Friction & The Difference Between Mass and Weight

The Difference Between Mass and Weight:
mass is the quantity of matter that makes up an object. it is measured in kilograms.
the weight of a body is the force it normally exerts on anything which supports it. a body exerts this force owing to the fact that it is itself attracted towards the earth by the force of gravity.
weight = mass x acceleration due to gravity
f     = mg
  w
a 1 kg object on earth weighs 1 x 9.8 = 9.8 N. the same object on the moon, where the acceleration due to gravity is 1 / 6 that of the earth, weights 1 x 9.8 / 6 = 1.63 N. the object has the same mass in both places, but the weight is different, due to the differences of the gravitational pull of the two planets.

Friction:
friction is a contact force that opposes motion. friction slows a moving object down. friction is also the force that stops stationary objects from moving. it plays an important role in our daily loves. walking would be impossible if there were no friction between the ground and the soles of our shoes. without friction, brakes would not work.
friction can be a nuisance, because when two surfaces rub together they convert kinetic energy into heat energy. when an object becomes hot, it expands and takes up more space. the inside of a car engine becomes very hot. it is necessary to reduce the heat generated, and consequently reduce the expansion of the engine components. we add oil to an engine to do this. a layer of oil between two surfaces acts as a cushion, reducing the friction between two surfaces rubbing together. air resistance is another example of friction. the greater the velocity of an object moving through the air, the greater the amount of air being pushed out of the way by the object. air resistance depends on the shape and the speed of the object. a truck with a large surface exposed to the air has to move a greater amount of air, therefore a greater force us needed to move at it at the same speed as a smaller object, such as a car. modern cars are designed to cut through the air with the least resistance, therefore they have small surfaces exposed to the air.