What is the Distance a Ruler Falls Before Being Caught Without Warning?

[JMWM]

Homework Statement

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)

Homework Equations

not sure what to use.

The Attempt at a Solution

I was thinking on using velocity? but I don't know, I don't think I have much info.
the answer is 19.6 cm
if anyone could please help me, set this up... I would truly appreicate it.

 

[cristo]

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.

 

[Ja4Coltrane]

how far does something fall in .2 seconds?

 

[JMWM]

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

 

[JMWM]

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

 

[cristo]

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?

 

[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?

 

[cristo]

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?

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

 

[JMWM]

ok many thanks!

 

[JMWM]

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

 

[Integral]

What are the units of your answer?

 

[JMWM]

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

 

[bob1182006]

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

 

[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.

 

[cristo]

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

 

[JMWM]

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

 

[cristo]

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.

 

[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. =)

 

[cristo]

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