In this study https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9784821/, the distance the punch travelled from start to impact is 0.49 meters and the time taken from start of punch (that's it, they define the start of punch as the moment the elbow first start to extend) to impact is 0.1 second. The velocity of the fist at impact is 8.9 meter per second. I use an online speed, time and distance calculator I got 4.9 meter per second. Why is that?
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$\begingroup$ Of course I did as everyone did $\endgroup$– SnoopyKidCommented May 13 at 6:35
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1$\begingroup$ Oh, it almost fits the standard kinematics relations! Not bad, for such a rough estimation that it would be. $\endgroup$– naturallyInconsistentCommented May 13 at 8:00
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1$\begingroup$ "Speed is equal to distance divided by time" No. The correct statement is "Average speed is equal to distance divided by time". $\endgroup$– Vincent ThackerCommented May 13 at 10:19
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3$\begingroup$ I'm a bit surprised this got closed. It is a perfectly reasonable question, and it has attracted an excellent answer. $\endgroup$– John RennieCommented May 13 at 15:05
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1$\begingroup$ @JohnRennie Agreed. Even more surprisingly a second close vote has been started. $\endgroup$– gandalf61Commented May 14 at 6:21
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1 Answer
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The average speed is 4.9 m/s.
However, since the fist starts from rest, you must expect that the final speed is faster than 4.9 m/s. (Or there is no way for the average to be 4.9 m/s.) Hence a final speed of 8.9 m/s is normal and expected.
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$\begingroup$ So, in order for the punch speed to be exactly 4.9 m/s is only when the speed of the punch is constant and unchanging? $\endgroup$ Commented May 13 at 6:36
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$\begingroup$ @SnoopyKid That is not the only way, but it is the simplest way to achieve that punch speed $\endgroup$– hftCommented May 13 at 17:25
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$\begingroup$ Any function will do. If you imagine an elbow joint as a spring-like mechanism for the fist punch, which roughly is for muscles, I guess, then the acceleration rate is not constant. Imagine the equations for acceleration of mass on spring. $\endgroup$– Radek DCommented May 13 at 22:07