# Reaction time

1. Mar 5, 2008

### lgen0290

1. The problem statement, all variables and given/known data
The figure below shows a simple device for measuring your reaction time. It consists of a cardboard strip marked with a scale and two large dots. A friend holds the strip vertically, with thumb and forefinger at the dot on the right in the figure below. You then position your thumb and forefinger at the other dot (on the left in the figure), being careful not to touch the strip. Your friend releases the strip, and you try to pinch it as soon as possible after you see it begin to fall. The mark at the place where you pinch the strip gives your reaction time.
(a) How far from the lower dot should you place the 50.0 ms mark?

2. Relevant equations

3. The attempt at a solution

I tried to use the equation h=1/2gt.
I got 24.5 cm=(.5)9.8m/s(5s)

Looking at it further, it seems that is 24.5 m, not cm, so I'm not sure. 24.5 meters is not right for sure, and neither is 24.5 cm.

2. Mar 5, 2008

### Shooting Star

It should be (1/2)gt^2.

3. Mar 5, 2008

### lgen0290

But wouldn't that make the number even bigger? Is the reaction time after 50 ms really 122.5 cm? It just doesn't sound right.

4. Mar 5, 2008

### Shooting Star

You are right -- it doesn't. But the value comes out to be 1.225 cm, which does. Please at least do your arithmetic correctly.

5. Mar 5, 2008

### lgen0290

Thanks. You could have simply pointed out that my conversion was incorrect on the seconds.

6. Mar 5, 2008

### Shooting Star

Sorry. I didn't mean to be rude, but so many people take our help for granted :grumpy: . I'm sure you are not one of them, and your mistake was genuine. We are , I hope.