# Can i ever achieve this?

1. Nov 4, 2005

### gunblaze

Think..
Can i ever make an object in vacumm and on a frictionless surface move with constant speed.. from rest?

2. Nov 4, 2005

### ZapperZ

Staff Emeritus
Shine light on it and stop after it moves.

Zz.

3. Nov 4, 2005

### gunblaze

how?
by using light particles as a resistive force?

4. Nov 4, 2005

### ZapperZ

Staff Emeritus
Photons have momentum.

Zz.

5. Nov 4, 2005

### gunblaze

So u mean by using photons, i can provide an opposing force on the accelerating object.. then as the resultant force on the ball reaches zero, i switch off the light.. the ball will move at constant speed.

But just another qn.. How long will this take?

6. Nov 4, 2005

### ZapperZ

Staff Emeritus
OK, now you lost me.

You said you want to know how to get an object that was originally AT REST to move with a constant velocity.

My response was shine a light on to it. This will make it move (i.e. accelerate). When you're happy, stop shinning the light. No more source of that acceleration. The object will then simply coast with constant velocity. You DID say this thing is in vacuum with no friction.

Zz.

7. Nov 4, 2005

### gunblaze

Yes. After shining the light, the body will accelerate.. but how by switching off the light can the object move with constant speed? the body will continue accelerate even with the light switched off since there is no opposing force on the object right?

Last edited: Nov 4, 2005
8. Nov 4, 2005

### inha

No it won't. After the force is "turned off" there's nothing to accelerate the body.

9. Nov 4, 2005

### Fe-56

now there is sum of all forces affecting the body zero.

So F=0=dp/dt.......so.......dp/dt=0......so.....p=const....v=const...and thats it.

10. Nov 4, 2005

### ZapperZ

Staff Emeritus
Please study Newton's First and Second Law of motion.

Zz.

11. Nov 4, 2005

### Pengwuino

It will continue moving at constant speed because all forces have equalized (ie. in this case, no forces exist) and acceleration only occurs when there is a net force.

By the way, since it is on topic, what is our best understanding as to why photons have momentum but no mass?

12. Nov 4, 2005

### Fe-56

hm, and what is mass

13. Nov 4, 2005

### Pengwuino

I think we can start off with defining that, very good.

14. Nov 4, 2005

### Fe-56

Could be good definition that mass is that m in the equotation F=d(mv)/dt?
but how can we define that F now? :-))

15. Nov 4, 2005

### Pengwuino

I was looking for more of an answer instead of a Q&A session :P

16. Nov 4, 2005

### ZapperZ

Staff Emeritus
Even without the photon picture, classical EM radiation also contains something called "radiation pressure" (see, for example, Jackson's Classical Electrodynamics). This is due to the presence of E-fields in EM radiation. It is why the often-imagined solar sails are usually made of "metallic" or mylar surface. The conducting surface interact with the E-field of the photon more efficiently than insulating surfaces.

Zz.