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### Course: 49ers STEAM education>Unit 2

Lesson 1: Force and motion

# Football physics

This content is provided by the 49ers Museum Education Program.
Physics is the study of matter and its motion through time and space, as well as its interaction with energy and the forces created by this interaction.
So, what is a force? A force is a push or a pull exerted on one object from another. Forces make things move. You can make something start or stop when you push or pull an object.
There are many different types of forces in action in football. A player kicking a football is a force that makes the football fly through the air. A quarterback throwing a football is another example of a force that makes the football fly in a game.
When studying the concept of force, we can look to history to find mathematical principles that guide the laws of motion. Sir Isaac Newton was one of the most famous scientists of the 17th century to study the laws of forces and motion. Through careful study of how objects react to various forces, Newton developed the Three Laws of Motion. Below are explanations of each law and how these laws can be applied to football.

## Newton's first law of motion: The law of inertia

Now, imagine a football sitting on Levi’s® Stadium’s 50 yard line. What do you expect to happen to the football? Based on your experience, you would probably expect the ball to just sit there unless someone picks it up or kicks it. The law of inertia tells us that the football will remain at rest unless someone or something moves it by a specific force.
The law of inertia also states that an object (like a football) in motion remains in motion unless acted on by an external force. Inertia is defined as the force that keeps an object at rest or in motion. The law of inertia also tells us that an object at rest, like the football at the 50 yard line, will remain at rest unless another force acts upon it.
If our quarterback picks up the ball and throws it, it will fly in the direction he threw it, at a specific speed based upon how much force he applies in his throw. Once the ball leaves the quarterback's hands, the first law tells us that if there are no other forces on the ball, the ball would continue to travel in the same direction and with the same speed until other forces affect its flight.
Based on our real life experience throwing things, we know that the ball does not continue to travel in the air with the same velocity indefinitely. That is because there are other forces that are acting on the ball, including gravity.

## Newton's second law of motion: Force and acceleration

As a football player, you might want to throw or kick the ball as far as possible, or you might want to throw it high enough that the player on the other team can't catch it. We can use Newton’s Second Law of Motion to figure out the best angle to apply the force on the ball so that it does what the football player wants it to. In fact, the football player has probably been doing physics in their head without even realizing it!
The second law of motion states that force on an object is equal to the mass of the object multiplied by its acceleration. If we apply this law to a football, it tells us that the amount that the ball accelerates depends on the force applied by the quarterback and the mass of the ball.
$\text{Force}=\text{mass}×\text{acceleration}$
We use the word mass to talk about how much matter there is in something. Matter is anything that takes up space.
Acceleration is the name we give to any process where the speed and direction of an object changes. There are only two ways for you to accelerate: change your speed (speed up or slow down) or change your direction—or change both.
To understand this physics law, let’s see it in practice through another football example. When a quarterback throws the ball, the player is applying a force on the object. This causes the football to accelerate. We know the football accelerates because it starts from a resting point in the quarterback’s hand and then speeds up after the appropriate force is applied to the football to reach the targeted receivers on the field.
The second law also tells us that acceleration of the football also depends on the mass of the football. If the player is throwing an extra heavy football, the acceleration of the football would decrease compared to a lighter football that is thrown with the same force.

## Newton's third law of motion

The third law of motion states that for every force applied there is an equal and opposite reaction force.
An illustration of this might be when a player is trying to catch a football from a very high kick. The football, coming down from above, exerts a force (a push) on the player as he catches it. The player then exerts a force that is equal in magnitude (the size of a force) and opposite in direction. This slows down the ball so the player can catch the football and bring it to rest.
Another example of Newton’s third law can be witnessed in the 49ers locker room. Picture a football helmet sitting on top of a player’s locker. Even at rest, the force of gravity is pulling on the helmet.
The locker is between the helmet and the locker room floor. Based on the third law, we know that the locker must be pushing up on the helmet with a force equal in magnitude. This keeps the helmet from falling due to the force of gravity.

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