Topics Covered:

• Definition of Forces
• SI Units of Force
• Free Body Diagrams
• Types of Forces
• The Normal Force

Forces

1. Definition: A force is usually described as a vector quantity with a definite magnitude and a definite direction which can be either a push or a pull on an object. 

2. Effects: Forces can have different effects on objects according to the type of material the object is made of.
   - the object could change its state of motion (make it move, make it go faster, stop it, or slow it down)
   - or it could change the shape of the object (deform it, resize it, reshape it...)

3. Fundamental Forces:  As far as what physicists know today there are only three or may be four fundamental forces
   - Force of gravity -- a very weak force
   - Strong Nuclear Force -- a very strong force
     (it holds the particles that make up the atoms and keeps them  together)
   -Weak Nuclear Force -- weaker than the strong nuclear force
     (it is responsible for the fact that some particles within the atom are released making the atoms radioactive)
   -Electromagnetic Force -- a strong force
    (made up of the electrostatic force of attraction between two charged particles at rest, and the magnetic
     force produced when these particles move)
   -Electroweak Force -- a new force
    This force is believed to be similar to the electromagnetic force.

Units of Force The metric (SI - International System) unit of force is The Newton.   Force is measured with a spring scale.  The amount of stretch on the spring within the spring scale is then calibrated in units of force. One Newton [1 N] is approximately the force of gravity ("the weight") acting on a medium sized apple falling under the influence of gravity alone, or the amount that a 100 g mass will stretch the spring of a spring scale by. The diagram shows a spring scale measuring the force of gravity on a medium-sized apple.  Its mass is about 100 g; its force of gravity is 1.0 N.  Another word for force of gravity is WEIGHT! Follow the link to find out more about force of gravity. Careful: Do not confuse the term Mass with the term Weight.  Although in everyday language they are used to mean the same thing, in Physics we use mass (a scalar quantity) to mean the "amount of matter in an object"; whereas weight is a vector quantity and it measures  the gravitational pull on an object (i.e. The Force of Gravity on an object).

Free Body Diagrams (F.B.D.)
A free body diagram is a simple sketch showing all the forces acting on an object when it is on its own (i.e. removed from its surrounding).  Here are some guidelines for drawing Free Body Diagrams.

• Forces are represented by vectors
• A relative scale should be used to show the relative size of the forces whenever possible.
• Only the forces acting on the object should be included not the forces that the object exerts on other objects.
• All forces (vectors) should be drawn with respect to the center of gravity (or center of mass of the object).  This point is usually located at the geometric center of the object.

Example:  Consider the apple hanging from the spring scale above.  To draw a free body diagram for the forces acting on the apple we would have to consider the apple alone, removed from the spring scale.