1. Balanced and unbalanced forces

2. Net force
Net forces is an overall force acting on an object. In order to calculate the net force the body is isolated and interacts with the environment or other constraints are represented as forces.
Example: accelerating car and a pulley system where one of the loads are accelerating.

3. Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy.
An example: rubbing both hands together to create friction

4. Inertia
he resistance of any physical object to any change in its state of motion (this includes changes to its speed, direction or state of rest). It is the tendency of objects to keep moving in a straight line at constant velocity.
Example: like a coin on a card and taking the card away
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5. Gravity
a natural phenomenon by which all things with mass are brought toward one another, including planets, stars and galaxies. Since energy and mass are equivalent, all forms of energy, including light, also cause gravitation and are under the influence of it. Example: when tossing a ball up in the air and it comes down