## Percussive Shock

A forum set up for physics questions from students in the courses PHYS1121, 1131, 1221 and 1331 at the University of New South Wales. It is intended for questions that cannot readily be answered in class,
either because they fall outside the main syllabus and therefore would be distraction (however interesting) or for other reasons.

Moderator: msmod

Jack0McN
Posts: 3
Joined: Sat Jul 13, 2019 9:41 am

### Percussive Shock

Hi,
So I came across this in one of my TV shows https://www.youtube.com/watch?v=BiXs3Pmb5-I. They attempt to knock off the lug from the car through a percussive shock?? I came across this article on Wikipedia https://en.wikipedia.org/wiki/Center_of_percussion. Where it notes that the centre of percussion is a bit below the centre of mass due to it needing to cancel out the translational and rotational energy. So, would it be correct to say that a percussive shock would be a shock hit at as far away as the centre of percussion that they can be to be able to rotate the lug off the tire?

Thanks in advance
joe
Posts: 755
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

### Re: Percussive Shock

The link about centre of percussion is good This concept is applied to objects (like bats) that rotate before collisions.

The TV show link in principle involves a rotating lever and a collision, but the shock in the man's hand is probably not the prime consideration, so we needn't consider the centre of percussion. Instead, think of two simpler, analogous problems.

First, their lever doesn't give enough torque. Obvious problem.
Second, they use a short collision to apply a large force. Let's do a simpler problem:

Driving a nail. You can apply a steady F<1 kN with your hand: not enough to drive it into wood.
So instead, you apply ~30 N over ~0.3 s to the head of a hammer (we'll omit the details of how you get it to the head of the hammer).
It now has ~10 kg.m/s of momentum.
If its collision with the nail lasts <1 ms, then the required force that the nail exerts on the hammer is >10 kN.

I leave you to try this with different values (and the rotation problem for swinging the hammer).