# Don't objects in free fall always experience one g?

1. Oct 31, 2009

### mr.physics

I recently read an article about Joseph Kittinger which stated that in a skydive, he was subjected to a G force of 22 m/s^2. Disregarding fluid resistance, don't objects in free fall always experience one g?

2. Oct 31, 2009

### Staff: Mentor

You can get a higher amount of g's when you open your chute. And 22 m/s² is barely over 2 g's.

3. Oct 31, 2009

### mr.physics

4. Oct 31, 2009

Staff Emeritus
Because the chute is pulling you up.

5. Oct 31, 2009

### LURCH

No, objects in freefall experience practically zero g. That's kind-of the definition of freefall.

Can you link to or reference the article? Or cut and paste the line containing that statement, along with enough text to put it in context?

6. Oct 31, 2009

### blkqi

It must be referring to the acceleration experienced from the parachute at opening, as velocity decreases about 50 m/s.

7. Oct 31, 2009

### mr.physics

8. Oct 31, 2009

### blkqi

Zero G in free fall. One G upwards at rest on Earths surface at sea level. For the skydiver, <1G in the aircraft, 0G in free fall, ~4-5G at parachute opening, >1G on landing, 1G on the ground.

9. Oct 31, 2009

### mr.physics

Why?

10. Oct 31, 2009

### blkqi

G-force is your acceleration relative to free fall. 0G is free fall is weightlessness.

11. Oct 31, 2009

### blkqi

Neglecting drag of course, in the case of the skydiver.

12. Oct 31, 2009

### mr.physics

So an object experiencing a G force of 1 G is acclerating at 19.6 m/s^2?

13. Oct 31, 2009

### essecks

Isn't 1G 9.8 ms-2?

14. Oct 31, 2009

### simpleton

Free Fall is just acceleration by gravity. However, the parachutist is also experiencing an acceleration by the opening of the parachute as mentioned (To slow down the speed of descent). If you add up the 2 accelerations (by vector addition) the acceleration is greater than the acceleration due to gravity.

And I think blkqi meant -1G experienced on when we are on the ground, so we have a net acceleration of 0.

15. Oct 31, 2009

### Staff: Mentor

No. What blkqi means is that if you measure acceleration using an accelerometer then you find that free-falling objects have 0 acceleration and an object at rest on the ground has an acceleration of g upwards. This is the acceleration that an object "feels".

16. Oct 31, 2009

### blkqi

1G on the ground. You can think of g-force as the vector difference between the acceleration relative to the field and the acceleration experienced during free-fall, relative to the magnitude of free fall acceleration.

Thus in free-fall:
$$\frac{-9.8-(-9.8)}{|-9.8|}=0G$$

And on the ground:
$$\frac{0-(-9.8)}{|-9.8|}=1G$$

Last edited: Oct 31, 2009
17. Oct 31, 2009

### Phrak

According to the usage of this acticle, http://en.wikipedia.org/wiki/Free_fall#Record_free_fall", quoted above, "free fall" is the following:

"Near sea level, an object in free fall in a vacuum will accelerate at approximately 9.81 m/s2, regardless of its mass."

and

"Record free fall

Joseph Kittinger starting his record-breaking skydive. According to the Guinness book of records, Eugene Andreev (USSR) holds the official FAI record for the longest free-fall parachute jump after falling for 80,380 ft (24,500 m) from an altitude of 83,523 ft (25,460 m) near the city of Saratov, Russia on November 1, 1962. Though later jumpers would ascend higher, Andreev's record was set without the use of a drogue chute during the jump."

They imply that Free Fall in air occurs throughout the fall, and includes terminal velocity, where air drag exerts a balancing upward acceleration of nearly 1g. I say nearly one g because the diver slightly decelerates as the air thickens.

Last edited by a moderator: Apr 24, 2017
18. Oct 31, 2009

### Phrak

Paragraph 3 is talking about accleration due to an object in rotation. The article,

"(he went into a flat spin at a rotational velocity of 120 rpm; the g-force at his extremities was calculated to be over 22 times that of gravity, setting another record)."

--nothing to do with the vertical acceleration.