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Q & A: Forces on objects going through the air

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Most recent answer: 10/22/2007
Q:
have two questions: 1. Why won’t parachute work at low altitudes? 2. How does a ball curve? Thank you for your time.
- Dianne (age 12)
Andrew Carnegie Middle School, Carson CA USA
A:
Hi Dianne,

1. Parachutes should work about as well or better at low altitudes as they do at high altitudes (that is, if you are falling to the ground with a parachute properly deployed, it will not somehow stop working as you near the ground, thankfully!).

It is very important, however, to make sure you use the parachute when you are falling from a large height rather than from a small one, for two very important reasons:

a) Parachutes usually have ten or so feet of cords between the parachute material and the parachutist. If you deploy the parachute when you are less than a few tens of feet from the ground, the parachute will just be beginning to stretch out when you hit the earth, providing no slowing force. A fall from a few tens of feet can be fatal. Even if the parachute cords are fully stretched, it will take some distance falling in order to get the fabric of the parachute fully taut and full of air.

b) If you are falling from a great height, say, from an airplane, you can fall for a while before pulling on the rope that will deploy the parachute. Skydivers use this time for doing outlandish stunts, like somersaults, riding surfboards, shaking hands and taking pictures, etc. If the parachutist waits too long before deploying the parachute, it can be fatal. The parachute may open, fill with air, and be functioning correctly, but there is still a limit to how much drag it will exert on the parachutist (which is good, otherwise it can yank the parachutist so hard when deployed as to be injurious). If the parachutist has not been slowed down to the new terminal velocity with the open parachute when he or she comes into contact with the ground, he or she may hit the ground much harder than is necessary.

2. Please check the search engine on our site for "Magnus force" and you'll get a good description of why curveballs curve.

Tom

(published on 10/22/2007)

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