We have forces acting on us everyday, though they likely don’t attract our attention. Gravity holds us to the earth and keeps our planet spinning in orbit. Forces within the atoms of our bodies keep the subatomic particles from flying apart. Frictional forces allow us to walk without sliding along the pavement. The magnetic force generated by our planet acts as a shield and protects the earth from solar radiation and space debris. Electricity works through the action of electrical force.

But what is a force? Are you able to explain it better than the folks in the following video?

In the simplest terms, a force is a push or pull on an object.  

A force is a push or pull upon an object resulting from the object’s interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction.

”The Meaning of Force” from The Physics Classroom, accessed 4/25/19.  https://www.physicsclassroom.com/class/newtlaws/Lesson-2/The-Meaning-of-Force

Given that definition, what forces do you think are acting upon you right now?  

Forces All Around Us

Sir Isaac Newton contributed much to our current understanding of forces.  In fact, the unit of force in the International System (SI), the newton (abbreviated N), is named after Sir Isaac.   Newton’s Laws of Motion describe the actions of forces.

Newton’s First Law of Motion

Newton’s First Law of Motion states that an object at rest will say at rest, and an object in motion stays in motion with the same speed and direction, unless acted upon by an outside force.This law is often called Newton’s Law of Inertia.  Though it sounds complicated, I’m sure you’ve observed the essence of the law yourself: an object doesn’t spontaneously move on its own unless it is acted on by an outside force (like a push or pull). Likewise, an object that is moving will continue moving in the same direction and at the same speed unless something acts upon it.  If you roll a toy car along the floor, it will continue moving in a straight line until it either runs into something, is moved by someone, or until friction (an outside force) slows it down.

Here’s a really fun experiment you can do to demonstrate the Law of Inertia (if you dare).

Newton’s Second Law of Motion

Newton’s Second Law of Motion explains the relationship between an object’s mass and the amount of force needed to accelerate it.  This law can be described by the equation:

F= m x a

where m is the mass of an object, a is the acceleration, and F is the force. If we have 2 objects at different masses and we push on each with the same force, the object with less mass will have a greater acceleration than the object with the greater mass.  This relationship also means that if we have two objects of different masses traveling with the same acceleration and they hit a barrier, the object with the greater mass will hit with a greater force than the object with less mass. Here is a video demonstrating Newton’s Second Law in a very interesting way.

The Difference Between Mass and Weight (An Application of Newton’s Second Law)

Many people use the terms weight and mass interchangeably. While an object’s weight and mass are related, they are by no means identical.  Mass is an indication of how much matter is contained within an object. Weight is a measure of the force with which gravity pulls on that same object. While an object’s mass (the amount of matter it contains) does not change depending on the object’s location, its weight may.  For example, if you weigh 100 pounds on Earth, you’d weigh only 38 pounds on Mercury. Why? Because the force of gravity is weaker on Mercury and doesn’t pull with the same amount of force as the Earth.

The relationship between an object’s mass is weight is actually explained using Newton’s Second Law.  In this equation, F represents the force with which an object is pulled toward the earth by gravity (in other words, its weight), while mass is the amount of matter contained within that object..  For a, we use the acceleration due to gravity on Earth (g). The value of g is the same no matter what the object’s mass (9.807 m/s2).  

F = m x a

Weight = m x g

Newton’s Third Law of Motion

Newton’s Third Law of Motion is often the trickiest to understand.  It states that for every action, there is an equal and opposite reaction.  When you sit down on a chair, your body exerts a downward force on the chair (the action), and the chair exerts an upward force of equal magnitude back on your body (the reaction).  As hard as that may be to visualize, it makes sense. If there were only the force of your body exerted downward on the chair without the equal and opposite force of the chair acting back on you, the downward force you exert would cause you to break through the chair and land on the floor.

This next video demonstrates Newton’s Third Law in space!

This video reviews Newton’s three laws of motion in ways that are easy to understand.

Types of Forces

Forces can be divided into two types.  Some forces, called contact forces, only exert their push or pull when they are in direct contact with the object they act upon. Contact forces include the types of forces we’ve been describing above (called applied forces), as well as frictional force, and tension force (like the pulling force on a string when it is stretched tight). Other forces can exert their push or pull even when they are not in direct contact with the object they act upon.  Examples of these action-at-a-distance forces include gravitational forces as well as electrical and magnetic forces.

Centripetal force is the force on an object on a circular path that keeps the object moving on the path.  

Centripetal force allows us to perform some pretty neat experiments, like the ones in the following videos. While these may appear to be “magic”, they aren’t.  Try them at home to see for yourself.

We may not think about it, but the air around us is pressing on us all the time.  We call this force air pressure or atmospheric pressure. From Newton’s Third Law, we know that if air pressure is pushing against our body (the action), then our body must be pushing back (the reaction).  Here are a couple of videos demonstrating experiments you can do with air pressure.

This next one is funny, but I disagree with playing tricks on restaurant servers.

Hopefully you come away from this post knowing a little more about forces than you did in the beginning, and a lot more than the folks from that first video!

Further Information

Marble Gravitron Experiment

Definition of Force in Physics

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