# Tipping a table

1. Jan 20, 2014

### ajcoelho

This is a very well known physics problem and i already know how to solve it.

How close to the edge of a 24kg table can a 66kg person sit without tipping it over?

I think everybody can see the table and what's happening in there.

MY QUESTION IS:

Why don't we use the normal reaction due to the farther suportting?? I mean, when we calculate total torque about one supporting, we still have the other support in which there's a reaction force ating. And i think this force is as legitimate as the gravity forces of the table and person...

I would be really glad with any answer trying to explain why is this approximation made or if I'm completely wrong in my reasoning!

Take care!

2. Jan 20, 2014

### tiny-tim

hi ajcoelho!
because it's zero!!

so long as the total reaction force from the floor (the resultant of the reaction forces from the four legs) goes through the base of the table (the rectangle with the legs at the corners), the table will not tip

the table will only start to tip when the resultant reaction force goes through the edge of the base

when that happens, the individual reaction forces through the farther legs will be zero

3. Jan 21, 2014

### ajcoelho

when you say edge of the base you're refering to the supporting that has become a "pivot"?

4. Jan 21, 2014

### tiny-tim

yes, when the reaction force reaches the edge, the tip will start about that edge as pivot