# Homework Help: Air resistance, maybe?

1. Dec 17, 2006

### JMWM

1. The problem statement, all variables and given/known data
Human reaction time is usually about .20s. If your lab partner holds a ruler between your fingers and thumb and releases it without warning, how far can you expect the ruler to fall before you catch it. (disregard air resistance g= 9.81 m/s^2)

2. Relevant equations

not sure what to use.

3. The attempt at a solution

I was thinking on using velocity? but I don't know, I don't think I have much info.

2. Dec 17, 2006

### cristo

Staff Emeritus
Use the kinematic equations, namely s=ut+(at^2)/2, where s is displacement, u is initial velocity, t is time and a is acceleration.

3. Dec 17, 2006

### Ja4Coltrane

how far does something fall in .2 seconds?

4. Dec 17, 2006

### JMWM

hm..I'm not sure, I'm still confussed...

5. Dec 17, 2006

### JMWM

or wait...I think I go it.
9.8 * 2?

6. Dec 17, 2006

### cristo

Staff Emeritus
Use the equation s=ut+(at^2)/2. Here, a=g, you are trying to find out s, and t is the reaction time. Substitute these into the equation, remembering that the ruler starts from rest [so you know what the initial velocity is]. Does this help?

7. Dec 17, 2006

### JMWM

ok, kind of there, but u- is initial velocity isn't that 0, at rest, when mulitplied to the time it goes to zero, doesn't it?

8. Dec 17, 2006

### cristo

Staff Emeritus

Yes, that's right, so you're left with just one term on the right hand side.

9. Dec 17, 2006

### JMWM

ok many thanks!!!

10. Dec 17, 2006

### JMWM

ok so its: (0)(.20)+((9.8)(.20)^2)/2 =.196
hm...what am I doing wrong?

11. Dec 17, 2006

### Integral

Staff Emeritus

12. Dec 17, 2006

### JMWM

oh well its supposed to be cm, do I have to convert or what?

13. Dec 17, 2006

### bob1182006

what units is the .169? cm?m?mm?

14. Dec 17, 2006

### JMWM

oh I don't know, I mean I just did it in the calculator. well s and then m/s^2 isn't that m? blah, I'm lost.
and in my answer its supposed to be 19.6 cm.

15. Dec 17, 2006

### cristo

Staff Emeritus
Yes, your answer is 0.196 m... what is this value converted into cm?

16. Dec 17, 2006

### JMWM

I move the decimal twice. I believe. Wow, I seriously need to memorize all the convertions.

17. Dec 17, 2006

### cristo

Staff Emeritus
Yes, but instead of "moving the decimal place twice" you should memorise the fact that there are 100cm in a metre, so to convert from metres to centimetres you must multiply by 100.

18. Dec 17, 2006

### JMWM

yes. thank you so much.
wow there's a lot of stuff I need to learn. but I'll get there. thank you so very much, I truly appreciate it. =)

19. Dec 17, 2006

### cristo

Staff Emeritus
You're welcome. Keep working and, like you said, you'll get there! Good luck!