- The weight of the body (w) , which is the force that the mass of this body exerts downwards, driven by the acceleration of gravity.
- The external force (F) , which is the force that acts on the body to move it in one direction.
- The friction force (f) , which is parallel to the external force and acts in the opposite direction to it, to counteract it.
- The normal force (N) , which is perpendicular to the motion.
The magnitude of the friction force is independent of the size of the surface . Rather, it is the substance of which the body and the surface are made, which defines how great this force is.
Types of friction
Friction can be of two types depending on its state of rest or movement:
- Static friction : It is the friction force that occurs when two bodies come into contact without any movement between them. They remain at rest with the force of friction between them, resisting external forces.
- Dynamic friction : It is the friction force that occurs between two bodies or surfaces that are kept in motion.
Friction force characteristics
- A body requires more force when it begins to slide than when it is already in motion.
- The friction force does not depend on the size of the contact surface between both bodies.
- The friction force does not depend on the speed at which a body is sliding.
- The greater the normal force, the greater the friction force.
- Depending on the materials that rub against each other when rubbed, this action can produce heat, an example of which is the friction that occurs between a match or match and a rough surface, or that which we make with our own hands to heat them up.
Three examples of static friction:
- The one that occurs between a cardboard box with a lot of weight placed on the wooden floor, being difficult to move and lift.
- The one that exerts a book on another when being together (glued), one with another in a bookcase.
- The one we do by leaning against a wall.
Three examples of dynamic friction:
- The one that occurs between an iron and the fabric that is ironed.
- The one that is observed in the water when furrowing a finger in it.
- The one that happens when the hands rub against each other (this produces heat).
- The one that occurs between the books of a bookseller, which allows them to hold each other.
- The one that the water exerts on the hand when sliding it.
- The one that a boat exerts in the water when moving over it.
- The one that we exert on the coffee (a liquid), by turning the spoon to stir the sugar.
- The one that exerts a cup of water on the table.
- The one that makes an airplane with the air when flying.
- The one we do with the air when we walk.
- The one produced by the waves of the sea on the sand of the beach.
- The one that occurs between the body of birds and the air when they fly.
- The one that occurs between a sandpaper and a piece of wood or metal that is sanded.
- The one that happens between the body of a fish and the water in which it swims.
- The one that happens between pencil and paper, when writing.
- The wheels of the car when moving on the pavement.
- A boat on the water when moving forward generates friction between the water and the surface of the boat.
- When one hand rubs against another, friction occurs that generates heat and warms the hands.
- River water produces friction on the river bed.
- When two objects rub, such as a stone with a piece of wood, friction occurs that generates heat.
- An ice skater can go very fast because the friction between the ice and the blades of the skates is low.
- An object that enters the Earth’s atmosphere from outer space, catches fire due to the friction that occurs between them.
- What happens when a box is pushed over a rough surface.
- The one that happens when a foot slides on the floor that is wet, (the water reduces the friction of the floor so it is easier to slip).
- A rope that slips on a piece of wood can burn it.