Newton’s 3 Laws of Motion
So much of the universe is governed by Newton’s Laws of Motion. Ask Science explains the impact that the laws of motion have on our lives using the most noble of sports, soccer.
Lee Falin, PhD
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Newton’s 3 Laws of Motion
If you glance through the books in your personal library, chances are you don’t have a copy of Principia Mathematica Philosophiae Naturalis. However, you might consider adding it to your collection because this book, written in 1687 by Isaac Newton, contains the definitions of some of the most important natural laws in the universe: Newton’s three laws of motion. Today we’ll take a look at those three laws and see how they impact the most noble of sports, soccer.
Newton’s First Law of Motion
Every soccer game starts with the ball sitting on the center spot of the field. It remains there until the team that won the coin-toss decides to kick it. The reason the ball just sits there and doesn’t roll off into a goal is because of the first law of motion.
The first law of motion is commonly referred to as the law of inertia. In its original form it states:
“Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.”
In other words, unless compelled to do otherwise, things tend to keep on doing whatever they were doing before. So the soccer ball, under no compulsion to do otherwise, just sits on the center spot. Waiting. Forever.
May the Force be With You (Newton’s Second Law)
Now eventually, someone will kick the ball and things really start getting exciting. When the ball is kicked, the force of the kick acts on the ball, causing it to accelerate. Forces and acceleration are covered handily by the second law, which Newton described as:
“The alteration of motion is ever proportional to the motive force impress’d; and is made in the direction of the right line in which that force is impress’d”
This wasn’t very catchy, so later on people just started referring to the second law as “Force equals Mass times Acceleration,” or “F=ma” for short.
Wind Resistance is a Real Drag
Let’s take a look at some of the other forces that are acting on the ball. First, we have the initial kick that propelled the ball out of its restful state on the center spot. As the ball soars through the air, it starts to experience another force, wind resistance or drag.
Drag is a force that acts on objects traveling through fluids. In science, we consider not just liquids to be fluids, but also gases such as air. This means that as the ball travels through the air, it is traveling through a fluid and is subject to drag forces. There are a couple of different factors that affect how much drag an object encounters as it flies through the air, including its shape and the exact composition of the air, but if those things remain equal, the main factor that affects drag is the relative velocity of the object and the fluid.
“Relative velocity” means the ball’s velocity compared with the air’s velocity. If the air is moving against the ball, then the ball will experience more drag than if the air is moving in the same direction as the ball.
The Gravity of the Situation
Another force that is constantly acting on the ball is the force of gravity. Gravity, as you might imagine, is always trying to pull the ball back down to the Earth. It’s important to note that gravity is always pulling the ball down, even back when it was sitting calmly on its center-spot.
The higher you kick the ball, the less gravity acts on it. However, that change is relatively small in terms of soccer ball kicks. Even if you could kick the ball as high as Mount Everest (or were playing a soccer game on the top of Mount Everest), the force of gravity would only decrease by about 0.2%.
Friction
Once the ball is back on the field, it starts to experience yet another force, friction. Friction occurs when one solid moves across another solid. How much friction is exerted on the ball and the ground depends on several factors, including the mass of the ball, the slope of the ground, and something called the coefficient of friction.
The coefficient of friction of two objects depends on what they are made of. The higher the coefficient of friction, the more friction those objects will encounter when moving against each other. For example, the coefficient of friction between rubber and asphalt is around 0.9, while the value for rubber and wet asphalt can be as low as 0.25, which is why your car is more likely to slide on a wet road than a dry one.
Newton’s Third Law
Last, but not least, we have the third law of motion. The third law is probably the most well known:
“Actioni contrariam semper et æqualem esse reactionem”
Or in English:
“To every action there is always an equal and opposite reaction.”
What this means, is that every time you kick a soccer ball, the soccer ball kicks you back with the same amount of force. So if that’s true, why don’t you go flying down the soccer field? Remember, that force is equal to mass times acceleration. Since your leg is considerably more massive than a soccer ball, it isn’t accelerated as much as the soccer ball is.
Conclusion
So that concludes our soccer-inspired tour of Newton’s laws of motion. One phenomenon we didn’t have time to talk about today, but which has heavy influence on the behavior of soccer balls, is the set of forces introduced when the ball spins. The physics behind this behavior are relatively complicated, but if you want to learn more, check out this great article by Rhett Allain at the National Geographic Science Blogs, or if you’re more curious you can read the original paper on the subject.
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Book image, schnaars at Flickr, CC BY-SA 2.0