11Aintro

Introduction to Mechanics

__**Mechanics**__ is the study of the forces that act on bodies to make them move (or not move).

Mechanics is sometimes divided into two parts :
 * **__Statics__** is where the body is stationary or at equilibrium
 * **__Dynamics__** is where the body is moving.

We will group both parts together and study it all under the topic of mechanics.

Forces


 * A __**force**__ is a vector quantity with magnitude and direction.
 * A force can act on a body as either a "push" or a "pull."
 * The force can be the result of a field such as a gravitational field or a magnetic field.
 * Or it can be the result of some direct contact on the body by an external agent, such as a rope or an engine or friction or air resistance.

The unit for force is usually **Newtons** (**N**) where 1 Newton = 1 kg.m/s 2.

Particles


 * For the purposes of this subject, we will continue to assume that all bodies are particles.
 * They have mass but no volume.
 * They do not spin or stretch or compress or absorb energy in any other way.

Assumptions

In this course, we make the following assumptions. They make the maths simpler. In more advanced work in later years, you will need to stop making some or all of these assumptions.


 * ** A light string **
 * the string has no mass at all.


 * ** Smooth surface **
 * the surface exerts no frictional force.


 * ** Inextensible string **
 * the string does not stretch.
 * All forces are transferred from one end to the other without loss.


 * ** Rigid bar or rigid rod **
 * the connecting bar does not compress.
 * All forces are transferred from one end to the other without loss.
 * The rigid bar can exert either a pushing force or a pulling force (strings can only pull)


 * ** Perfectly elastic string or perfectly elastic spring **
 * the spring or string does stretch or compress but it will not be permanently deformed.
 * It will return to the same shape/length when the force is removed.
 * Further, it will compress or stretch with the same degree of elasticity at all lengths.
 * a string can only pull, a spring can either push or pull

Resultant Force

.
 * The __**Resultant Force**__ is the vector sum of all the individual forces acting on the object.
 * The motion of an object depends purely on the **Resultant Force** acting on the object.