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Friction originates from forces between atoms and molecules when surfaces are
in contact. For example, friction occurs when a body moves on a rough surface
or through a fluid medium (water, air, etc.). There are
two types of friction:
Consider a block on a rough surface. Apply an external force Fext to the
- The Static force of friction ( fs ) is the force of friction
between two objects when there is no motion.
- The Kinetic force of friction ( fk ) is the force of friction
between two objects when there is motion.
Fext < fs(max) the block won't move
- as Fext increases, fs will increase until it reaches its maximum
Fext = fs(max) the block will start to move (this is called
the point of slipping).
- Once the block starts to move, the force of friction is given by fk .