# Convert g forces to height

1. Aug 1, 2014

### Deina

A certain laptop says it can withstand 1500 gs.

Assuming that it's dropped onto a hard surface, and ignoring air resistance, how would I calculate [STRIKE]how many times Hubby can come home late without calling[/STRIKE] the max "safe" drop height?

2. Aug 1, 2014

### Simon Bridge

Welcome to PF;
That's good thinking. You basically want to know if 1500g is a lot or a little or what? The number doesn't say anything unless you understand it.

The "g" rating refers to an acceleration - or, in this case, a deceleration, in multiples of the acceleration of gravity.
The harder the surface, the faster the deceleration when you drop it.

Back of envelope, you can work out the average acceleration the laptop undergoes if you know it's initial and final velocities and the amount of time it spends decelerating.

You can get the speed just before it hits by kinematic equations.

The time you want would be the "contact time".
If we guess that the laptop and surface are hard enough for a collision (that does not break things) is very nearly elastic, then the final speed is about the same as the initial speed after the contact time is up: i.e. you expect the laptop to rebound. (This is usually a bad approximation - look up "coefficient of restitution", but it should be good enough to get the idea.)

So you can write a=2v/T (average) if T is the contact time, and v is the speed from falling height h.
The value of T depends on the details of the surface and it is something that gets measured. You can look up typical contact times for hard surfaces ... usually of the order of a few milliseconds.

(You will want to look up "collision physics")
It looks to me like you laptop is not rated for drops bigger than 1cm, but I could have misplaced a decimal point.

From experience, laptops do not long survive striking hard surfaces like husband's heads even from quite short drops like from above a door. Just saying.

Last edited: Aug 1, 2014