# Free fall

Hello, I was wondering under what conditions does free fall need to comply to in order to take effect? I am pretty certain that for an object to be in free fall it needs to be void of any friction, but what about air resistance? Or is air resistance a form of friction? Thank you in advance.

Hello, I was wondering under what conditions does free fall need to comply to in order to take effect? I am pretty certain that for an object to be in free fall it needs to be void of any friction, but what about air resistance? Or is air resistance a form of friction? Thank you in advance.

A freely falling body has no force other than gravity acting on it. That is why bodies in orbit are in free fall.

Drakkith
Staff Emeritus
Air resistance is a form of friction yes.

fluidistic
Gold Member
Hello, I was wondering under what conditions does free fall need to comply to in order to take effect? I am pretty certain that for an object to be in free fall it needs to be void of any friction, but what about air resistance? Or is air resistance a form of friction? Thank you in advance.

Drakkith is right and so is Fewmet.
Most of the time we say a body is in free fall even if it falls within the air. It's because in some situations air resistance is negligible and hence the total force acting on the falling body is almost the same as the force of gravity, but a very bit lesser due to air resistance.

Air resistance is a form of friction yes.

I don't suppose there is a rigorous definition of friction, but I question saying that air resistance is a form of it. I think of friction as arising from two surfaces moving side-by-side. While there is an element of that acting on a body moving through air, I suspect the third law force from pushing air out of the way plays a significant role.

It would be an interesting thing to measure experimentally. Maybe I'll talk some students into it.

Drakkith
Staff Emeritus
You are correct Fewmet, it is both.

Thank you all so very much.

"While there is an element of that acting on a body moving through air, I suspect the third law force from pushing air out of the way plays a significant role."

If air wasn't pushed out of the way the body would not be falling anymore.

How do you guys quote only part of the post?? Quote message in reply option is disabled !!!

Drakkith
Staff Emeritus
I just hit quote and then delete what I don't want to quote, or I copy and paste a section and use the quote button once I highlight it.

I think there is air/fluid friction at the boundary layer that sticks to the body and imparts momentum to the transitional layers of relatively still air. A more serious friction occurs where there is turbulence destroying the boundary layer and energy is wasted driving the turbulence.

An interesting question.....
Post #2 is not very satisfying....reading a bit more in the Wikipeida article helps, but I'm not especially impressed:

http://en.wikipedia.org/wiki/Free_fall

Other perhaps more subtle insights are here:

http://en.wikipedia.org/wiki/Weightlessness

This offers some insights:

In all inertial reference frames, while weightlessness is experienced, Newton's first law of motion is obeyed within the frame. Inside the frame (for example, inside an orbiting rocket or free-falling elevator), objects in linear motion stay in motion, at the same speed and direction. Objects at rest tend to stay at rest. Objects not in contact with other objects "float" freely.

Air resistance is a form of friction called drag.

The frictional force is proportional to the normal force of the two surfaces touching, while drag is proportional to the velocity and area of the object perpendicular to he velocity.

I would argue that when objects initially begin to fall in the earth's atmosphere, they are just about in free fall because of the negligible drag. However, as the velocity increases, the drag force increases, and the object would no longer be in free fall.

Drakkith
Staff Emeritus
Technically anything falling within the atmosphere is not within free fall.
Free fall is any motion of a body where gravity is the only force acting upon it, at least initially.

Due to friction and having to move air out of the way, gravity is NOT the only force acting upon a falling body inside a medium. (Atmosphere, water, ETC)
However the use of Free Fall is still common for these situations.

Technically anything falling within the atmosphere is not within free fall.

Due to friction and having to move air out of the way, gravity is NOT the only force acting upon a falling body inside a medium. (Atmosphere, water, ETC)
However the use of Free Fall is still common for these situations.

At first, the amount of friction is negligible so I would say that one is in free fall, but the drag will quickly build.

But yes, I think technically free fall can only happen within a vacuum.

a massive point object falling can be considered as the most practical way of experimenting with free fall. Air resistance depends on shape while drag on square of velocity or something.