Static and Kinetic Friction

Introduction

The amount of friction force between two surfaces in contact depends on the type of the surfaces in contact and the amount of compression between the surfaces. Static friction is the force that is acting against your force before the object begins to move. If you exert a small push on the box, the box will not move because static friction is directly opposite to the push. If you apply a greater force than the static friction force, the friction increases to match the magnitude of your push. There is a limit to the magnitude of static friction, so eventually you may be able to apply a force larger than the maximum static force, and the box will move. The maximum static friction force is …show more content…
By starting the box from rest and timing a known distance, the acceleration can be calculated. From a free-body diagram, the relationship between the angle of incline, the normal force, and the friction force can be determined. The limiting angle of repose is the smallest angle at which an object will just being to slide down an inclined plane without being pushed to get it started. The coefficient of static friction is equal to the tangent of the limiting angle of repose. When actual sliding is taking place, the frictional force acting is slightly lower than the maximum static frictional force that acts just before the object starts to slide. Thus, the coefficient of kinetic friction is lower than the coefficient of static friction.
The friction forces that arise between any two contacting surfaces are divided into two broad categories based on the relative motion of the surfaces. Static friction exists whenever there is no relative motion and kinetic friction whenever one surface moves relative to the other. The magnitude of each friction force depends on the number of the chemical bonds formed between the two surfaces and their average strength. The direction of each friction force is always opposite the horizontal component of the applied force. Suppose you wish to slide a heavy box of mass m along a horizontal surface with a force F. As you increase F, an equal and opposite static friction force fs arises to keep the box at rest until F equals the maximum value of