# Free fall definition

1. Jul 10, 2007

### MIA6

I am not so clear about the definition of 'free fall'. If an object is an example of free fall, then must its acceleration be 9.81m/s^2? For example, if a sky diver jumps from an airplane, because earth has atmosphere, the rate of acceleration decreases due to air resistanc. so now the acceleration is not equal to the gravity that the earth pulls it down, then is it still a free fall? Second example, a parachutist descending at a speed of 10.0m/s loses a shoe at an altitude of 50.0 m. My teacher said it was not a free fall because it didn't start at rest. Must a free fall motion start at rest? but the acceleration is still 9.8m/s^2. so even when it is not a free fall, its acceleration can still be equal to gravity that pulls it downward?
thanks a lot.

2. Jul 10, 2007

### nrqed

There does not seem to be complete consensus.
First, there is a difference between the meaning of the term in the context of physics and in the context of everyday language. But even within the contect of physics, I have been surprised to note differences.

My personal definition (and I think the most used one) is that free fall implies that only the force of gravity is acting. And the initial velocity has nothing to do with it (I am surprised your prof said the shoe would not be in free fall! It makes no sense to me since what one defines as the initial time is arbitrary. If one notices for the first time an object when the object already has some velocity, should one say that the object is not in free fall only because it had some velocity when it was first observed?). And according t my definition, when there is air friction, we do not have the conditions for free fall.

On the other hand, in everyday language, free fall includes often air friction but no parachute open (so it's not entirely consistent as an open parachute only introduces more air friction, not a new type of force).

Just my two cents.

Last edited: Jul 10, 2007
3. Jul 10, 2007

### Danger

I agree with both statements. It just doesn't seem practical to consider it free-fall if air resistance is involved. If you did, where do you draw the line when person A is dropping naked, person B has puffy clothes, person C has a parachute, and person D has a hang-glider?
As for the second part, a captured meteoroid that goes into orbit is considered to be in free-fall, but it certainly didn't start at rest relative to Earth.

4. Jul 10, 2007

### MIA6

btw, Is skydiving free fall? I don't think so because the air resistance is involved.

5. Jul 11, 2007

### Staff: Mentor

nrqed gave a good answer to that question.....

6. Jul 11, 2007

### mgb_phys

I thought the definition of skydiving was rather expensive fall

7. Jul 11, 2007

### MIA6

Pardon, what does that mean?

8. Jul 12, 2007

### Danger

Unless you're in the military, skydiving lessons are very expensive. You can learn how to fly a plane for the same price that it takes to learn how to jump out of one. (And the survival rate for failed pilots is a lot higher than that for failed parachutists.)

9. Jul 12, 2007

### mgb_phys

That's what I always thought. I'm a scuba diver, if anything fails I generally float up - in skydiving if anything fails you don't 'float' down!

10. Jul 12, 2007

### MIA6

ok, but anyway, Is skydiving a free fall? Is the path of free fall a vertical line or a parabolic path?

Last edited: Jul 12, 2007
11. Jul 12, 2007

### Staff: Mentor

Reread nrqed's response to this question. For a short answer, using the typical physics definition: no, skydiving is not free fall because forces other than gravity act on the falling object.

If the freely falling object has a horizontal component of velocity, it's path would be parabolic; if not, vertical.

12. Jul 12, 2007

### Danger

As stated, skydiving is not free-fall because of air resistance.
The path is irrelevant as long as it's toward a gravitational source. A satellite can be in a circular orbit or any number of elliptical paths. Something coming straight in from space would technically be on a straight trajectory, but it would appear curved from Earth because of our rotation. (Unless, of course, its path was curving around something else such as the sun.)

Last edited: Jul 12, 2007