Physics Help Forum Kinetic energy relation to mass of an object.
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 Special and General Relativity Special and General Relativity Physics Help Forum

 Jul 5th 2017, 09:31 AM #1 Senior Member     Join Date: Feb 2017 Posts: 203 Kinetic energy relation to mass of an object. I will tell you what i understand and correct me if i am wrong. Anything with mass cant travel at the speed of light because at the speed of light mass would become infinite since infinite kinetic energy is required to make mass travel at speed of light. My question is that: Why do we require infinite amount of energy to travel at the speed of light?
Jul 5th 2017, 01:38 PM   #2
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 Originally Posted by avito009 I will tell you what i understand and correct me if i am wrong. Anything with mass cant travel at the speed of light because at the speed of light mass would become infinite since infinite kinetic energy is required to make mass travel at speed of light. My question is that: Why do we require infinite amount of energy to travel at the speed of light?
The faster an object is moving the more massive it is. If you'd like to follow the derivation then see: Work-Energy Theorem

 Jul 5th 2017, 10:32 PM #3 Senior Member   Join Date: Apr 2017 Posts: 428 We are told by relativity , that mass of an object will increase as its speed does .. suppose we have an object with mass at rest of 1 kg ... As speed increases we see at 86%c it's mass is 2Kg ....at 98%c its mass is 6 Kg .. as its mass increases it requires more energy to increase its speed . close to c the mass is nearly infinite .. you can never reach c because the mass then is infinite ... That's the theory ... some doubt it ...
 Jul 6th 2017, 08:40 AM #4 Senior Member     Join Date: Feb 2017 Posts: 203 My interpretation. The Work-Energy Theorem: The principle of work and kinetic energy (also known as the work-energy theorem) states that the work done by the sum of all forces acting on a particle equals the change in the kinetic energy of the particle. So does it mean that the work has to be done and this work is increase in kinetic energy. This work done would be low if mass increases. When mass increase, velocity decrease. When mass decrease, velocity increase just to put up that total KE value constant.
 Jul 6th 2017, 02:06 PM #5 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 538 The change in kinetic energy is "offset" by the change of mass according to: $\displaystyle E_k = \large \frac{\Delta m \space c^{2} }{\sqrt{1 - \frac{v^{2}}{c^{2}}}}$ This change in mass affects work done by inertial forces (ie F = ma) but not work done by gravity (F = GmM/r^2) (and this is where I get somewhat confused myself so I might be wrong on that last statement).
Jul 6th 2017, 08:46 PM   #6
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 Originally Posted by avito009 The Work-Energy Theorem: The principle of work and kinetic energy (also known as the work-energy theorem) states that the work done by the sum of all forces acting on a particle equals the change in the kinetic energy of the particle.
Yes

 Originally Posted by avito009 So does it mean that the work has to be done and this work is increase in kinetic energy.
The work done results in increased kinetic energy of the mass

 Originally Posted by avito009 This work done would be low if mass increases. When mass increase, velocity decrease. When mass decrease, velocity increase just to put up that total KE value constant.
No ... The rate at which a force accelerates an object is dependent on it's mass ...with increasing velocity mass increases so the acceleration will be less.

Jul 6th 2017, 08:59 PM   #7
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 Originally Posted by kiwiheretic The change in kinetic energy is "offset" by the change of mass according to: $\displaystyle E_k = \large \frac{\Delta m \space c^{2} }{\sqrt{1 - \frac{v^{2}}{c^{2}}}}$ This change in mass affects work done by inertial forces (ie F = ma) but not work done by gravity (F = GmM/r^2) (and this is where I get somewhat confused myself so I might be wrong on that last statement).
The increase in mass will cause the object to resist being accelerated by a force ...

It will also increase the gravitational pull the object feels from other masses

Don't muddle up force and work ! if a force (say from gravity) acts , work is only done if that force moves the object through a distance and brings the two objects closer ..

When a rigid satellite is orbiting a riged planet in perfect vacuum no work is done , the gravitational pull is there , but the satellite stays at the same distance from the planet .... it will orbit forever at the same distance , no work is done.

Jul 6th 2017, 10:02 PM   #8
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 Originally Posted by oz93666 The increase in mass will cause the object to resist being accelerated by a force ... It will also increase the gravitational pull the object feels from other masses Don't muddle up force and work ! if a force (say from gravity) acts , work is only done if that force moves the object through a distance and brings the two objects closer .. When a rigid satellite is orbiting a riged planet in perfect vacuum no work is done , the gravitational pull is there , but the satellite stays at the same distance from the planet .... it will orbit forever at the same distance , no work is done.
Yeah, I was a little confused about the differences between inertial mass, gravitational mass and relativistic mass after watching this video:

However it does make more sense if they are the same.

Jul 7th 2017, 06:09 AM   #9
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 Originally Posted by oz93666 Yes The work done results in increased kinetic energy of the mass No ... The rate at which a force accelerates an object is dependent on it's mass ...with increasing velocity mass increases so the acceleration will be less.
I think this one thing people missed saying is that as mass increases when we move near the speed of light the inertia of the object also increases. And we know the more inertia an object has the more difficult it is to move it. So more kinetic energy is required to move this massive object.

Jul 7th 2017, 09:09 PM   #10
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 Originally Posted by avito009 I think this one thing people missed saying is that as mass increases when we move near the speed of light the inertia of the object also increases. And we know the more inertia an object has the more difficult it is to move it. So more kinetic energy is required to move this massive object.
Nobody missed saying it because mass is exactly what quantifies inertia. I.e. mass is a measure of inertial.

Also inertial mass, gravitational mass and relativistic mass are not the same thing.

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