# Dynamics: The Forces and Why Motion Occurs

Dynamics is the branch of mechanics that deals with the effect that forces have on the motion of objects. It explains why objects move.

#### 1. Newton’s Laws of Motion

• First Law (Law of Inertia): An object at rest stays at rest, and an object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced external force.
• Second Law: The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to its mass. This law is often written as F = ma.
• Third Law: For every action, there is an equal and opposite reaction.

#### 2. The Concept of Force

A force is a push or pull upon an object resulting from the object’s interaction with another object. Forces only exist as a result of an interaction. It is a vector quantity, having both magnitude and direction.

#### 3. Friction

Friction is a force that opposes the motion of an object. It acts parallel to the surfaces in contact. There are two main types of friction: static friction (which prevents an object from starting to move) and kinetic friction (which acts on moving objects).

#### 4. Circular Motion

When an object moves in a circle at a constant speed, it is experiencing uniform circular motion. The acceleration of the object is directed towards the center of the circle and is called centripetal acceleration. This motion is caused by a centripetal force.