# Mass and resistance

1. Mar 14, 2012

### nouveau_riche

well i am trying to understand the basic of mass and i got myself into a deadlock of questions
firstly the definition of mass as i know is :quantitative measure of an object's resistance to the change of its speed.

assumption:particle have charge but no mass and they just fall into the significant field of each other,there is nothing they interact with except each other and a force F.

now consider a situation where a particle with charge say -q comes into significant field influence of another particle with charge +Q,it then follow a particular path P,suppose a force F is used to the bring a change in path of -q,the force F does so in two ways:at first it tries to change the path of -q by change it's state of motion(either by accelerating or deceleration him) ,thus the resistance offered to F relative to him appeared because of the electromagnetic interaction between the charges.
In the second case the force F tries to change the trajectory of -q but by not changing it's state of motion(maintaining it's velocity),in this case also the resistance offered was due to the electromagnetic interaction.

so according to the definition of mass the resistance offered in first scenario should add to mass but not second,though it can be clearly seen that the resistance offered is because of the electromagnetic interaction under action.

SO WHAT IS WRONG WITH THE PICTURE DEPICTED?

2. Mar 14, 2012

### Drakkith

Staff Emeritus
There are two "types" of mass. The first, inertial mass, is just like you described it, resistance to change in speed. The second is gravitational mass, which is the magnitude of the gravitational force an object exerts and experiences.
No, the charge on the particles causes a force to be felt on each of them. It does not cause a resistance. It is the mass of the particles that causes them to resist the force applied and thus taking time to accelerate. (A particle of zero mass would accelerate instantly to infinite speed)

3. Mar 14, 2012

### sophiecentaur

The word "resistance" is not the best one to use because that implies dissipating / losing energy - as with electrical resistance. A better word would be "reaction against", which also ties in with Newton's Third Law - 'Action and reaction are equal and opposite'.

Also, you may find it easier to talk of purely mechanical forces (pulling with a force meter or string) so that we don't get bogged down with electric forces, which is just adding more complication because the Coulomb force depends upon separation.

I can't actually see the distinction between your two scenarios. If you are trying to change a particle's "state of motion" then you need to apply a force, by some means or another. Acceleration is a rate of change of Velocity, which is a vector quantity, in which direction and magnitude are both important.
f=ma does not distinguish between change of speed or change of direction. The existence of mass will be equally apparent if either quantities is changed.

4. Mar 15, 2012

### nouveau_riche

the electromagnetic force causing an opposition to force F will be measured as resistance,what you suggest applies well to object that has mass,but in the situation i gave you the particles have no mass initially they just have started to feel the field of the other one
or what i mean to speak of is inertial mass of particles observed knowing the fact that before the moment of observation they were mass less

5. Mar 15, 2012

### nouveau_riche

i am applying a force F as mentioned

6. Mar 15, 2012

### sophiecentaur

Sorry, I am looking for a 'distinction'. That answer has only 'one side' to it. CAn you explain the difference?

7. Mar 15, 2012

### sophiecentaur

If there is no mass then there will be no force - the movement will be instantaneous and can't be discussed in the context of Mass. I think you may be making some implied assumptions in the model you have in your head.

8. Mar 15, 2012

### nouveau_riche

you are still missing the electromagnetic interactions(giving an perception of force)

9. Mar 15, 2012

### f95toli

But even the effects of "electromagnetic interactions" depend on mass, F=ma hold regardless of what is the cause of F, if there is no mass there can't be a force,

10. Mar 15, 2012

### nouveau_riche

suppose there are two particles(mass less but have charge)
when they just start to interact,describe the situation

11. Mar 15, 2012

### DrewD

You cannot deal with massless particles in the framework of Newtonian Mech. If there is ANY net force, and mass is zero, there would need to be infinite acceleration. That is silly when dealing with classical mechanics, but you can probably convince yourself the a massless particle, when considered in the framework of relativity, would move at the speed of light.

Lets pretend the particle is not massless, but instead has very little mass.
There is no "resistance" from the EM interaction, just another force. The "resistance" to acceleration from mass is intrinsic and is called inertia. The EM interaction is an external force that will accelerate the particle, not keep it moving at constant speed.

Note that you don't have mass because you are on earth in a gravitational field. You have mass no matter where in the universe you are. Your inertia is the same if you were speeding along through empty space, but when on earth, there is also a force (which oddly is also proportional to your mass) that keeps you on the surface of earth.

There is no answer to your question, but I hope this clears up why there is no answer.

12. Mar 15, 2012

### nouveau_riche

so according to you there should be no place for any mass less charged particle?

13. Mar 15, 2012

### DrewD

No: Massless particles cannot be dealt with in classical mechanics.

14. Mar 15, 2012

### Drakkith

Staff Emeritus
No, you are adding TWO forces to the issue, not just one. This is covered in classical physics. You have to find the magnitude and vector of the net force from adding the two forces and their vectors together.

15. Mar 16, 2012

### nouveau_riche

and if you go outside classical mechanics?

16. Mar 16, 2012

### nouveau_riche

then how is the mass of a quantum particle measured?

17. Mar 16, 2012

### f95toli

You need to use some fairly advanced QM, such as the Dirac equation.

That said, there are as far as we know no charged, massless, particles. The only thing I can think of that comes close is e.g. quasiparticles in graphene (which are described by the Dirac equation), but they are of course not "real" particles.