Notes

vocab
force push or pull action on an object
gravity force of attraction between all objects in the universe
work product of force moving through a distance

FORCE
Motion is the moving of matter from one place to another. Everything in the world is made of matter. Matter is in constant motion. Atoms and molecules that make up matter also are moving. Even the electrons inside the atoms are moving. They move around the nucleus. The protons and neutrons within the nucleus also are in motion. All things in the universe move. The Earth, planets, and stars continually move. In this lesson you will learn about force and how it affects motion.

FORCE AND WORK
Force is defined as a push or a pull on an object. When you want to move an object, you use force. Gravity is the force that pulls things toward the center of the Earth. To lift an object, you must exert force. Your force must overcome the force of gravity. More force is usually required to lift an object than to push it along a smooth surface because of the force needed to overcome gravity. There are many different types of forces. Some forces involve objects which are physically touching each other. These types of forces are called contact forces. When you push a wheelbarrow or lift a box you are using a contact force. Friction, air resistance, and tension are other examples of contact forces. Other forces involve objects that are not in direct physical contact. These types of forces are called action-at-a-distance forces. The Earth, for example, is not physically touching the sun, yet it is affected by the sun's gravitational pull. Electrical, magnetic, and gravitational forces are examples of action-at-a-distance forces. Force has both magnitude and direction and thus is a vector. Magnitude refers to the amount of force. It is measured using a standard metric unit called the Newton (N). However, the Newton only describes half of the picture. Imagine a person is pulling a wagon filled with bricks using a force of 10 Newtons. We know how much force they are using, but we do not know in what direction they are moving. Force also has a direction. It is more accurate to say a person is pulling a wagon to the right with a force of 10 Newtons or to the left with a force of 10 Newtons. For this reason, many force diagrams will include an arrow called a vector indicating the direction of a force and the magnitude of the force. In some instances, two or more forces acting on an object will balance, or cancel each other out. For example, imagine a book on a table. The downward force of gravity and the upward force of the table are equal. When the book is not moving, the forces balance each other.



The force diagram below shows forces that are added together because they are in the same direction.

A car has a force of 10 N to the west. Then a second force of 10 N to the west is added. The car is being pushed with a total net force of 20 N west. This could also be written as 20 N to the left.



This force diagram shows the two 10 N force vectors acting on the car. Conventionally, the two 10 N forces are added and represented by a single force vector:



Work
Force is used to do work. In science, work has a special definition. Maybe you think of work as a job or a means of earning money. Scientists do not think of it in this way. According to science, work is only done when a force moves an object. If you try to push a piano and are unable to move it, you have done no work. To accomplish this work, you must apply a force that causes motion. You may have spent energy trying to move the piano, but you have accomplished no work unless you have moved it.