 # Measuring Reaction Time with Gravity ### Lab Safety

Always wear eye protection during science experiments. Never handle sharp objects, open flames, chemicals, or other dangerous objects without adult permission and supervision.

Visual reaction time is the amount of time it takes for us to see something and react to it. The average reaction time is 250ms, or 1/4 of a second. That's pretty fast--it takes 300-400ms to blink.

Because reaction time is so fast, it's hard to measure without a slow-motion camera or a computer. In this experiment, however, we're going to learn how we can measure time by using gravity to our advantage. We can do this by using the fact that the earth's gravity acts on objects at a constant acceleration, -9.8m/s2.

## Procedure

1. ##### Step 1

First, find a labmate. This experiment isn't possible to do alone!

2. ##### Step 2

Either sitting or standing, hold your hand out like you're ready to grab something, but with enough space for the meter stick to fall between your fingers.

3. ##### Step 3

Have your labmate hold the meter stick straight up and down so that the 0cm mark is right at the top of your hand.

4. ##### Step 4

When you are ready, and without telling you when, have your labmate drop the meter stick so that it starts falling through your fingers.

5. ##### Step 5

As quickly as you are able to, after the meter stick has begun to fall, close your fingers around the meter stick and hold them shut, so that the meter stick can no longer move.

6. ##### Step 6

Observe the measurement where you grabbed the meter stick and record it in your lab book. Record this measurement in meters (1cm = .01m)

7. ##### Step 7

Repeat steps 2-5 a few more times and calculate an average of the measurements you make.

8. ##### Step 8

To calculate your reaction speed, we're going to use a special physics equation that relates acceleration to distance and time: Distance=.5*Acceleration*Time2 (Don't worry if this is confusing, it's from college-level physics!). In our experiment, gravity is the force that's accelerating the meter stick downwards, distance is just the number that we measured in the steps above, and time is what we want to figure out.

By plugging in our number for gravity, 9.8meters/second2, and rearranging the equation, we get: Time2=2*Distance/9.8. Plug in your value for distance and use a calculator to figure out the right side of the equation. Then, find the square root and that's your reaction time!