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Question Number 5122 by FilupSmith last updated on 16/Apr/16
Lets say person 1 punches a bag and   the punch is fast from start to finish.    Lets say person 2 does a punch but  only the final part of the punch is fast.    How will the forces differ in these punches?    (sorry if this is hard to understand.  this is something i am questioning  in real life)
$$\mathrm{Lets}\:\mathrm{say}\:\mathrm{person}\:\mathrm{1}\:\mathrm{punches}\:\mathrm{a}\:\mathrm{bag}\:\mathrm{and}\: \\ $$$$\mathrm{the}\:\mathrm{punch}\:\mathrm{is}\:\mathrm{fast}\:\mathrm{from}\:\mathrm{start}\:\mathrm{to}\:\mathrm{finish}. \\ $$$$ \\ $$$$\mathrm{Lets}\:\mathrm{say}\:\mathrm{person}\:\mathrm{2}\:\mathrm{does}\:\mathrm{a}\:\mathrm{punch}\:\mathrm{but} \\ $$$$\mathrm{only}\:\mathrm{the}\:\mathrm{final}\:\mathrm{part}\:\mathrm{of}\:\mathrm{the}\:\mathrm{punch}\:\mathrm{is}\:\mathrm{fast}. \\ $$$$ \\ $$$$\mathrm{How}\:\mathrm{will}\:\mathrm{the}\:\mathrm{forces}\:\mathrm{differ}\:\mathrm{in}\:\mathrm{these}\:\mathrm{punches}? \\ $$$$ \\ $$$$\left(\mathrm{sorry}\:\mathrm{if}\:\mathrm{this}\:\mathrm{is}\:\mathrm{hard}\:\mathrm{to}\:\mathrm{understand}.\right. \\ $$$$\mathrm{this}\:\mathrm{is}\:\mathrm{something}\:\mathrm{i}\:\mathrm{am}\:\mathrm{questioning} \\ $$$$\left.\mathrm{in}\:\mathrm{real}\:\mathrm{life}\right) \\ $$
Commented by Yozzii last updated on 16/Apr/16
The forces depend on the change in  momentum of your fist upon impact  with an object. From Newton′s second  law of motion, the force F is given by                        F=((Δp)/t)=((mΔv)/t)  where m=mass of fist  Δv= change in velocity (recoil velocity  of fist minus impact velocity of fist)  t= time for which the fist is in contact  with the object.   From this we see that the motion  of the fist before impact is only as  important as what impact velocity the  fist has just before collision. In the  first case the fist is moving at some  velocity v. In the second case the fist  accelerates, possibly to v also.   Depending on the relative positions  of the front area of the fist and the  shape of the target object, the value  of t and recoil velocity are affected.
$${The}\:{forces}\:{depend}\:{on}\:{the}\:{change}\:{in} \\ $$$${momentum}\:{of}\:{your}\:{fist}\:{upon}\:{impact} \\ $$$${with}\:{an}\:{object}.\:{From}\:{Newton}'{s}\:{second} \\ $$$${law}\:{of}\:{motion},\:{the}\:{force}\:{F}\:{is}\:{given}\:{by} \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:{F}=\frac{\Delta{p}}{{t}}=\frac{{m}\Delta{v}}{{t}} \\ $$$${where}\:{m}={mass}\:{of}\:{fist} \\ $$$$\Delta{v}=\:{change}\:{in}\:{velocity}\:\left({recoil}\:{velocity}\right. \\ $$$$\left.{of}\:{fist}\:{minus}\:{impact}\:{velocity}\:{of}\:{fist}\right) \\ $$$${t}=\:{time}\:{for}\:{which}\:{the}\:{fist}\:{is}\:{in}\:{contact} \\ $$$${with}\:{the}\:{object}.\: \\ $$$${From}\:{this}\:{we}\:{see}\:{that}\:{the}\:{motion} \\ $$$${of}\:{the}\:{fist}\:{before}\:{impact}\:{is}\:{only}\:{as} \\ $$$${important}\:{as}\:{what}\:{impact}\:{velocity}\:{the} \\ $$$${fist}\:{has}\:{just}\:{before}\:{collision}.\:{In}\:{the} \\ $$$${first}\:{case}\:{the}\:{fist}\:{is}\:{moving}\:{at}\:{some} \\ $$$${velocity}\:{v}.\:{In}\:{the}\:{second}\:{case}\:{the}\:{fist} \\ $$$${accelerates},\:{possibly}\:{to}\:{v}\:{also}.\: \\ $$$${Depending}\:{on}\:{the}\:{relative}\:{positions} \\ $$$${of}\:{the}\:{front}\:{area}\:{of}\:{the}\:{fist}\:{and}\:{the} \\ $$$${shape}\:{of}\:{the}\:{target}\:{object},\:{the}\:{value} \\ $$$${of}\:{t}\:{and}\:{recoil}\:{velocity}\:{are}\:{affected}. \\ $$
Commented by FilupSmith last updated on 16/Apr/16
So, in general, which would most likely  produce the most force?
$$\mathrm{So},\:\mathrm{in}\:\mathrm{general},\:\mathrm{which}\:\mathrm{would}\:\mathrm{most}\:\mathrm{likely} \\ $$$$\mathrm{produce}\:\mathrm{the}\:\mathrm{most}\:\mathrm{force}? \\ $$
Commented by Yozzii last updated on 16/Apr/16
You ought to maximise your  change of momentum, minimise  contact time and probably hit an   object with minimal area contact during  the punch.   If we introduce the pressure concept  ⇒P=(F/A)=((Δp)/(At))   we see that P is greatest if A×t is as  small as possible and Δp is as large as possible.  So, get a fist with a small face−area,  high density(⇒ high mass and little  volume) and high impact velocity.
$${You}\:{ought}\:{to}\:{maximise}\:{your} \\ $$$${change}\:{of}\:{momentum},\:{minimise} \\ $$$${contact}\:{time}\:{and}\:{probably}\:{hit}\:{an}\: \\ $$$${object}\:{with}\:{minimal}\:{area}\:{contact}\:{during} \\ $$$${the}\:{punch}.\: \\ $$$${If}\:{we}\:{introduce}\:{the}\:{pressure}\:{concept} \\ $$$$\Rightarrow{P}=\frac{{F}}{{A}}=\frac{\Delta{p}}{{At}}\: \\ $$$${we}\:{see}\:{that}\:{P}\:{is}\:{greatest}\:{if}\:{A}×{t}\:{is}\:{as} \\ $$$${small}\:{as}\:{possible}\:{and}\:\Delta{p}\:{is}\:{as}\:{large}\:{as}\:{possible}. \\ $$$${So},\:{get}\:{a}\:{fist}\:{with}\:{a}\:{small}\:{face}−{area}, \\ $$$${high}\:{density}\left(\Rightarrow\:{high}\:{mass}\:{and}\:{little}\right. \\ $$$$\left.{volume}\right)\:{and}\:{high}\:{impact}\:{velocity}. \\ $$$$ \\ $$
Commented by FilupSmith last updated on 16/Apr/16
Thank you very much!!!
$$\mathrm{Thank}\:\mathrm{you}\:\mathrm{very}\:\mathrm{much}!!! \\ $$

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