# Parachute acceleration

Tags:
1. Dec 30, 2014

• moved from general physics forum
this is an exam question and i am not sure about my answer:

a man with a mass of 60 kg is a parachutist :

calculate the acceleration if the air resistance is 1200N

this is how i solved it:

we must take the wieght into consideration so his wieght is : w=mG G=approximatly 10ms^-2

60(10)=600N

so 1200-600= 600

600=60a

10=a

please tell me is this right?

2. Dec 30, 2014

### PWiz

Um the question sounds kinda funny. You're solving it right (except that the resultant force will be 600-1200 since you took the value of g to be positive, so the answer should be -10) but with a 1200N air resistance force, the man would actually fly upwards at 10m/s^2! Are you sure you've copied the question correctly?

3. Dec 30, 2014

### Staff: Mentor

That was my thought also. The math is right, but I don't often see parachutes falling up!

4. Dec 30, 2014

yes i dont think so either

5. Dec 30, 2014

yes i am 100% sure it is the question and i know it will be -10ms^-2 but i put it 10ms^-2 because it does sound wierd

6. Dec 30, 2014

### haruspex

It need not be falling up, merely accelerating up. Hint, how fast was the parachutist falling before releasing the chute?

7. Dec 30, 2014

his acceleration is 10m/s^2 because of gravity

8. Dec 30, 2014

### BvU

When you fall out of a plane, you accelerate -10 m/s2. After a while, you have picked up quite a bit of speed. Down (negative in physics parlance employing the usual orientation of + = up).

Parachute opens and decelerates you appreciably (this case +10 m/s2). Air resistance drops when speed drops and you hope to end up descending at a reasonable, as good as constant speed (i.e. air resistance 600 N for this bloke).

9. Dec 30, 2014

### Staff: Mentor

D'oh! Of course you are right.

10. Dec 31, 2014

### PWiz

Imagining a person decelerate with a magnitude of $g$ made me wonder whether the question was correctly copied. I mean practically the decelerating effect is never close to g, since it takes much longer to reach terminal velocity than it takes for air resistance to become noticable. You rarely "slow" down as quickly as you "speed" up while falling. If however, 1200N is the maximum value air resistance reaches (instead of an average force as it appears to be in this case), then this logic is certainly plausible.

Last edited: Dec 31, 2014
11. Dec 31, 2014

### PeroK

You certainly do when you hit the ground!

12. Dec 31, 2014

### PWiz

"You rarely "slow" down as quickly as you "speed" up while falling.."

13. Dec 31, 2014

### PeroK

I was going to say that I'd bet a skydiver does decelerate greater than 1g, and, for what it's worth, Wikipedia agrees:

"During a normal deployment, a skydiver will generally experience a few seconds of intense deceleration, in the realm of 3 to 4 g, while the parachute slows the descent from 190 km/h (120 mph) to approximately 28 km/h."

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

14. Dec 31, 2014

### PWiz

Well I did say rarely. And I'd recommend that you try to avoid Wikipedia, anyone can edit the facts! (unless there's a quotation to a reputable site)

15. Dec 31, 2014

### jbriggs444

I'd say that the number of times a parachutist experiences a deceleration in excess of 1 g from air resistance will tend to be least as high as his number of drops minus one.

With rare exceptions.

16. Dec 31, 2014

### PWiz

How silly of me, I was constantly imagining the parachute to be closed!

17. Dec 31, 2014

### jbriggs444

Pop the parachute and you're going to decellerate fast.

Fail to pop it and you are going to decellerate even faster. But that doesn't count as "air resistance".

18. Dec 31, 2014

### PWiz

Yes you're right, the parachute definitely has to be open if the frictional force has a value causing such upward acceleration! If only they sold common sense boosters in the market...

19. Dec 31, 2014

### haruspex

That's a little unfair. PWiz was referring to a net deceleration greater than 1g. That would only be vital if free fall lasted longer than the time with parachute deployed.
That said, I have no trouble believing that the drag on the chute is more than double that on the person. If the chute is deployed when at near terminal velocity, the drag on the person is already mg, so the drag on the chute at that speed will exceed 2mg.