# I 1N Force and going near the speed of light?

Tags:
1. Jan 20, 2017

### Arman777

Lets suppose we are in a space and theres no external force that affects our system,which our system is simply one object which we can think it is a box and it has 1 kg mass and there is us.

The object ,lets call it A,Its initally rest relative to us so it can be our inertial referance frame So We pushed the A and we applied a force to the A ( Lets call it 1N).It started to accelerate.(I am not sure at this point A will be still our inertial referance frame or not).

In this case we are observer and we will think we are stationary so the A will start to accelerate with constant acceleration $\vec F=m \vec a$ (The motion is only in one direction like +x) so, $F=1N$ and $m=1kg$ so $a=1\frac m s^2$

We know that theres no external force affecting our system.So after some time the objects speed will increase.And after a period we will use relativity theories to calculate A 's speed.The thing is I just wanted know In this circumtances , With $1N$ force object can reach $0.9999999....c$ if its given enough time ?

If you also give an answer to the question that I wrote in bold I'll happy,

Thank you

2. Jan 20, 2017

### oz93666

You say .... "We pushed the A and we applied a force to the A " well that makes it accelerate away from you , so how are you going to continue to apply a force ??

But if you could find a way of applying a 1N force to a 1kg mass ... then yes , in theory you could get it to 0.9999999 c

3. Jan 20, 2017

### Merlin3189

I don't think A can be an inertial frame of reference, as it is accelerating.

Since you say, we think we are stationary observer, then you are defining yourself, the observer as the frame of reference. Since you are applying the force to A, then A is applying an equal and opposite force to you (Newton's 3rd law) and you are also accelerating. So you also are not an inertial reference and neither can be any other frame in which you are stationary.

4. Jan 20, 2017

### Arman777

Well It gained some acceleration right.Theres no external force so it will keep its state of motion.It will accelerate with constant $a$.I dont need to apply force constantly.
Thats my idea

5. Jan 20, 2017

### Merlin3189

Keeping it's state of motion, means no acceleration: constant velocity.
If you want acceleration, a force needs to be applied. $a=\frac{F}{m}$ if the force is 0, so is the acceleration.

6. Jan 20, 2017

### ZapperZ

Staff Emeritus
Your "idea" is wrong. When there's no longer an applied force, it will not accelerate (Newton's First Law).

Zz.

7. Jan 20, 2017

### Arman777

Ok I got it thanks.I also understand the referance frame case thanks for that too.It will accelerate and gain some velocity (lets call it v) but when I cut force there will be no acceleration so it will move with a constant velocity v.
For referance frame.A cannot be an inertial referance frame cause its accelerates.We need constant velocity object to choose an inertial referance frame.

8. Jan 20, 2017

### Staff: Mentor

...but you cannot get to 99.999...C

9. Jan 20, 2017

### Arman777

İs it some kinda of joke .I didnt understand

10. Jan 20, 2017

### A.T.

Not even to 0.999... c

11. Jan 20, 2017

### Arman777

sure we need huge energy a lot of energy.Even for an atom we need huge amount of energy for a 1 kg object we need a lot

12. Jan 20, 2017

### Staff: Mentor

Oops, lol.

13. Jan 20, 2017

### Staff: Mentor

Badly played joke. In math language, 0.999...=1 and since you can't go C, you can't go 0.999...C either.

14. Jan 20, 2017

### Staff: Mentor

Accelerating 1 kg to 99.99% the speed of light needs 6*1018 J, about the world energy consumption of a week.
Accelerating a proton to 99.99% the speed of light needs a large accelerator. The SPS and the LHC, Tevatron (and its preaccelerator), HERA and RHIC can/could do that. The energy per proton is tiny - 0.01 microjoule - but getting that energy per proton is not easy.

Accelerating a proton (well, actually 6*1014 protons) to 99.9999990% the speed of light (1.06 microjoule per proton, 600 MJ in total) is done at the LHC, it is the current record for accelerators.

15. Jan 20, 2017

### oz93666

You're figures are way off mfb ... at .9999c mass is 10 tonnes needs 4.5 * 10 ^20 J...

but at 0.9999999 c the original 1kg mass would be so much greater .....trillions of tonnes I would guess...

16. Jan 21, 2017

### weirdoguy

17. Jan 21, 2017

### oz93666

So are you saying the 1kg mass at 0.9999999c would not be "so much greater in mass"?
Are you saying the equation in post 15 is not valid?
If so , what is the mass at 0.9999999c?

18. Jan 21, 2017

### weirdoguy

1kg. Read again the article that I gave you.

19. Jan 21, 2017

### oz93666

Perhaps you can explain it to me ....I don't understand it.

20. Jan 21, 2017

### ZapperZ

Staff Emeritus
It looks like we are back on this issue again.

There's nothing wrong with the equation. It is the "concept" of "relativistic mass" that is the issue. Please note that even the "originator" of this idea of relativistic mass, Albert Einstein, stopped using it after he realized that this might be problematic. Read this post as well as that Insight article: