On Newton’s Laws of Motion and the Person in a Motor Vehicle Accident

This blog post is not of my doing, it is reproduced here with the kind permission of the author. Mary is a nurse with an interest in Physics from the U.S. Any additions that I have made to the post will be in [square parentheses], I have also made some amendments to the spelling from American to English.

Mary uses the term “accident” in her post, however, other than in the title I have changed this to be collision or crash. Accident is a standard word used for collisions in the US however the problem I find with using the term is that it implies that nothing could have been done to avoid such an incident, however on the road there are very few accidents. Greater than 99% could be easily avoided if all parties drove appropriately and/or complied with road law.

The original post can be read at http://maryspad.com/on-newtons-laws-of-motion-and-the-person-in-a-motor-vehicle-accident/ along with her other blogs.

*Sam’s Club is a membership discount supermarket, similar to Costco except non members are also allowed to shop but with a 10% surcharge.


I’ve been wanting to write this post for a long time but never got around to it. Since I don’t want to do what I’m supposed to be doing at this moment, I figure I could write this post now then I won’t feel too bad about procrastinating doing that other thing I’m supposed to be doing.



When a person has a motor vehicle [collision] and presents to the [ED], several questions emerge: “Were you wearing a seat belt?” is probably the most important one, followed by “Were you intoxicated?” And then other questions follow, such as did you lose consciousness (to determine the possibility of head injuries), etc. The one that I want to discuss is of course, “Were you wearing a seat belt?”

Have you ever wondered why seat belts are so crucial, and more importantly, why they work and are the one biggest reason for reduced mortality in car [crashes]? Well, the answer is actually pretty simple. It’s because they restrain your motion and prevent you from being tossed and banged around too much when a car lurches suddenly, comes to a sudden stop, or is involved in a collision. Simple, right?

Well, let’s look at it from the point of view of Newton’s laws of motions, which have been tested and proved over and over again for years. Newton’s laws of motion state (I am rephrasing them for simplicity):

  1. A body will stay in motion or continue to move at uniform speed in a straight line unless acted upon by an external force.maxresdefault
  2. The force that acts on a body is directly proportional to the rate at which that body is accelerating. The constant of proportionality is the body’s mass, therefore mathematically, F = ma, where F is the force acting on the body, m is its mass, and ais its acceleration.newton
  3. To every action there is an equal and opposite reaction.

Newton_s Third Law of Motion


So what does this have to do with the patient who presents with injuries resulting from a car [crash]? Everything.


Newton’s first law of motion tells us that if you’re moving at a particular speed, you will continue moving at that speed indefinitely unless an external force acts on you to stop you. Now think about this in the context of sitting in a moving car. You are moving at the same speed as the car that you’re in, even though you appear to be stationary relative to the car. If the car comes to a sudden stop, you will continue to move forward.  Here’s the part that really sucks – you continue to move forward at the same speed the car was moving at! This is why you always slow down before stopping the car, so that your body slows down too and comes to rest with the car. But if the car stops suddenly, such as when it hits a pole, or whatever, your body has not had time to slow down and keeps moving at the same speed the car was moving at before its sudden stop, until some external force stops it, such as the dashboard or the windscreen.

You can mitigate the damage incurred in a motor vehicle [collision] and explained by Newton’s first law with two simple strategies:

  • Don’t speed – this will reduce how fast your body will be moving if you have to come to a sudden stop.
  • Wear a seat belt – let the seat belt provide that external force that stops your motion, and not the dashboard or the windscreen. It’ll hurt less.


Newton’s third law of motion states that to every action there is an equal and opposite reaction. In other words, if you slam your fist into a wall with a force of 40lbs [18kg], the wall will slam your fist right back with a force of 40lbs [18kg]. Ouch! In case you’ve ever wondered, this explains why we don’t sink into the earth, because we exert our weight on the surface of the earth, and the earth pushes right back with an equal force. This is also why it hurts when you jump from a particular height above the ground. You hit the ground with a particular force, and it hits you right back. Anyway, so this means that if you’re hurled from a vehicle and you hit the ground, or a pole, or even the dashboard or windscreen, the object that you hit hits you back with a force of the exact same magnitude as the force you hit it with. The impact of this becomes more apparent when you think about Newton’s second law.

Newton’s second law of motion tells us that the force that acts on you or that you exert on another object depends on your mass and how fast you’re moving, to be more accurate, how fast you’re accelerating. Let’s look again at you sitting in a moving vehicle. The car you’re in is building up speed, accelerating, and then it hits an object, say another vehicle, or a pole. You weren’t wearing a seat belt so you continue to obey Newton’s first law and you continue moving, and hit something, the dashboard, or the windscreen, or whatever.  You will hit that object with a force that is the product of your mass and the acceleration of the vehicle that you were in. And according to what we said about the third law, the thing that stops your motion will hit you back with the same force. This is why you can get injuries even from a seat belt to your tissues or internal organs, because that seat belt hits you with the same force you hit it with, and that force depends on your mass and the acceleration of the vehicle.

The reason airbags are effective is that they have give, they don’t hit you back quite as hard because they’re bouncy and they take in some of that force and absorb it, instead of hitting you right back with all of it.

So how do you mitigate injuries resulting from a car [crash] relative to Newton’s second and third laws?

  • Don’t speed – that only increases the potential force with which you’re inviting external objects to slap you with.
  • Use a seat belt – it hits you just as hard but not as broadly, and it only affects a small part of you, and not your head or other places.
  • Make sure your airbags are working – they are kinder and don’t hit as hard.
  • Did I say don’t speed? Yes, don’t speed. Ease up on that pedal.
  • I could add, lose mass, or lose weight, but the speed is way more of an issue than that piece of cake I had in the [ED] today. However, it does matter, and I will explain more in another post.


You cannot change Newton’s laws of motion, they are what they are, and you can’t escape them. You can only make sure they don’t screw you up. So, not to belabour the point, but here are the main points again.

  1. Don’t speed
  2. Always wear a seatbelt
  3. Make sure your airbags work
  4. If you’re riding a bike, wear a helmet, because you’ll usually hit the ground head first, with a force equal to your mass times the acceleration of your bike, and the ground will hit your head back at the same force.



  • Newton was a cool guy
  • Physics is better than Everything bagels from Sam’s Club.*