If I asked you to define the word force, what would you say? You probably have a feeling for what force means, but you may have trouble putting it into words. It's kind of like asking someone to define the word “and” or “the”. Well, this lesson is all about giving you a better feeling for what the word force means. We'll be talking a lot about forces in many lessons to come. The simplest way to define force is to say that it means a push or a pull like pulling a wagon or pushing a car. That's a correct definition, but there's a lot more to what a force is than just that.
There are four forces in the universe that make everything move, shift, explode, zoom, wiggle, and dance. As two of these forces require a nuclear reactor in your garage, we'll just focus on the other two for now: the electromagnetic and the gravitational. (Actually, scientists think they found a fifth, but we'll stick with the basics to get started.)

## Scientific Concepts:

• A force is a push or a pull.
• There are four fundamental forces. In order of strength they are strong nuclear force, electromagnetism, weak nuclear force, and gravity.
• A force field is an invisible area around an object within which that object can cause other objects to move.
• A force field can be attractive (pull an object towards it) or repulsive (push an object away).
• The closer something gets to the object causing the force, the stronger the force gets on that object. This is the inverse-square law.
• The four basic force fields are gravity, magnetic, electric, and electromagnetic.
• An object will be pushed or pulled in the direction in which the overall net force is acting on it.
• The net force is the sum of all the forces on an object.

## Select a Lesson

 Detecting the Gravitational FieldOk, sort of a silly experiment I admit. But here's what we're going for - there is an invisible force acting on you and the ball. As you will see in later lessons, things don’t change the way they are moving unless a force acts on them. When you jump, the force that we call gravity pulled you back to Earth. Detecting the Magnetic FieldRemember, there are four different kinds of forces: strong nuclear force, electromagnetism, weak nuclear force, and gravity. There are also four basic force fields that you come into contact with all the time. The Electromagnetic FieldThe electromagnetic field is a bit strange. It is caused by either a magnetic field or an electric field moving. If a magnetic field moves, it creates an electric field. If an electric field moves, it creates a magnetic field. Detecting the Electric FieldYou are actually fairly familiar with electric fields too, but you may not know it. Have you ever rubbed your feet against the floor and then shocked your brother or sister? Flying Paper ClipHave you ever been close to something that smells bad? Have you noticed that the farther you get from that something, the less it smells, and the closer you get, the more it smells? Well forces sort of work in the same way. Force-full CerealDid you know that your cereal may be magnetic? Depending on the brand of cereal you enjoy in the morning, you'll be able to see the magnetic effects right in your bowl. You don't have to eat this experiment when you're done Net ForcesIt is very rare, especially on Earth, to have an object that is experiencing force from only one direction. A bicycle rider has the force of air friction pushing against him. He has to fight against the friction between the gears and the wheels. Building BridgesWhat keeps building from toppling over in the wind? Why are some earthquake-proof and others not? We're going to look at how engineers design buildings and bridges while making our own. Barrel RoofThis roof can support over 400 times its own weight, and you don't need tape! One of the great things about net forces is that although the objects can be under tremendous force, nothing moves!