energy transfer in relativistic contexts
  • Watch educational videos on kinetic energy and relativity for visual understanding
USEFUL FOR

This discussion is beneficial for physics students, educators, and anyone interested in understanding the complexities of kinetic energy and the principles of relativity in motion.

jlennoxg
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I've just started getting interested in physics and I am only a few (basic) books deep but there is something that has been distracting me that I can't come to a logical conclusion about. I assume this means there is a flaw in my basic understanding and wish to fix it asap. Here goes.

I am assuming the following statements to be true (as is my understanding which, as I've said, is likely flawed).

*The kinetic energy of an object is greater the faster it is moving.

*As motion is relative, Person A on the ground can observe Person B whizzing through space close to the speed of light and claim that they are stationary and it is Person B who is moving. Likewise, Person B can claim that they are stationary and it is indeed Person A who is moving close to the speed of light (along with the Earth which Person A is standing on).

What I don't understand is where kinetic comes into this. If Person A is stationary they have no kinetic energy and they perceive Person B to have a high amount of kinetic energy. But at the same time Person B perceives them to have a high kinetic energy similar to that which Person A thinks Person B has. They can't both have high kinetic energy AND no kinetic energy can they?

I hope that makes sense and I've not overcomplicated it too much. Sorry if this is a stupid question or has an obvious answer, it's just something that has been bugging me! Thanks.
 
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jlennoxg said:
If Person A is stationary they have no kinetic energy and they perceive Person B to have a high amount of kinetic energy. But at the same time Person B perceives them to have a high kinetic energy similar to that which Person A thinks Person B has. They can't both have high kinetic energy AND no kinetic energy can they?
Kinetic energy is frame dependent. It is not an intrinsic property of the object. It is a number assigned to the object based on the reference frame, just like position and velocity.

Consequently the transfer of KE is also frame frame dependent:
http://www.youtube.com/watch?v=fXr3O1cxYrc#t=90s

If you internalize those facts, you will never be confused asking yourself where the energy comes from to drive one of these:
 
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