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I will describe my question using a scenario since I don't know exactly what to ask that will convey my question properly.

Let's say two identical faces were to get punched by a fist that generates the exact same amount of energy, 1000 joules. (same fist size, same impact point). The only difference being one of the faces is on earth, and the other is on a planet with much much higher gravity.

Once the fist has already hit the face and the energy has been transferred to the face, would gravity make a difference on that transferred energy? Assuming everything was identical up until the impact on faces, would gravity make a difference on how much damage each face takes? Would the face on the higher gravity planet take less damage, or would it be the exact same?

If gravity won't have any impact in this scenario, why not? Doesn't energy have mass and therefore affected by gravity?

curious
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    A force of $1000 J$? Joules is a unit of energy, not force. – Heisenberg Jun 01 '22 at 16:57
  • 1000 J is an energy, not a force. One cannot "transfer" force from one object to another. One can only exert a force during some amount of time. Energy can be transferred, however, so perhaps that's what you're talking about. What reason do you have for thinking that gravity should somehow affect how much energy is transferred during the punch? Being more precise about the mechanism here might help you come up with the answer, but it also will help us answer as well. – march Jun 01 '22 at 16:57
  • @march I guess what I am trying to know is, once a punch is done (fist has hit the face), does the force that went into the face get affected by gravity? Like gravity pulling that force down making it less effective and thus the face absorbing less of the power from the punch? Like imagine the force from a punch travelling through the face, but gravity pulling that force down and making it travel less far in the face? Or is that force unaffected by gravity. Sorry for making it confusing, I am not familiar with these terms. – curious Jun 01 '22 at 17:03
  • I feel like you need to be more precise about your terms, or learn more about what these terms mean. Gravity is a force that acts on objects. A force is not an object, so gravity can't "pull" on a force. In addition, once the punch is done, the force applied by the fist is gone. Again, I think you mean energy when you are talking about force, but even then, gravity can't act on energy, because gravity acts on objects with mass. What do you really mean by "gravity pulling that force down and making it travel less far in the face"? What thing is "traveling less far"? – march Jun 01 '22 at 17:06
  • @march Let's say two identical faces are hit with a 500J and 1000J punch respectively, then wouldn't the 1000J punch have more impact (damage going deeper into the face) then the 500J face? That's what I meant by gravity making a punch's impact travel less far (again not sure if gravity affects a punches impact at all). I want to know if gravity would impact the damage of a punch given the force generated by the fist was the same when it hit the face. – curious Jun 01 '22 at 17:15
  • I still don't understand why you think gravity should have anything to do with the punch. That is what I want you to clarify. Yes, the punch that transferred more energy will have more of an effect, but the details of the collision matter for determining the recoil of the head (if that's what you mean by "traveling farther"), but again, this has nothing to do with gravity. – march Jun 01 '22 at 17:19
  • @curious 1. punching upwards (upper cut) and punching downwards (strike downwards) with full power are different muscles. 2. If I assume you use the same muscle. Then the maximum power you could exert in both cases are different (it would be more in the downward punch). – More Anonymous Jun 01 '22 at 17:20
  • @march why do I suspect you haven't had the privilege of teaching physics to a nephew/niece? :P – More Anonymous Jun 01 '22 at 17:24
  • @march I think I am under the assumption that just like normal objects, gravity also pulls energy towards it. Is that not the case? When I plugin 1000J in an online converter it shows it has a mass of 1.112650056e-14 kg. – curious Jun 01 '22 at 17:26
  • Not especially important to the physics, but a 1 kJ punch would almost certainly be fatal to the recipient and cause irreparable injury to the punching hand. – g s Jun 01 '22 at 17:48
  • @march I will rephrase the question to see if it makes sense to you. I want to know if the same punch (same amount of joules delivered on impact, same fist size, same impact point) on identical heads would have less/different of an effect in a planet with way more gravity – curious Jun 01 '22 at 18:49

2 Answers2

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If somehow we are not putting in more energy during the punching, then I believe it would depend on the angle the punch makes with the horizontal. If you punch horizontal to the ground, the force of gravity won't do any work on your hand and you would land a blow without losing energy by doing work against the force of gravity as $W = F s \cos{\theta}$. Here the angle $\theta$ would be $90^\circ$ and so the work done against gravity would be zero. If you were to punch upwards, you would be doing work against gravity and hence would land a weaker punch and if you punch downwards, gravity would do positive work on your fist and you would land a stronger punch.

But all of this assumes the simplification that you put in all the energy at the beginning of the punch and your posture and physique has no effect on the punch which is obviously not true.

Mechanic
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  • I understand gravity affecting the hand, but what about gravity AFTER the fist has already hit the face? Like what exactly causes the face to take damage from the fist? And whatever energy that causes the damage to the face, does that get affected by gravity? – curious Jun 01 '22 at 17:17
  • I'm not exactly sure where the confusion lies. The damage taken by the face will depend on several factors like the impulse of force, the area the force acts on etc. Gravity doesn't work on energy, not on the classical Newtonian scale. It won't decrease the magnitude of energy. I think you are confusing some aspect of mass-energy equivalence with this problem. – Mechanic Jun 01 '22 at 17:28
  • I was under the impression, just like regular objects, energy would also be pulled towards the earth, since energy also has mass? – curious Jun 01 '22 at 17:31
  • @curious That won't happen. If that were the case, we would continuously be losing energy to the ground even if we didn't move a muscle. – Mechanic Jun 01 '22 at 17:33
  • @curious Just to clarify, it's the kinetic energy that does damage to the face when you take a punch right? And that kinetic energy isn't pulled towards the ground by earth's gravity at all? – curious Jun 01 '22 at 17:35
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    @curious I'm a bit concerned about your reasons for asking this question. – More Anonymous Jun 01 '22 at 17:37
  • @MoreAnonymous I want to know if the same punch (same amount of joules delivered, same fist size, same impact point) on same head would have less of an effect in a planet with way more gravity. – curious Jun 01 '22 at 17:39
  • @curious Before landing the punch, the kinetic energy of your punch is a part of your body. Your body is being attracted to the centre of the earth, and so is the kinetic energy. But it won't leak away from your body to decrease in magnitude. When you land the punch, it would transfer to the dude whose face you smashed. – Mechanic Jun 01 '22 at 17:40
  • @Mechanic What I want to know is, once the punch is already transferred to the face of the other guy, would gravity be able to affect the transferred kinetic energy? I want to know if the same punch (same amount of joules delivered, same fist size, same impact point) on same head would have less of an effect in a planet with way more gravity. – curious Jun 01 '22 at 17:43
  • Gravity won't have any effect whatsoever on the transferred energy. – Mechanic Jun 01 '22 at 17:46
  • @Mechanic Can I ask why not? Doesn't energy have mass and therefore shouldn't gravity attract it towards it too? – curious Jun 01 '22 at 18:46
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Let's say you punch with $1~\text{kN}$ force horizontally (perpendicular to gravity field), then gravity will have no effect on your punch. But if you punch vertically,- aligned to Earth gravity field, then punch and gravity forces must add to net force :

$$ F_{pv} = F_{ph} \pm m_{a}g $$

Where $F_{pv}$ is vertical net punching force, $F_{ph}$ - your "standard" punching force, i.e. in horizontal line, $m_a$ is about your forearm mass.

If for example your forearm has $2~kg$ mass, then gravity will "assist you" or will be stopping you by additional $\approx 20 N$ force, depending if you launch punch upwards or downwards.

Agnius Vasiliauskas
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  • "force horizontally (perpendicular to gravity field), then gravity will have no effect on your punch" It's a rigid body surely there would be torque involved? – More Anonymous Jun 01 '22 at 17:28
  • For answering how exactly torque(s) of your arm and gravity will be involved/interact,- one needs full information on exact punch trajectory and setup. As it is now, it's undecidable from the current problem statement. – Agnius Vasiliauskas Jun 01 '22 at 17:35
  • I don't think this answer addresses reality very well. A good boxer can punch with several times his body weight in force. Nobody alive can lift momentarily off of the ground by thrusting their hand downward through air and conserving momentum. The conservation of linear momentum for the punch has to be being paid for, in advance of or during the strike, by the planet, via the normal force and static friction. Modeling the punch as being propelled by a force that's direction invariant and adding the influence of gravity misses the most important effects shaping the phenomenon. – g s Jun 01 '22 at 18:29
  • @AgniusVasiliauskas I am sorry for using confusing terms. My question was more about once the punch has already taken place. What I want to know is, once the punch is already transferred to the face of the other guy, would gravity be able to affect the transferred kinetic energy? I want to know if the same punch (same amount of joules delivered to the face, same fist size, same impact point) on same head would have less of an effect in a planet with way more gravity – curious Jun 01 '22 at 18:42
  • @gs. Seems you miss my point. It doesn't matter how strong good boxer can punch or "lifting of the ground by punching down" - this nobody has said either, it's just your straw man fallacy. The point was that net force principle applies to everything, including punch. Imagine that you are very weak and can only punch with $20~N$ force, now try to punch up in the air. Will you succeed it ? Or I would like to see how these professional boxers would "easily" punch upwards in gravitational environments like close to black hole (or even at Jupiter). – Agnius Vasiliauskas Jun 01 '22 at 20:20
  • @gs Btw, I congratulate you on buying "reality" passport, now you can pass all checks :-) Jokes aside,- nobody has reality exclusive ownership. We can only get a grip, here and there. And I believe that we can only approach reality only asymptotically. And this is a good thing, because absolute knowledge of reality would do more harm than good. – Agnius Vasiliauskas Jun 01 '22 at 20:42