Newton's 3rd Law of Motion: Is the Punching Bag punching back?

[paulb203]

Homework Statement

Is punching a punch bag the same as the punch bag punching you?

Relevant Equations

F=ma?
Force A = - Force B?

Boxer Bob punches a punch bag with a force of 430 N on the bag (the action force).

The bag simultaneously exerts a force of 430N on Bob (the reaction force).

We can see this force on Boxer Bob by the compression of his glove while it is in contact with the bag (assuming that the conditions mean that this is glove compression is the only visible effect on Bob, e.g, he isn't moved backwards).

Q. Instead of the above, imagine if Bob just stood with his arm extended (matching the stance/pose he adopted in the scenario where he punched the bag) with the bag in the same position too. Then we applied a F of 430N to the bag in the direction of Bob's fist, would the compression of Bob’s glove be identical to what it was when Bob punched the bag? And would the bag respond to this in the same way it did to Bob punching it?

Is Bob punching the bag the same as the bag ‘punching’ Bob?

Is x striking y the same as y striking x, generally?

 

[anuttarasammyak]

Homework Statement: Is punching a punch bag the same as the punch bag punching you?
Relevant Equations: F=ma?
Force A = - Force B?

Is x striking y the same as y striking x, generally?

Yes, I think you have got a point.

 

[BvU]

Hi,

The scenario you describe is already pretty complicated ! That is because the punch bag can deform in two ways: elastically and inelastically -- in various proportions. Bob's glove as well.
Physically, we have a collision. The energy transferred in the elastic part is converted into kinetic energy (the bag swings backward) and the energy transferred in the inelastic part is converted into deformation and heat.

Another complication is the duration of the collision: the whole thing doesn't happen in a zero time interval, but in a few milliseconds.

Physicists try to simplify things by considering limiting cases, sometimes rather exotic.
In this scenario case a close-to-limiting case would be a solid steel glove hitting a solid steel cylinder (poor Bob ). Pretty close to a fully elastic collision (cf Newton's cradle). Conservation of momentum and conservation of kinetic energy.

Fully inelastic cases include cars crashing into a rock wall. Or identical cars, same speed, crashing head on.
Conservation of momentum only.

Close to fully inelastic is the ballistic pendulum. Some similarity with your scenario !

Only fully elastic is reversible. An inelastic part is not reversible.

$\$

 

[jbriggs444]

In fact, there is nothing that distinguishes between an "action" force and a "reaction" force. Neither is cause. Neither is effect. They come into existence together and always coexist.

The second part of Newton's original wording is superior to the first: "the mutual actions of two bodies upon each other are always equal, and directed to contrary parts"

would the compression of Bob’s glove be identical to what it was when Bob punched the bag?

The simple answer is "yes". If the force between glove and bag is 430N then the compression of the glove is fixed. If you put the two into a C clamp then then it does not matter which way the clamp faces. 430 N is 430 N.

It does not matter if you tighten with the screw on the left or flip the C clamp and tighten from the right.

That considers the situation from a static point of view. Perhaps you are concerned with the dynamics. Punching someone in the face is not the same thing as squeezing their face into a boxing glove using a vice.

Those are the two things that you are trying to compare:

Then we applied a F of 430N to the bag in the direction of Bob's fist

 

[Lnewqban]

Homework Statement: Is punching a punch bag the same as the punch bag punching you?

It is the same, but results are different for each.

The bag has been set to yield or oscillate in response to the punch.

The boxer has adopted a legs and body position such that his weight will help him to conserve that position, which will be advantageous for the following attack.