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

March 20th, 2008 · 13 Comments

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 accident and presents to the ER, 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 accidents? 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.
  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 it’s mass, and a is it’s acceleration.
  3. To every action there is an equal and opposite reaction.

So what does this have to do with the patient who presents with injuries resulting from a car accident? 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 it’s 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 accident 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, the wall will slam your fist right back with a force of 40lbs. 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 accident 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 ER 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 belabor 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.

Tags: Nursing · Physics · Random Postings

13 responses so far ↓

  • 1 Charles Dunn // Sep 29, 2008 at 1:59 pm

    From across the water in the UK. I read with interest your article on Newtons law. I liked it very much and would like permission to reproduce the article for our new Road Safety web site under Tips and Advice.

    I look forward to hearing from you

    Best wishes

    Charles Dunn.

  • 2 Mary // Sep 29, 2008 at 2:20 pm

    Thank you for your interest in this post. You are more than welcome to use it on your website as long as proper credit is given and a link is provided to this post.

    I look forward to seeing your website when it’s up.


  • 3 yeon // Dec 6, 2009 at 8:56 pm

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  • 4 Deborah // Dec 15, 2009 at 9:50 pm

    Great and practical explanation of Newton’s laws!

  • 5 Mary // Dec 15, 2009 at 10:03 pm

    Thanks Deborah, glad you found it interesting. I’m constantly surprised at the ways in which my knowledge of Physics crosses over and holds my hand when I’m dealing with patients or thinking about ways in which our bodies are impacted by and impact the world around us.


  • 6 Harsha // Mar 8, 2010 at 9:29 pm

    Excellent writing style. Really drew my attention.


  • 7 Hanan // Mar 23, 2010 at 10:12 am

    WOW, thanks! you just helped me with my essay for physics class :) good to know some people out there understand SOMETHING.

  • 8 Mary // Mar 23, 2010 at 10:52 am

    Glad the post was helpful to you :)


  • 9 Georgy- Mary // Mar 29, 2010 at 4:19 am

    I’m a sixteen year old High school student in Australia. I would just like to say that i have an essay tomorrow, and that i have learnt more about physics by reading your blog then sitting there all year in lessons and having it go straight over my head.

    so thank you for simplifying it for me,


  • 10 Y // Jan 25, 2011 at 4:01 pm

    that was amazing thank you

  • 11 Ann // Mar 16, 2011 at 3:45 pm

    Ditto Georgy! Great stuff Mary!

  • 12 Emily // Apr 4, 2011 at 10:16 am

    I agree with Georgy. I have learned by reading your blog, then sitting in physics class! Thankyou, this post has helped me very much with my paper due tomorrow.

  • 13 Larry // Oct 24, 2014 at 8:57 am

    Great blog post in plain English! The only thing I’d change is that concerning the 2nd Law, it’s not your acceleration *before* impact that gets you, but rather *during* – how quickly you go from moving to stopped (your “60-to-0” time if you will). Very short times = very large forces (cushions make the time much longer, which makes the force much smaller).

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