# Does the force of gravity ever have a reaction force?

1. Oct 25, 2007

### pakmingki2

for example, a book is at rest on a table.

One might say that the normal force is the reaction force.

BUt reaction forces always have to be equal in magnitude, and you put an object on the book, then the normal force would increase to keep the book at rest, therefore the normal force is not the reaction force to gravity because it doesnt have to be equal in magnitude to it.

So what would be the reaction force to gravity?

2. Oct 25, 2007

### D H

Staff Emeritus
Regarding a book at rest on a table: the normal force is not the reaction force to gravity. The normal force is the reaction force to the book pressing on the table. This pressure can in turn come from gravity or from a clamp if the table is upside down or floating in space.

The reaction force to gravity is gravity. The Earth exerts a two pound force on a two pound book, and that two pound book exerts a two pound force on the Earth.

3. Oct 25, 2007

### pakmingki2

but if you can say the reaction force to gravity is gravity, how can you say the reaction force to friction is friction?

so if you push the book lightly foward and it doesnt move, there must be a frictional force opposing the force of your hand. But your hand and the friction are not action reaction pairs. WHat is the reaction force to friction in this case?

4. Oct 25, 2007

### Staff: Mentor

OK, Newton's second law tells us there's a friction force on the book.
The table exerts a friction force on the book; the book exerts a friction force on the table.

5. Oct 25, 2007

### D H

Staff Emeritus
... and the book exerts a friction force on the hand. There are several action-reaction pairs in this example of a book resting on a table being pushed by a hand. To make the list a bit smaller, I am placing the book directly on the Earth's surface:
• Hand pushing on book / book pushing back on hand
• Book pushing horizontally on table / table pushing back on book
• Earth gravity pulling vertically downward on book, book gravity pulling Earth upward
• Book pushing vertically downward on Earth's surface / Earth's surface pushing upward on book.
The list becomes even longer when one places the book on a table.

6. Nov 3, 2007

### Scorpixx

I do not understand your question, and that part. I'd see gravity as an acceleration, not a force. Adding books, masses are changing, then the forces are changing, but gravity stays the same.

7. Nov 3, 2007

### Staff: Mentor

If you put an object on top of the book, that object exerts a force on the book, which then exerts the same force on the table and the table exerts an equal force on it. All the force pairs remain equal.

8. Nov 3, 2007

### ineedmoney

Gravity is a meauserment of acceleration and it's reaction force is the ground or a table holding a book above seawater. So whether the reaction force is the gravity or the table's molecules ability to bond. Friction is the force of two surfaces in contact. It is not a fundamental force, as it is derived from electromagnetic forces between atoms.

9. Nov 4, 2007

### vjraghavan

All known forces are found to obey the third law. As rightly pointed out earlier, "Gravity" is the reaction force of "Gravity," as the law itself mentions that for every force, there is "equal(both qualitative and quantitative)" and "opposite(in direction)" reaction.

10. Nov 4, 2007

### craghunath

let us consider a body of mass M in free space, is in contact with a smaller mass 'm'.
both are accelerating in such a way that (m is in front of M) OR M follows m ............ gravitational forces do not exist in this setup.

In this condition what is the action and reaction pair?

M exerts a force "Ma' on smaller mass, if 'a' is the acceleration of the system....
The only reactionary force I can think is "inertia of the mass m" to the force Ma.

Any thoughts.....??

11. Nov 4, 2007

### Staff: Mentor

If M & m are in free space, why are they accelerating? There must be an external force acting on them. If, for some reason, M is exerting a contact force on m (perhaps they collided), then m exerts an equal and opposite contact force on M.

12. Nov 4, 2007

### Scorpixx

Correct. But note that even without contact, they also have gravitational forces acting on each other, described by Newton as F = G (universal gravity constant) x M x m / r² (distance separating the two objects).

13. Nov 4, 2007

### Staff: Mentor

Have you been taught how to draw a free-body diagram? If so, draw two free-body diagrams, one for each object. In Newtonian mechanics if the bodies are interacting then there will be some force on one diagram that will have the same magnitude and opposite direction as a force on the other. This is the action-reaction pair. They are always of the same "kind". Frictional forces are equal and opposite other frictional forces, normal forces to other normal forces, gravitational forces to other gravitational forces, etc.

14. Nov 4, 2007

### craghunath

thank you Doc Al!:)
I got it, if there is no reaction pair simply one body will merge into other. Just as knife gets into the butter.

15. Nov 4, 2007

### rcgldr

Using the example of two objects in space. There is an attractive gravitational force between them, absent any other force, the net force on each object is imbalanced, and the two objects accelerate towards each other. Their rate of acceleration at any point in time will be equal to the gravitational force divided by their respective masses. Each objects mass times that objects rate of acceleration could be considered a reaction force.

Now assume that each object has a rocket engine attached and that the distance between the objects is held constant. The force from the rocket engine cancels the force from gravity, there's no acceleration and there is no reactive force in this case, both forces are real. Gravity could be replaced with a string producing an inwards tension force to oppose the outward thrust forces from the pair of rocket engines. Again the forces in this case are real and there is no reactive force.

Then switch the example to that of a single object with a rocket engine causing the object to accelerate. The force is not opposed, so the object accelerates. The reactive force of the object is equal to it's mass times it's acceleration at any point in time.

16. Nov 5, 2007

### Staff: Mentor

This is incorrect. In the rocket engine there is a reactive force: the rocket exhaust exerts a force on the rocket engine which is equal and opposite the force that the engine exerts on the exhaust. There is always a reactive force in classical mechanics.

17. Nov 5, 2007

### Staff: Mentor

My point was that having M and m in free space yet accelerating together implies that there's some external force acting on them. With no external force, the center of mass of the system will move at constant velocity.

If m and M do interact, they will exert equal and opposite forces on each other.

18. Nov 5, 2007

### Staff: Mentor

You must be using the term "reaction force" in some sense other than the OP. Here "reaction force" just means the 3rd law pair in an interaction, also known as "action/reaction" pair.

True, if the net force on the object is zero, then it will have no acceleration. But this has little to do with action/reaction pairs. The force from the rocket on the object has a reaction: the force of the object on the expelled gases; the force of gravity on the object has a reaction: the gravitational force of the object on the other.
All of these forces have action/reaction pairs.

Again, you are using a different meaning of "reactive force".