Softening a stop
A fielder pulls the hands back when catching a fast ball. Why?
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Pulling the hands back makes the stop last longer. A longer stop needs a smaller average force.
The rule
The velocity drops by the same amount. A longer stopping time means a smaller acceleration. So the average force needed is smaller.
In every catch, the ball goes from 20 m/s to 0. Only the time changes.
Acceleration is the change in velocity divided by the time. Then F = ma gives the average force on the ball. The ball pushes the hands with an equal force. The table shows sizes only. The force on the ball points against the ball's motion.
| Catch | Stopping time | Acceleration | Average force |
|---|---|---|---|
| Stiff hands | 0.02 s | 20 ÷ 0.02 = 1000 m/s² | 0.16 × 1000 = 160 N |
| Hands give a little | 0.05 s | 20 ÷ 0.05 = 400 m/s² | 0.16 × 400 = 64 N |
| Hands go back | 0.1 s | 20 ÷ 0.1 = 200 m/s² | 0.16 × 200 = 32 N |
A crumple zone is a car part that folds in a crash. Crumple zones, airbags and gym mats do the same job. Each makes the stop last longer.
A glass dropped on tiles stops very quickly. The floor must push very hard. That large force can crack the glass. A thick mat gives a longer stop. So the force is smaller.