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Baseball Science Fair Project: Center of Percussion

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Author: Christopher Crockett

Hitting a baseball with a bat is said to be one of the most difficult things to do in sports. The ball comes at you at speeds exceeding 80 mph, and as the ball travels from the pitcher’s mound to home plate, you have about a quarter of a second to decide what to do. When you do strike the ball with the bat, you’ll want to do so at the sweet spot—the point on a baseball bat where the hit feels good, the crack of the bat fills the park, and the ball sails off deep into the outfield.

What is the sweet spot? Is it just a myth, or is there actually something to the idea? In this project, you’re going to try and find the legendary sweet spot and see what physics can tell us about the perfect hit.

Problem: Learn how the sweet spot correlates with movement and vibration.

Materials

  • 2’ long wooden plank (a 2”x4” plank works well)
  • U bolt
  • Drill
  • Smooth rod along which the U bolt can slide (e.g., shower curtain rod)
  • Rubber mallet
  • Tape measure
  • Baseball bat

Procedure

  1. Drill a hole through one end of the 2”x4” plank large enough to insert the U bolt. Use the bolt to hang the 2x4 from the curtain rod.
  2. Mark a spot exactly halfway along the length of the wood. Strike it with the mallet in the direction parallel to the curtain rod. How does the wood move in response? Note how far along the curtain rod it moves.
  3. Strike the end of the 2x4 farthest from the U bolt with the mallet. Is the movement different from last time? Which way does it move? How far? Note how far along the rod it moves now (taking note of the direction as well).
  4. Starting at the far end, strike the wood at different distances from the end, moving towards the center each time. Keep track of how far you are from the edge of the wood and by how much the wood moves along the rod after each strike. Is there a spot where the wood barely moves along the rod at all?
  5. Grab the baseball bat in one hand (holding it roughly where you would normally hold it if you were up to bat) and let it hang towards the ground.
  6. Have a friend use the mallet to strike the bat at different distances from the far end of the bat. Take note of how the bat moves in your hand, taking particular note of any sensation of vibration. How does the sensation change as you strike the bat at different spots? Is there a spot where the vibration is minimal? It may help to close your eyes and concentrate on the vibrations you feel.

Results

When the wood is struck at the center, it moves sideways along the curtain rod but doesn’t rotate. When struck at the far end, the center doesn’t move, but the wood pivots around it, sending the U bolt moving in a direction opposite the direction of the blow. At some point along the wood, about 2/3 of the way down from the top, there will be a spot where the wood does pivot but it doesn’t move along the curtain rod.

Likewise, on the bat there will be a spot where the vibrations are smallest, probably around 17 cm from the end.

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