# Is gravitational force affected by intervening medium?

If we leave a iron ball and feather into the water, feather returns to the surface and floats or moves into the water slowly. On the other hand, iron ball (of certain mass greater than mass of feather) moves into the water directly. Here, both iron ball and feather are accelerated towards the earth to the same extent. Then why does feather moves slowly into the water than iron ball? Shouldn't they reach the earth at the same time.

If we assume that water medium is making the feather to accelerate to lesser extent than iron ball, then doesn't it mean gravitational force is affected by intervening medium?

Similarly if we consider a feather and iron ball to be left from certain height, I hope iron ball reaches the earth first and then feather. The reason I think is because of air resistance, doesn't this mean air medium or intervening medium is affecting gravitational force?

We know that gravitational force is not affected by intervening medium, but it is contrary to the above situations. Is it that gravitational force is affected by intervening medium?

I might have misunderstood sometimes, if so, pardon me and explain.

• Its important to note that magnetic force and electric force are affected by intervening medium. Apr 3, 2014 at 17:26
• innisfree basically answered the question, but basically the answer is "yes, gravitational force is not affected by intervening medium". The reason the feather drops more slowly in water than iron has to do with viscous fluid damping, not gravity. Apr 3, 2014 at 23:07

Your thought experiment of dropping an iron ball and a feather need not be in water; in fact, it is more commonly considered in air, but the pertinent facts are the same.

All objects, regardless of their mass or composition, are accelerated identically by gravity.

But within a particular medium, the acceleration of particular objects might be impeded by greater resistance than that of other objects. In ordinary air, your feather will fall more slowly than an iron ball, because of air resistance.

• youtube.com/watch?v=-4_rceVPVSY Apr 3, 2014 at 15:59
• Thank you for the answer. Did you mean that gravitational force is affected by intervening medium? Yes or no:) Apr 3, 2014 at 16:59
• I feel you haven't answered my question yet. Still +1 for your precious time. I hope we will enjoy discussion with you from here onwards:) Apr 3, 2014 at 17:06
• It's not that the force of gravity is affected by the medium, but that the medium exerts a different force on objects that move through it. Apr 3, 2014 at 17:11
• @GeorgeG: lets consider that we leave the iron ball from certain height, it falls on the ground at time t. Now lets say there is no air, would the ball reach at the same time? I think it will not. I hope it will reach at time less than t. Doesn't this mean force is affected by intervening medium? I will be happy to know if there is anything wrong here:) Apr 3, 2014 at 17:22

As you said, if feather has a stronger decelaration, is due to air friction (not because iron is more dense than feather or something).

The same should hold in absence of gravity: if you give a boost to these balls, then the feather one will stop first.

From this last consideration, I'd say that this medium does not affect gravitational force, but just it adds its effect to the gravitational force one.

• (+1). Good contradiction to OP's idea. Apr 17, 2014 at 12:59

To analyze whats going on you need to sum all forces that affect the objects in question. For each object, the feather and the ball, the forces of gravity are identical. The other forces are not.

Even though the people who replied are correct, fun fact: the speed that gravity is propagating is very slightly affected by mediums. Here are 2 pages from Kip Thorne's lectures at Les Houches. Gravitational waves are affected by a medium through a dispersion relation, but the effect is ridiculously small and is neglected.

• Did you mean gravitational force is affected by intervening medium? (+1) Apr 3, 2014 at 17:34
• No the force strength is the same. It was just a fun fact. The speed that gravity is mediated is the quantity that changes. What I mean is, gravity will take longer to affect you in a medium. Apr 3, 2014 at 17:39
• Gravitational force does not depend on intervening medium Apr 4, 2014 at 9:36
• Yeah thats what I said Apr 4, 2014 at 9:52

There are multiple forces involved in your thought experiment. Namely, the gravitational force and the 'force' of air resistance. This does not mean that the presence of the air affects the gravitational force.

As another thought experiment, consider two planets orbiting in space. Somewhere between these two planets there is a point where the gravitational force goes to zero (side note: there are actually 5 of these points in a co-rotating frame, known as Lagrange points). At this point the net force on an iron ball or a feather is zero even though the gravitational force from both planets is still acting in full.

• (+1) If there was no air medium, there would not have been air resistance, then both feather and iron ball would had reached at the same time. Because there is intervening medium, they reach at different time. Then doesn't this mean, intervening medium is affecting force between the two? Apr 3, 2014 at 17:11

Let's use a spring scale in water s.t the spring scale is sealed in a air tight glass box containing only vacuum so as to remove the buoyancy. Putting both the feather and the iron ball of same masses on the scale we notice that the spring shows the same reading(say $a_0$) for both feather and iron ball. Hence the gravitational force on both the feather and iron are same.
Now let's repeat this experiment by putting the spring scale enclosed in the box in the air(or oil or some other material). We notice that the spring scale again gives the reading $a_0$ for both feather and iron ball.
Hence we conclude that the medium does not affect gravitational force.

• (+1) I am understanding what you are trying to say than others had said. I have certain things in my mind, I will get resolved from them to accept your answer. Apr 17, 2014 at 13:45
• @GODPARTICLE Your question is good. It was unfairly downvoted (+1). Infact I got the idea from Antonio's answer. Antonio's idea is: Apply same force on feather and iron in water and notice the difference in acceleration, measure the difference again in due to gravitational force. My answer is similar to his. Apr 17, 2014 at 13:53
• "Hence we conclude that the medium does not affect gravitational force." You conclude incorrectly, there is a buoyant force in the system now, but the weight of each object is unaffected. Apr 23, 2014 at 3:48
• @dmckee Thankyou for notifying the mistake I have edited my answer. Apr 23, 2014 at 12:53