What is the physical explanation of "recoil" force when used to amplify bodily movements?

Question

Example 1: Say you are standing with a chest-high boulder in front of you. Rather than jumping up from rest, it is easier to climb on top by first performing a small hop and jumping out of the "bounce." This is very intuitively understood when practiced.

Example 2: A punch can connect with greater force by actually jerking backwards briefly just before impact rather than following through. I am not a martial artist, but again this seems intuitive when attempted. You can feel the difference. I can also break more solidly in a game of pool by using this technique.

So the question: clearly this phenomenon exists, and I would like to know what the explanation is in terms the physics involved. It seems to me to be a recoil force of some sort, but I'm having trouble understanding why it doesn't work in the opposite direction. How does pulling back on a striking blow make it stronger? How does jumping out of a downward motion result in greater lift than jumping from rest?

Thx

No answers? :(

Aren't there some smart people here who can handle this?

Steve: Other than for you ego, your answer is more useless than "I don't know," so you are correct assuming I don't like it. I gave a very demonstrable example (jumping) of what I'm talking about so why don't you put down your books, get out of your wheelchair, and try it. No laboratory is needed. I am an engineer schooled in physics as well, which is why the question perplexes me. From a practical perspective the phenomenon is intuitive, from an academic perspective it is counter-intuitive.

There are other examples as well, such as a wrestler throwing a suplex or a housecat rocking its hips before pouncing. I believe Bruce Lee actually spoke on this technique of "recoiling" a punch. I think I'll post this in the martial arts section to see if they can help. They may not be physicists, but they might not be such jackasses either.

dkrudge: I think inertia may have something to do with it. This may be more a question of the physiology than the physics.

Answer 1

You won't like the answer.

Your perception of what's happening is backed up by nothing. You have no experimental evidence to show that what you say is true, and your statements are completely contrary to the physical sciences governing these kind of phenomena.

"....clearly this phenomena exists..."

NOT

Answer 2

Actually this isn't recoil force. It is wrong to say that the punch is stronger if you jerk backwards before impact. This is just done so that it will not hurt much and it would be convenient for the puncher. I am sure that your punch would be stronger if you won't jerk your hand backwards (try it with a wall :p). This also has something to do with impulse. The power increases if the time the force is applied is shortened.

As for the hopping before jumping, this is called inertia. It is hard to overcome inertia from rest so one must be on the move so as to easily overcome it. If you don't know what nertia is, it is the property of objects at rest to stay at rest and the property of objects at motion to tay in motion until some other force acts against it (or just google it)

Answer 3

The examples are not related, even though they seem alike.

The first example is energy stored in the springiness of the legs. A lot of insects make use of this, where they pre-load their legs before letting fly. If muscles did not have the ability to store potential energy in this fashion, acting as a kind of a spring, this wouldn't work.

The 2nd example is providing for a longer stroke in which to deliver power. Think of a handgun versus a rifle. Rifles usually deliver higher muzzle velocities because the pressure behind the bullet has more time to exert a force on it before the bullet is on its way. For a pool stroke, for example, for a given power shot, if done with only a short stroke, the player would have to snap his stick more quickly to achieve the final velocity, and that tends to make shots inaccurate. For martial arts blow, it's a tradeoff: A windup before a punch may deliever a higher final velocity for a given muscle exertion, but the price is telegraphing your shot; I think an experience martial arts practitioner would prefer shorter but quicker punches, because the opponents have less time to react to them.

Answer 4

Take for instance a car on the highway. If it crashes into bushes it will slowly stop. If it hits a wall, it il get crushed. The reason for that is that the same energy is lossed, but the rate at which it is loss (power, J/s) will have a very different effect. So, that's what you do when snap a punch. You shorten the time on which the energy is transferred, ths increasing the power. And most materials have problems with high power, not energy.

it's akin to f=ma. You increase acceleration (lose your kinetic energy faster) and thus increase the force. It's all linked.

Answer 5

Since force is equal to the rate of change of momentum, when you pull your hand back, you forcibly change your momentum even more quickly than it would change had you not pulled it back.

One moment your hand has momentum in one direction. When you pull back, the momentum suddenly goes changes so drastically, that the sign is reversed. This means that the momentum has changed really fast. By consequence, the acceleration is also far greater. Since F = ma, this works out why the force delivered is greater.

Answer 6

I certainly challenge the punch. Pulling it at the last second reduces the velocity and therefore the momentum and force delivered at the instant of impact. I did study martial arts, mine taught us to follow through to deliver the maximum force.

Answer 7

My guess would be that when you pull back it creates more energy then to push forward in the opposite direction.. Same as why its easier to push furnature than to pull it somewhere.. maybe its something with muscles also.. Just a guess though