Students observe how frictional forces are different for stationary and moving objects and determine the various factors affecting the force of friction.
After completing this tutorial, you will be able to complete the following:
Friction can be felt in the action of tires on pavement, the strength required to drag a heavy file cabinet, and the warmth produced by rubbing hands together. Friction is defined as a force that resists the motion of objects against each other, when the surfaces of those objects are in contact. The force of friction always acts parallel to the contact surface and opposite the applied force.
When a force is applied against an object resting on a surface below a certain threshold, no motion will result. The opposing force balances the applied force and is consistent with Newton's third law of motion. The magnitude of this frictional force must equal the applied force, up to a maximum above which the friction is overcome and motion ensues. The greatest frictional force that successfully opposes the applied force (i.e., the maximum force that can be applied without moving the object) is referred to as the maximum force of static friction.
Static friction acts on objects at rest. If, once in motion, a force of the same magnitude were continuously applied to the object, acceleration would result. This is because the applied force exceeds the force of friction acting on the moving object. This type of friction is known as kinetic friction, and is smaller in magnitude than static friction. Applying a force that exactly opposes kinetic friction results in a constant velocity. Friction is directly proportional to the coefficient of friction between two surfaces, as well as the object's mass, because normal force presses the two surfaces together. Friction does not, however, depend on the surface area of an object.
|Approximate Time||20 Minutes|
|Pre-requisite Concepts||Students should be familiar with the concept of force and how to measure force.|
|Type of Tutorial||Concept Development|
|Key Vocabulary||coefficient, dynamic friction, force|