Effects of Gravity G - force
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.
The energy of an impact is used to compact and fold metal while dissipating in the process. The solid cars of the 50’s might have felt tough, but with nowhere to dissipate the energy, it was dissipated into us instead.
In the formulae below, we see that
Where a is the rate of deceleration and g is acceleration due to gravity.
Note how the rate of decelerations (a) determines the g-force experienced. So if deceleration is slowed by crumpling metal, we greatly reduce this effect.
There’s a great example on YouTube, where crash tests are performed on a 1959 Chevrolet into a 2009 Chevrolet
In Monaco in 1973 David Purdy decelerated from 173 KPH to 0 is a distance of 660mm and experienced a G-force of 214 g! He survived.
This is the highest survived, g-force crash ever recorded.
||To hark-back to my cell phone (sorry, yes I am geek-obsessive) my protective case was chosen because I’m a klutz! The soft aluminium end-caps don’t actually touch the phone itself. Rather the phone is only clamped at the sides, and hence the energy of an end/corner fall is dissipated through the rear polycarbonate which is connected to the caps.
In a similar way, I have a tempered glass screen protector bonded to the screen. There is lively discussion on internet forums about the merits or lack thereof, of glass protectors. The rationale being that the phones own screen is tougher than the protector and hence the protector does not offer protection.
My opinion is that the relative weakness of the screen protector is a good thing; offering energy dissipation as discussed above.
At Homershams we import Industrial Engineering Adhesives and sometimes when I’m browsing the datasheets and see enormous figures (e.g. 16 MPa) for bond strength I wonder about when this might be needed and this thinking of g-force has crystallised this thought process.
As a general rule-of-thumb, Cyanoacrylates (‘super’ glues) are poor at high impact strength. There are specialised exceptions, and we have special acrylics that have some flexibility to absorb and dissipate energy (eg ET515) so talk to us if you have special requirements.
The confluence of thinking about phones, impact and car crashes is a timely reminder. We want to keep you as readers and customers. Please don’t use your phone while driving :) !
A new feature in IOS11 is “Do not disturb while driving”. We applaud Apple for this new innovation.
193 KPH into a solid wall (BBC 5th Gear) https://www.youtube.com/watch?v=R7dG9UlzeFM
Big vs small. Toyota Yaris vs Holden Berlina (NZ) https://www.youtube.com/watch?v=Fn53MGa1v_o
http://hyperphysics.phy-astr.gsu.edu/hbase/flobi.html Source of Calculator
5m/s = 20 km/h