# The Physics of Knocking Things Over

1. Apr 17, 2013

### Peppino

I've encountered a problem that I do not believe I am able to answer using my very basic knowledge of classical and calculus-based physics, dealing with knocking objects over.

Say we had a cylinder of length L and radius R and mass M. And suppose we shot a bullet of mass m at the very top of the cylinder, and suppose the bullet immediately bounces off the cylinder.

In terms of the above quantities, can we find the minimum speed necessary to knock over the cylinder? Is there anything else that needs to be determined?

I have found that the cylinder must be lifted greater than 45 degrees off the ground or else gravity will restore it, but I am unsure what the rotational inertia would be of this sort, among other things.

Any help would be greatly appreciated!!!

2. Apr 17, 2013

### pnorm91

correct me if im wrong, but isnt there a correlation between the length of the cylinder and the angle at which it will begin to fall? also, im assuming you're speaking of a cylinder of uniform volume, and not thicker at the top, middle, or bottom? additionally, it would seem to me that you would need to know precisly where on the cylinder the force was being applied. It would tip much easier if it were at the top edge versus the bottom edge. just a few things to consider.

3. Apr 17, 2013

### Peppino

Possibly, but on the various objects I have tested (a marker, a textbook, a can of Dr Pepper, a sliced cucumber) the 45 degree rule seems to uphold. If only I had a large range of various sized cylinders could this be tested.

And the bullet is direct towards the very top of the cylinder, and everything is uniform.

4. Apr 18, 2013

### Staff: Mentor

Ever tried cutting down a tall (twenty meters or so) tree? It doesn't take anywhere near 45 degrees for it to be going over.

Find the center of gravity of the cylinder... When you tilt the cylinder enough that a vertical line through the center of gravity intersects the ground outside of the base of the cylinder, it's no longer stable and will tip over. The longer and thinner cylinder, the smaller the angle at which this happens.