# Homework Help: RainDrop + air resistance

1. Sep 23, 2006

### shakejuhn

i am doing my hoem work and this question is confusing the hell outta me.

A 4.35 * 10^-5kg raindrop falls vertically at constant speed under the influence of gravity and air resistance. After the drop has fallen 76m, what is the work done by gravity? After J is found for gravity, what is the work done by air resistance in units of J.

i have no idea wheer to sart or what formula to plug in to or even what laws to apply.

can some one please show me a set of detailed instrusctions on how to find these to answers.

2. Sep 23, 2006

### FunkyDwarf

Well i would have thought they would give you either a time amount or a radius of the drop, otherwise i cant see a way to solve it.

if you have a time value you can work out how far it should fall under normal gravitational conditions, and compare it to how far it did fall and use the difference to work out the energy used.

3. Sep 23, 2006

### shakejuhn

nope its was not in the question, all i got was the weight of the drop.

this is so confusing.

4. Sep 23, 2006

### durt

First of all, what is work?

5. Sep 23, 2006

### shakejuhn

it does not say what work is.

its a 2 part problem.

i know first i must find the work done by gravity after the drop has fallen 76m

after i find that i use the information i found to find J done by air resistance.

i hope this makes it a little bit clearer so i can get some kind of help.

thanks again all.

6. Sep 24, 2006

### FunkyDwarf

Work is the intergral of a force function. So you use f=ma to get the work done by gravity. as for the work done by air resistance i have no idea with this amount of info

7. Sep 24, 2006

### shakejuhn

i still confused, i use teh formula F=MA

where

M= 4.35*10^-5

what would A be, it says that the drop fall at a constant speed.

8. Sep 24, 2006

### durt

Work is force applied through a distance. Here W=F*s, where F is the magnitude of the force on the drop and s is the distance, because the force and direction of travel are constant. The fact that the droplet is falling at a constant speed is a hint that the air resistance balances its weight.

9. Sep 24, 2006

### shakejuhn

ok so if i go W= F * S

i get

W= 76* 4.35*10^-5

is this setup correct?

10. Oct 6, 2006

### TrippingBilly

what would A be, it says that the drop fall at a constant speed.

You're right, there is no net acceleration. But, there is still work being done by gravity, so you should use a=9.8m/s^2. When trying to find the air resistance, keep in mind what you said about the drop falls at a constant speed, no net acceleration (what does this tell you about the net force).

Last edited: Oct 6, 2006