Force of Gravity

Every time a new flagship phone is released, we hear about how the latest version of super-glass is being used. Whether it’s Gorilla Glass 5, heat-tempered or dual ion exchange ‘uber-glass’, I still pick up my fallen phone with dread and trepidation to see not if, but what damage I’ve caused.

And so I started to think about the forces that my long-suffering phone is being exposed to.
From high school we learned that objects falling to earth accelerate at a rate of 9.8 metres per second, per second. But what forces does my phone endure?

Turns out that there is quite a lot of study done on the physics of falling bodies. The impact force of gravity on a body is called the g-force. It’s a measure of the amount of times the force of gravity is multiplied.

Here’s some examples

• Standing on the earth                                1g
• Super scary roller coaster                         3.5 to 6g
• Top Fuel-dragster                                      4.2g
• Dodge Demon                                           1.8g
• Maximum turn in a fighter                          9g
• NASA centrifuge                                        20g
• Worst Formula 1 Crash ever                     200g
• Unconsciousness                                      >7g (typ.)

So perhaps we need to be a little more forgiving when we find that our poor cell-phone might be exposed to 200 g or more when falling as shown in the images above.

The factors affecting the force on a falling body are

• Its mass
• The distance fallen
• The time taken to decelerate
• The surface area of the impact point

The formulas are a bit arcane (but are shown at the end for the masochistic) but in summary, dropping my 129 gram iPhone 6S from shirt pocket height will take half a second and see it hitting the ground at 20 KPH!

If it stops within 6mm, it will decelerate at a rate of 8000 KPH and if falling on a corner as in the example above, will experience a force of 250-g’s!

So no matter how fancy the new ion-exchange glass is, 200g’s is 200g’s, and the secret to safely surviving the energy of a falling phone, or indeed a crashing car, is to allow the energy to dissipate in a safe and non-destructive way.

Perhaps I should blame Sir Isaac Newton rather than Steve Jobs!

The First Law of thermodynamics tells us that energy can’t be destroyed, but only changed in form. We need to give the energy somewhere to go.

Those of us (well over!) fifty may sometimes bemoan the tin-can-like softness of modern cars, but then with further thought, perceive that the crumpling design of modern cars is part of their safety system.