# Do objects resist acceleration when falling?

1. Dec 28, 2013

### inertiaforce

If I try to accelerate an object to the left, its mass resists that acceleration. If I try to accelerate an object to the right, the same resistance occurs. If I try to accelerate an object forward or backward, the same resistance occurs. This resistance to acceleration is known as inertia.

Gravity is a constant acceleration. If I drop an object, its velocity is constantly increasing under the force of gravity, which means it is constantly accelerating.

Therefore, since it is constantly accelerating, the object's mass should theoretically resist that acceleration the same way it did when it resisted acceleration to the left or to the right or forward or backward.

This resistance to acceleration applies a force against the accelerating force.

Therefore, this means that there is a force against gravity, since gravity is the accelerating force.

Is this correct? Is there a force against gravity?

Last edited: Dec 28, 2013
2. Dec 28, 2013

### TumblingDice

Hey there inertiaforce!

Good point for a new thread and a place to take one step back. At least that's how I felt after reading your post. (LOL)

Gravity originates from mass. The mass you're speaking about has its own gravitational effects on other mass. The gravity you mention might be Earth's, but wherever it also originates from mass.

So if you *really* want to drill into this one , I'm thinking that the two significant interactions are the two gravitational forces of each mass. Mutual attraction. Each one will affect the other, even though the Earth is awfully large and doesn't show it...

3. Dec 28, 2013

### TumblingDice

Which just might be a better way to view this. IOW, the Earth would appear to be putting up resistance to the gravitational effect of the mass you spoke of (Object A?).

Must be proportional to mass whether calculated from who accelerates more or who resists more. (6 of one...?)

4. Dec 28, 2013

### DrStupid

Forget this "resistance to acceleration" and refer to Newtons Laws of motion.

5. Dec 28, 2013

### AlephZero

+1.

The basic rule of PF is: this is a site about mainstream physics. If you want to ask questions about Newton's laws of motion, that's fine. If you want to have a historical discussion about what Newton meant by "vis insita" (Principia, definition III) that's also fine - so long as you actually read the relevant parts of Newton's "Principia" first.

But woolly notions of your own about "resistance to acceleration" are something else, IMO.

6. Dec 29, 2013

### Buckleymanor

The object's mass does not resist acceleration by gravity the same way as the resistance to the left or right forewards or backwards.
You can test this by dropping two objects of different masses they both accelerate at the same rate.
If you take the same two objects and try to accelerate them to the left or right at the same rate they will require a different amount of force, the larger one will require more force to accelerate it at the same rate as the smaller one.

7. Dec 29, 2013

### Staff: Mentor

This is incorrect. As in the previous thread "resistance to acceleration" is mass. Mass does not apply a force against gravity.

For gravity $F_g=GMm/r^2$. So each mass contributes to the force, it does not oppose it.

8. Dec 29, 2013

### Crazymechanic

exactly mass attracts all other mass, and the only force that opposes that has to be exerted on the mass which we want to move , because without that force nothing will happen

and inertia works in all frames whether moving something upwards against gravity like a space rocket or when someting falls , like a rock from a mountain top.the only difference is in the amount of inertia.which depends on the mass of the object and the velocity.

Last edited: Dec 29, 2013
9. Dec 29, 2013

### sophiecentaur

If you are looking for a reactive force to satisfy Newton 1, you can consider the force of attraction, towards your mass, which is acting on the Earth.

10. Dec 29, 2013

### voko

Historically, the following played a role in developing the modern ideas of gravity.

Every body has "gravitational mass". The force of gravity on a body (i.e., its weight) is proportional to its "gravitational mass".

Every body has "inertial mass". The inertial mass resist any force acting on the body; the acceleration is inversely proportional to its "inertial mass".

A series of careful experiments (by Eötvös) established that the "inertial mass"of a body was very accurately equal to its "gravitational mass". Thus the proportionality and the inverse proportionality cancel each other, and the acceleration of a body due to gravitation does not depend on its mass.

11. Dec 29, 2013

### Jilang

The best way to think about it is that the acceleration is affected by the mass
a=F/m
So the bigger the mass the smaller the acceleration will be for a given force.
In the case of gravity, the force is also proportional to the mass, so the formula for the acceleration has the mass on the top of the formula and also in the denominator. The m's then cancel out and the acceleration due to gravity is the same for all m.

12. Jan 1, 2014

### inertiaforce

The same is true when accelerating under gravity. The larger mass requires more force to accelerate it than the smaller mass.

The only reason two objects of different mass fall at the same exact rate is because the ratio of the gravitational force to the inertia is the same. For example, a penny has little mass, so it has little gravitational force acting on it. That's why it weighs very little. On the other hand, a car has a lot of mass, so it has a lot of gravitational force acting on it. That's why it weighs a lot.

Although the penny has little gravitational force acting on it, it's small mass also has very little inertia, so it is very easy to accelerate. On the other hand, although a car has a lot of gravitational force acting on it, it's large mass also has a lot of inertia, so it is very hard to accelerate. Therefore, the ratio of gravitational force to inertial resistance is the same for every object, and that's why every object falls at the same rate. But the force acting on the larger mass to get it to accelerate is much greater than the force acting on the smaller mass to get it to accelerate.

So when you say "If you take the same two objects and try to accelerate them to the left or right at the same rate they will require a different amount of force, the larger one will require more force to accelerate it at the same rate as the smaller one", this is also true with downward acceleration due to gravity as well. The larger mass will require more force than the smaller one. This is not limited to horizontal acceleration. The larger mass has a much larger accelerating force acting on it even in the downward direction than the smaller mass does. The only reason they fall at the same rate is because the ratio of the accelerating force to the inertia resisting the acceleration is the same. That's why they accelerate at exactly the same rate.

13. Jan 1, 2014

### inertiaforce

Guys, let me try to clarify my question.

If you accelerate an object to the left, it will resist your attempt to accelerate it. By resisting your attempt to accelerate it, a force is produced against the accelerating force. This force against the accelerating force is equal in magnitude to the accelerating force and opposite in direction to the accelerating force.

If you accelerate the same object to the right, the same effect occurs, a force is produced equal in magnitude but opposite in direction to the accelerating force. The same thing happens if you try to accelerate the object forward or backwards.

This force that is equal in magnitude and opposition in direction to the accelerating force is produced by the object's mass resisting acceleration (its inertia).

Since the object's mass resists acceleration forward or backward, or left or right, then theoretically, it should also resist acceleration downward.

Since gravity is a downward accelerating force, this means that the object would produce a force equal in magnitude but opposite in direction to the downward accelerating force, the same way it produced a force equal in magnitude but opposite in direction to the leftward accelerating force, rightward accelerating force, forward accelerating force, or backward accelerating force.

Since the downward accelerating force is gravity, this means therefore that the object is producing a force equal in magnitude but opposite in direction to the force of gravity.

Is there such a force, produced by an object's inertia, equal in magnitude but opposite in direction to the force of gravity?

14. Jan 1, 2014

### sophiecentaur

This has already been pointed out. There is an equal force on the Earth, upwards,to the force, downwards that is pulling the mass. Because it is so massive, the Earth doesn't move appreciably. If we replaced the force of gravity with a piece of string under tension, the action and reaction forces would be more obvious, perhaps.

15. Jan 1, 2014

### inertiaforce

Ok so the answer is yes? There is a force, produced by an object's inertia, equal in magnitude but opposite in direction to the force of gravity?

16. Jan 1, 2014

### sophiecentaur

If you want the answer "yes", then yes, there is.

17. Jan 1, 2014

### inertiaforce

That is amazing.

And what you are saying is that this force, equal in magnitude, but opposite in direction to the force of gravity, pulls back on the earth then? But because the earth is so massive, the earth doesn't move under that force?

18. Jan 1, 2014

### Staff: Mentor

It sounds like you are talking about Newton's 3rd law here, not his 2nd law. A 3rd law reaction force acts on a different body, and it is not due to the first body's inertia. It is due to the interaction force between the two bodies.

I have never seen that characterization of the 3rd law. Specifically, I have never heard it as being due to the inertia of the other object. Do you have a reference for this?

It does. The object pulls up on the earth with an equal and opposite force to the force of the earth pulling down on the object.

No, it is caused by the object's gravity, not its inertia.

Last edited: Jan 1, 2014
19. Jan 1, 2014

### TurtleMeister

There's nothing special about gravity in this respect. Try working with two objects of less extreme mass difference and you can understand it better.

20. Jan 1, 2014

### inertiaforce

When I asked sophiecentaur the question "There is a force, produced by an object's inertia, equal in magnitude and opposite in direction to the force of gravity?"

Sophiecentaur responded with "If you want the answer "yes", then yes there is."

Now you're saying no there isn't. You're saying it's caused by the object's gravity.

So who's right lol?