Unit 1 - Kinematics - Experimental Design

A student wishes to find their reaction time. However, they don’t have a charged cell phone or other timing device. How can they determine their reaction time?

Create a short procedure that the student could follow, be sure to include ways to reduce uncertainty increase accuracy. Assume you have commonly available lab equipment, but no timing devices.

Please post your responses by Thursday (4/2). I’ll post some critiques and a rough scoring guide on Friday (4/3)

  1. Acquire a smooth, flat surface of negligible friction, a small and smooth ball, a photogate, and a meterstick.

  2. Place the meterstick next to the expected path of the ball. Place the photogate in front of the meterstick.

  3. Roll the ball toward the meterstick. Make sure the photogate reads a constant velocity.

  4. Raise your hand slightly above the surface, just high enough to be a little above the ball if it were to pass under your hand.

  5. Slap your hand down over the ball right when you think it reaches the o.5 m mark. Keep the ball at this location. Record this displacement from the 0m mark.

  6. Calculate the time it takes to reach 0.5 m using x = vt, t = x/v.

  7. Calculate the time it takes to reach your recorded displacement using x = vt, t = x/v.

  8. Find the difference between the time it takes to reach 0.5 m and the time it takes to reach your recorded final displacement. This is your reaction time.

This looks really good! Just a few tips for improving it:

  1. You only mention using 1 photogate, but ideally you want 2 so that you can get the change in time for the ball to travel the 1m. Alternatively you could use a motion sensor to get the velocity of the ball directly.

As an alternative method, you could have another student hold the meterstick vertically while the 1st student placed their hand loosely around it, with the top of their hand at the 0m mark. Without prompting, then 2nd student lets go of the meterstick and the 1st student tries to catch it. Recording the distance the meterstick fell allows us to use x = xo+vot+1/2at^2 (simplifies to x = 1/2gt^2) to calculate t.

Thanks for the feedback! I really like the alternative method you suggested.

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