An Informal Study of the Physics of Running Fast and Becoming Faster
In my last article, I was out to reveal the lie being sold in the Speed-Agility-Quickness industry. With that behind us, the question looms: How do you really get faster?
In an effort to be concise, three examples, three principles, and three steps to becoming faster.
Quick definition: acceleration, as used in this article, is to cause an object to move faster.
EXAMPLE #1: How do we fire a faster bullet?
The instant a bullet leaves the barrel, it is losing speed. To increase the average speed at which a bullet travels, its speed when exiting the barrel must be increased. The bullet begins in the chamber at zero velocity (not moving). For it to be moving at a higher speed at the end of the barrel, a larger force has to be applied at the very beginning.
Principle #1: Newton’s Second Law of Motion – to accelerate an object of given mass, force must be exerted against that object.
Translation of Principle #1: to accelerate a body from rest, I must apply force to that body.
Application of Principle #1: to move yourself from rest, you must push against the ground. That push – how hard you push and in what direction – determines how much you accelerate. Each step you take while pushing hard enough in the correct direction allows you to accelerate more. The harder I can push, interestingly enough, the longer I can push in the correct direction and, thus, the more I can accelerate. The more I can accelerate, the faster I run.
EXAMPLE #2: How do we skip a stone farther?
When a thrown rock hits water just right, friction between rock and water creates a little water ramp which launches the rock into the air again. Well, that’s half of the story. The other half: water is dense and, as anyone who has belly-flopped into a pool can testify, it hits high-speed objects pretty hard. Part of skipping a stone really far is keeping it in the air longer after each bounce. In addition to the water ramp phenomenon, which is caused by the speed of your initial throw, the harder the water “hits back” against the rock, the higher it bounces; the higher it bounces, the longer it’s in the air.
Principle #2: Newton’s Third Law of Motion – in the interaction of two objects, force exerted by one produces an equal and opposite reaction by the other.
Translation of Principle #2: to move an object farther across a surface, I should provide a big initial push and take advantage of “bounce” off that surface.
[Aside: rocks slow down because of friction, which diminishes speed, which reduces the effectiveness of the water ramp, which reduces how high the rock can fly, which reduces how hard it can bounce.
At slow speeds, water doesn’t hit so hard, so the rock eventually sinks.]
Application of Principle #2: to accelerate effectively, I need a big initial push (see principle one) and I need to be airborne while moving forward as long as possible. A flying rock cannot choose to push against the water. It ONLY has friction and gravity as acting forces – friction from its spinning surface against the water (negative: it slows the rock down; positive: it creates the water ramp) and gravity pulling it down toward the water. A human body has terrific advantages: muscles and bones which can CHOOSE to push against the ground. Once your foot is on the ground, it has two jobs: support your body from falling down (because gravity pulls you down!) and push your body forward.
Fun fact: every single foot contact is an opportunity to PUSH YOUR BODY FORWARD. The harder you can push, after subtracting the amount of force needed to support your body against gravity, the more force you have left over for pushing yourself forward.The harder I can push, the longer I can push in the correct direction and, thus, the more I can accelerate. The more I can accelerate, the faster I run.
EXAMPLE #3: What makes a skydiver STOP accelerating?
To stop accelerating means you’ve reached a steady, unchanging speed (whether that speed is zero or 120 mph is irrelevant). A skydiver, like a skipping rock, has exactly TWO acting forces: gravity and friction. But this time, they are working in opposite directions. Gravity pulls the skydiver down. Friction – the resistance of air against that falling body – effectively pushes the skydiver up. Eventually, those forces become equal.
Let’s call the amount of each of those forces “100.” Now let’s do some math: 100 – 100 = ?
ZERO. And zero (net) force, means zero acceleration.
(See Principle #1! Also, this is Newton’s First Law in action, so in three simple laws, he pretty much figured out speed for all of us. In 1686. When was that “revolutionary” SAQ product made again?)
So a skydiver reaches terminal velocity. That means, because no force is left over to accelerate her, she cannot possibly go any faster. That also means, because no force is left over to resist her, she cannot possibly go any slower. (Hence parachutes. Or nets, for your entertainment.)
But a running body has THREE forces acting on it and – tragically – they don’t all simply go in opposite directions. The tragedy of this fact raises Principle #3…
Principle #3: Eventually, you absolutely WILL slow down.
Translation of Principle #3: A running body would stay at terminal velocity forever IF it could stay airborne AND IF the force pushing it forward AND the force pushing it backward were exactly in balance…but those conditions are impossible forever.
The bullet stays airborne but still slows down. Why?
There’s no more push forward!
Humans can push every time a foot is on the ground but we still slow down. Why?
In order to push, we can’t be airborne!
But in order to run at maximal velocity forever, we have to stay airborne AND push against the ground…
so… um… what the hell?
Application of Principle #3: You will inevitably slow down and it’s your brain’s fault.
By accelerating from rest, the body is now moving forward at a certain speed. That speed isn’t very fast (compared to bullets or skyscrapers) and the body isn’t being resisted by very much air (compared to airplanes or buses), so it doesn’t take much force to keep pushing the body forward, despite friction. But gravity is a nefarious, persistent force pulling you down toward the ground.
Your muscles don’t need to push you forward very hard.
They need to push you UP hard enough to keep you flying.
Here’s the really ugly part: the faster the body is moving, the less time your foot is allowed to be on the ground pushing.
(That’s why a skipping rock is necessarily a bouncing rock – it can’t pause on the water without sinking.)
You have to push just as hard as gravity is pulling you down (plus a little bit of push to keep yourself going forward), like accelerating from rest, but now you have LESS time to do it.
That’s a complex job for your brain and muscles. And that’s why you slow down.
Eventually, your brain and muscles can’t do the job in that tiny amount of time. Your brain gets tired.* You don’t push as hard. Because your brain is tired and you don’t push as hard, you’re only barely covering that gravity debt – you don’t have any left for pushing forward. So you slow down. Eventually, all the momentum you had fades away.
So, to have a higher terminal velocity, you need more momentum. To get more momentum, you have to have already accelerated well, because your brain and muscles are pretty used up by around 7 seconds of pushing. You have to push hard in the beginning, when everything is fresh. The harder I can push, meaning the amount of force left over after dealing with Mean Ol’ Mr. Gravity, the longer I can push in the correct direction and, thus, the more I can accelerate. The more I can accelerate, the faster I run. The faster I run, the more momentum I have, The more momentum I have, the longer it takes to slow down.
So what do you need to do to run faster?
1. Push harder.
2. Push in the correct direction.
3. Push hard faster.
(And if you have to run longer than 7 seconds…
4. Hold that precious momentum as long as possible.)
How can you accomplish those things?
1. Become stronger.
2. Learn correct mechanics for acceleration.
3. Be more explosive.
Sprinters, preserving momentum depends on a special quality (“bounciness,” in simple terms) and is beyond the scope of this article.
So, how to really run faster?
Lift heavier weights, lift them faster, and practice the skill of sprinting. That allows you to push harder in the correct direction for longer. That lets you reach a higher speed, from which it takes longer to slow down.
Speed development is simple.
Speed development takes time.
Speed development takes hard work.
As Dan John quips, “I said it would be ‘simple,’ not ‘easy.'”
BONUS QUESTION: Why do sprinters start in blocks?
Because the correct direction for pushing is BACKWARD – and blocks create a WALL to push BACK against.
* It cannot be said conclusively that this coordination failure is not also related to metabolic interference or motor unit fatigue.