Friction

• Is the force which opposes the motion of two objects or surfaces that are in contact with each other
• Fore example, a box sitting on a floor. The force holding the surfaces together is called the Normal force.
• The Normal force is defined as the force which acts perpendicular to the contact surface.

                                                   Fgravity = w = Fn

• The amount of frictional force is determined by the amount of force holding the two surfaces together. It is also determined by the roughness of the two surfaces.

Identifying forces: If the box moves across the floor at a constant speed, the forces acting on it are balanced.

• Frictional force = Ff, this force oppose the motion of the box
• Normal force = Fn, this hold the contact surface together. It is perpendicular to the contact surfaces.
• Applied force = Fa, is the force moving the box across the floor.

Coefficient of friction

• Is a constant for any two surfaces in contact with each other
• It is number determined by the roughness of the surfaces.
• The rougher the surfaces are, the higher the coefficient of friction.
• It is calculated using this formula: μ = Ff / Fn
• μ (mu) is the coefficient of friction, it is unitless
• Ff is the force of friction
• Fn is the normal forces

Example: A box of 15kg being pushed forward at a constant speed. The applied force is 50 N. What is the coefficient of friction?

     Fn = w = mg = 15kg (9.8 m/s^2) = 147 N

     Ff = Fa, μ = Ff/Fn = 50 N/ 147 N = 0.34