Friction Friction is a force that opposes the motion, or tendency of motion, of an object. Friction is caused by the electromagntic interactions of particles within molecules at the surfaces of objects in contact. While these interactions can be rather complex, their combined effects can be reasonably approximated with a decent amount of consistency.

Basic Types of “dry” Friction – Static friction exists between the surfaces of non-moving objects Opposes applied forces that would otherwise cause motion…maintains equilibrium Maximum static friction refers to the most force that can be applied before the object starts to move – Kinetic friction Exists between the surfaces of objects when there is relative motion between the objects – Rolling friction

Coefficient of Friction The coefficient of friction is the ratio of the magnitudes of frictional force to the normal force acting between two surfaces. μ = f/F N Since this is a ratio of force to force, there are no units for the coefficient of friction This is an experimentally determined value for any two types of surface combinations. Its value depends on the nature of the materials and is roughly independent of surface area or speed

Coefficient of Friction The coefficient for static friction (μ s ) is generally larger than that of kinetic friction (μ k ) between surfaces. A common substitution to be made in problem solving will be f = μF N. – If working with static friction, this equation represents a maximum possible value.

Example – kinetic, constant speed The coefficient of friction between a 12 kg wooden crate and the floor is How much force is needed to push this crate across the floor at a constant speed?

Example - static A 24 kg crate initially at rest on a horizontal floor requires 75 N of horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.

Example – accelerated motion A 5.0 kg box is pushed horizontally across the floor with a force of 25.0 N. If the coefficient of kinetic friction is 0.24, what is the acceleration of the box?

When the applied force is acting at an angle… Remember, a component of that force acts vertically and a component acts horizontally An upwards component will tend to separate surfaces reducing the normal force and thus reducing frictional force since f = μF N What do you think would happen if there was a downward component to the applied force? FaFa F a y = F a sinθ F a x = F a cosθ θ

Example – applied force at an angle A 15 kg sled is dragged across the level snow at a constant speed by a force of 40.0 N that is applied 28° above the horizontal. Determine the coefficient of friction between the sled and snow.