Force - an action which tends to cause a change in motion. A push or pull
Forces come in two basic flavors; contact forces and field forces.
Contact forces occur as the name implies - when contact between two object is present. You cannot push your book across the table without contacting the book. A tennis racket cannot change the motion of the ball unless it contacts the ball.
By contrast, a field force tends to change motion at a distance. The earth exerts a gravitational force on an object even when the object is not touching the earth. Two magnets will affect each other before they come into contact.
Until a few years ago scientists believed that all forces could be categorized into five classes:
- Gravitational force - the force of attraction between any two objects with mass.
- Electric force - a force of attraction or repulsion between charged objects.
- Magnetic force - a force of attraction or repulsion between ferro magnetic objects.
- Strong force - the force holding protons and neutrons together in the nucleus.
- Weak force - the force which causes radioactive decay.
This means that all forces, whether they are contact forces or field forces, are either a form of electrical force or gravitational force. When you push your book across the table it is the electrons in the atoms of your skin which are repelling the electrons in the atoms of the book. If you think contact is "actually touching" then you never really touch the book. The effect is the same no matter how you define it.
Force tends to cause a change in motion of an object. The definition of a force is an operational definition; it is defined by what it does. Remember that changes in motion are measured as acceleration. Forces tend to cause acceleration.
The unit of measurement of force is the Pound (lb) in the English system and the Newton (N) in the metric system. You are most familiar with pounds, like the measurement of your weight. Newtons will be new for you.
To get an idea of the size of a Newton, one Pound is the same force as 4.45 Newtons. Roughly speaking a Newton is a quarter Pound. A 100 lb person would weigh 445 N. That's one reason we don't use the metric system to express weight. No one wants to weigh 445 of anything.
Notice that the phrase "tends to cause" has been used because, as you will see later, a single force may not cause a change. It is only when the total of all the forces acting on an object has a non-zero value that the object will change its motion.
Now here's an interesting fact. Forces never operate alone. They always occur in pairs. Isaac Newton stated this as one of his basic laws back in the 1600's. We often refer to these two forces as action/reaction forces. You can't tell which is the action and which the reaction, but it doesn't make any difference.
Simply stated I can not push on you without you pushing back on me. Our pushes are the same size (magnitude) , bit they are in opposite directions. I push on you. You push on me.
If you throw a ball against the wall the ball pushes on the wall, and at the same time, the wall pushes on the ball. The wall pushes on the ball with the same amount of force that the ball pushes on the wall. One of the forces is on the ball, the other force is on the wall.
Think of this example: A small car has a head on collision with a large truck. Does the car get "hit harder" than the truck?
Your first inclination would be to say "yes, the truck hits the car harder than the car hits the truck". Guess again. There is no doubt that the car will have more damage than the truck, that the people in the car may suffer more injuries than for those people in the truck, but it is not because the car gets "hit harder". As we will see in the next unit, the reason for the inequity in the collision is due to the smaller mass of the car. They both hit each other with the same force.
When a golf club hits a ball, the force of the club on the ball is the same amount of force the ball exerts on the club.
When a big senior runs into a small freshman in the hall the force of the senior on the freshman is the same as the force of the freshman on the senior.
When you stub your toe on a table leg the force of your toe on the table is the same force as the table on your toe.
Think about it, let it sink in. Whether you believe it or not, it's true.