The Force of Friction Friction opposes the applied

The Force of Friction

Friction opposes the applied force Force Applied

Friction opposes the applied force Force of Friction

Very Important Friction Video

Static Friction (fs) The resistive force that opposes the relative motion of two contacting surfaces that are at rest with respect to one another. When an object is at rest, the static friction is equal to and opposite in direction to the applied force. As the applied force is increased, the static friction increases (and when the applied force decreases, the static friction force decreases) to keep the object at rest.

Static Friction (fs) When the applied force is greater than the static friction force, then the object moves in the direction of the applied force! fs < FA

Kinetic Friction (fk) Kinetic Friction is less than static friction. When an object is already in motion the kinetic friction is less than the applied force. fk < fs < FA

Dependency of Friction depends on the weight of the object. ◦ Consider the amount of force to move a feather as opposed to moving an M 1 A 1 Abrams Battle Tank! Friction depends on the composition of the surfaces that come into contact with each other. ◦ Consider a new hockey puck on a freshly smoothed ice rink! Friction depends on the quality of the surfaces that come into contact. ◦ Consider a newly waxed floor at an ice cream stand as opposed to a floor that is very dirty and covered in ice cream!

The Coefficient of Friction The quantity that expresses the dependency of frictional force on the particular surfaces in contact is called the Coefficient of Friction. The Coefficient of Friction is defined as the ratio between the force of friction and the normal force between two surfaces.

Coefficient of Kinetic Friction The coefficient of kinetic friction is the ratio of the force of kinetic friction to the normal force:

Coefficient of static friction The coefficient of static friction is the ratio of the force of static friction to the normal force:

Frictional Force the value of m and the normal force on an object are known, then the magnitude of the force of friction can be calculated directly: If

Example: A 24 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. Given: fs = 75 N m = 24 kg Find: ms Solution: ms = fs/FN = 75 N / 24 kg(9. 81 m/s 2) ms = 0. 32

Drag Resistance (Air Resistance) Friction is when a solid comes into contact with another solid. Drag is when a solid comes into contact with a fluid (a liquid or gas). Drag Resistance is calculated: Rd = Drag Force (Fd) / speed (v) Streamlining = process by which you reduce the drag resistance.