The force that one surface exerts on another when the two rub against each other is called friction.
The strength of friction depends on two factors:
–1. the type of surfaces involved
2. how hard the surfaces push together.  
The Usefulness of Friction
•Without friction, we would not be able to walk!
•We would only slip when we try to move.
•In fact, we often try to increase friction.
–Tire chains in winter for better traction on ice.
–Basketball shoes for quicker stops and starts.
 Types of Friction
•Sliding friction occurs when solid surfaces slide over each other.
•Rolling friction occurs when an object rolls over a surface.
–The force needed to overcome rolling friction is much less than that of sliding friction.
•Fluid friction occurs when an object moves through a liquid or gas.
–The force needed to overcome fluid friction is usually less than that needed of sliding friction.
Lubricants replace sliding friction with fluid friction.   
•Gravity is the force that pulls objects toward each other.
•Free fall occurs when the only force acting upon a falling object is gravity. 
Air Resistance
•Objects falling through air experience a type of fluid friction called air resistance.
•Air resistance is not the same for all objects.
–The greater the surface area of an object, the greater the air resistance.
•Air resistance increases with velocity.
•Eventually, the air resistance equals the force of gravity (balanced forces).
–When these forces are balanced, there can be no acceleration.
–Even though the object continues to fall, its velocity no longer increases.
–This velocity, the greatest velocity the object reaches is called terminal velocity.
•In a vacuum, where there is no air, all objects fall with exactly the same rate of acceleration 
Projectile Motion
•An object that is thrown or launched is called a projectile
•When a projectile is launched horizontally, it is in free fall.
Since weight is a force, we can rewrite Newton’s second law of motion. 
 Weight = Mass X Acceleration due to gravity

•Weight is usually measured in newtons, mass in kg, and acceleration due to gravity in m/s2.

Universal Gravitation
•The law of universal gravitation states that the force of gravity acts between all objects in the universe.
–Any two objects in the universe, without exception, attract each other.
–Attraction between other objects is not noticeable because the gravity between us and Earth is much greater than other objects.
–The farther apart objects become, the weaker the gravitational force becomes.  
The force of attraction between two objects varies with mass and distance.If mass is doubled, force increases.If distance is doubled, force decreases.

If distance is halved, force increases.