Gravity is a force or just curve space.

[mah123]

1)Nasa uses gravitation force of planets to propel space craft & also to change direction of space craft.

Gravity is just space curvature then it can only change the direction of space craft but to accelerate it. I think, we must require force because kinetic energy is get added into it.

2)This addition of kinetic energy is only done by work done in the direction of motion.

3)Can work done is possible with out any force in the direction of motion?

4) In this situation energy of space craft increases then who looses the energy. Co-ordinate system looses the energy which is in direct contact with space craft or anything other looses the energy.

[studiot]

It's not an 'either or' situation.

 

If you want to consider energy you are better off working in terms of forces.

 

Using the curvature tensor is substantially more difficult.

 

This is a very common situation in Science and Technology.

 

There is more than one method of analysis.

 

Lazy people like me choose the easiest to get the job done.

[swansont]

To add to what studiot has said:

 

After deciding which approach to take, you need to be consistent is the application of the physics — not mixing things that hold in one system but not the other. Similar to when choosing a frame of reference or coordinate system in which to solve a problem — you pick what makes solving the problem the easiest.

[The Almighty]

To change direction, internal energy of space craft is used.

 

3)Can work done is possible with out any force in the direction of motion?

Work done is the dot product of Force and Displacement. So a component of force in the direction of motion is only necessary.

 

4) In this situation energy of space craft increases then who looses the energy. Co-ordinate system looses the energy which is in direct contact with space craft or anything other looses the energy.

Energy of spacecraft is not increasing. Chemical energy from fuel is converted to kinetic energy

[mah123]

Without burning fuel acceleration happens:-

Just consider some space craft is fired from earth & moving with velocity more than escape velocity of Mars & it is passing through gravity of mars with its fuel tank empty. Then

1) Definitely due to Mars gravity it will first accelerate & attend the velocity much more than escape velocity & near to Mars, it will move in curve path. This spacecraft will again escape from Mars gravity. Now, as it will go away from Mars, its velocity will definitely get reduce to some extent.

2)When spacecraft is approaching the Mars increase in the kinetic energy happens, means work is done. Who is doing the work?

Gravity as force doing the work

or

curve space applying force & doing the work.

3) Ultimately who is loosing the energy co-ordinate system directly in contact is loosing the energy or Mars apply the force & loose the energy when kinetic energy of space craft increases..

[MigL]

These are two different models, but both make valid predictions for their area of applicability.

Their use is dictated by the circumstances, and generally, the Newtonian force is an extremely good approximation ( and much more simple ) of Einsteinian space -time curvature at the low mass-energy ( or low space-time curvature ) limit.

[mistermack]

1)Nasa uses gravitation force of planets to propel space craft & also to change direction of space craft.

Gravity is just space curvature then it can only change the direction of space craft but to accelerate it. I think, we must require force because kinetic energy is get added into it.

I don't think that follows. Space curvature causes objects to accelerate every day. Just drop a brick on your foot, and you will discover this.

When two massive objects move closer together, the curvature changes, and I believe that the kinetic energy comes from the change in curvature of the system, which is equivalent to a change in the potential energy of the massive system.

Edited June 26, 2017 by mistermack

[mah123]

You are true but change in kinetic is happens due to change in momentum & rate of change of momentum is force. So, how could we avoid any force action in this event.

Edited June 27, 2017 by mah123

[mistermack]

You are true but change in kinetic is happens due to change in momentum & rate of change of momentum is force. So, how could we avoid any force action in this event.

Basically, you are talking about the same thing. The only difference is the method you use to describe it.

There is a thing called gravity that causes mutual attraction between masses. You can model it as a force, or as curvature of space time. But you are talking about the same thing.

General relativity works as a model of gravity, to a high degree of accuracy.

 

If you look on the wikipedia page for general relativity, you will find this telling passage :

 

" Some predictions of general relativity differ significantly from those of classical physics, especially concerning the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light. Examples of such differences include gravitational time dilation, gravitational lensing, the gravitational redshift of light, and the gravitational time delay. The predictions of general relativity have been confirmed in all observations and experiments to date."

 

So if you want to stick with classical physics, and ignore GR, you run into problems.

[mah123]

Thanks but all this time dilation & curving geometry can explain change in kinetic energy of a substance because if it is just the motion of matter in this curve geometry without any action of force (or rate of change of momentum is zero in that curve space) then its kinetic energy will have to remain same.

 

 

Then, this is just the motion of object in its own path by its own way.

Edited June 30, 2017 by mah123

[TakenItSeriously]

Thanks but all this time dilation & curving geometry can explain change in kinetic energy of a substance because if it is just the motion of matter in this curve geometry without any action of force (or rate of change of momentum is zero in that curve space) then its kinetic energy will have to remain same.

 

 

Then, this is just the motion of object in its own path by its own way.

As I understand it, how a space probe gets some net gain in speed from a gravity assist is that the probe does a near flyby crossing behind the orbital motion of the planet.

 

Because the probes flyby is behind the planets orbital motion therefore the planet slows down an infinitessimal small amount but the probes momentum is curved towards the motion of the planet so its direction is changed but also its net velocity is considerably greater.

 

Or at least thats what I think must be happening from various graphic representations that I've seen.

Edited July 21, 2017 by TakenItSeriously