Why it’s a good idea to wear your seatbelt

by Deb on October 30, 2009

Most people know that Newton came up with 3 laws of motion a long time before there were cars.  They are:

  1. If a body is at rest or in constant motion it will remain so unless acted upon by an unbalanced force. English – Things won’t move unless you push them. Or if they’re already moving, they need a push to stop them.
  2. The force acting on an object equals its mass multiplied by its acceleration. English – Assuming it stays the same size, making it go faster needs more force and energy.  The same for stopping it.
  3. For every action there is an equal and opposite reaction. English – If you’re pushing on something it will be pushing back on you.  So if the chair you’re sitting on didn’t push back, it would collapse.

So what do these mean for you in a car?

Obviously, we need to put energy in in the form of fuel to accelerate the car (1).  That’s converting chemical energy to kinetic (movement) energy.  More acceleration means more force (2), so the faster you are going the more force the car has and the more force it needs to stop it.

But that’s the car – as the passenger you are in a slightly different position.  The fuel doesn’t accelerate you, but as you sit in your seat it pushes you along.  Your seat is pushing on you (3), and the faster you accelerate the harder it pushes (2).  This is why it’s good to have a soft seat – it has some give in it as it pushes you along.  When you stop accelerating and are going at a constant speed the seat back stops pushing against you, but the seat still pushes you up against gravity.

When the car brakes it is braking on the wheels, and you aren’t attached to them.  So what is actually stopping you?  Because according to rule 1 you should keep going unless something stops you personally.  In fact if you drive with a helium balloon in the car you will find all sorts of interesting things happen.  What stops you in most cases is the friction between you and the seat.  For those of us old enough to have sat on vinyl seats as kids you might have memories of sliding off, especially if you were wearing your trendy polyester trousers or skirt.  That’s why old vinyl seats had little patterns on them – to try to make them a bit rougher and increase their friction. But friction will only work when the deceleration is slow, because then the force is small (2).

When you brake quickly the deceleration is high so it’s a large force (2) and friction isn’t strong enough to stop you.  This is where the seatbelts come in.  As you continue travelling forward (1) you push on the seatbelt, and it pushes back (3). That is the force that stops you.

An interesting point here is about pressure.  Pressure is based on how much force there is spread over an area.  The force needed to stop you in an emergency is set – your mass times how quickly you are stopping (2).  But that can be distributed in different ways.  If your seatbelt is nice and wide, there is a large area to spread the force over.  If your seatbelt is twisted, there is only a little bit actually in contact with your body and the same force is all going to hit you along one line.  That’s asking for internal injuries.  In fact people often do get internal injuries from seatbelts in crashes.  But the other thing in their favour is that they are nice and soft and you are mostly soft, so all that pushing against each other has a bit of cushioning.  If you’re not wearing one, there’s no cushioning at all as your solid head hits the solid windscreen, with exactly the same force all focussed on one tiny little area.  Of course if you aren’t stopped by the windscreen the force is a bit less, and then you can bounce off the equally solid bonnet and road.

This also explains why it is safer to have babies facing backwards – in an emergency stop, they are being stopped by the back of the chair and the force is spread across their whole body.  It’s a good reason for harnesses too.

So please wear your seatbelt and work with Newton, rather than against him. Have you had any experiences with everyday forces?  Not just life and death ones!

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{ 3 comments… read them below or add one }

Coran October 30, 2009 at 8:47 pm

Also, when you decelerate rapidly and your kineric energy gets deposited into the seatbelt, that energy is expended by deforming the seatbelt – the seatbelt ‘gives’. That’s why seatbelts need to be replaced after an accident.

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Deb October 30, 2009 at 9:44 pm

Absolutely. It’s called plastic deformation because it doesn’t go back to the way it was before. That’s another reason it’s important to make sure seatbelts are untwisted – if they are then they don’t stretch properly.

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Tara November 2, 2009 at 9:16 am

I think that it’s very sad that in this country more publicity is not given to the importance of rear facing seats. Surely if the government explained it as sincinctly as you just did more people would take notice and babies would be safer. I shudder now to think that I turned my now 3.5 year old way too soon as I didn’t know any better than the manufacturers recommendations. Our second will have a miserable but safe time riding backwards till 12kg and no less.

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