# PHYS 2211 Module 6.2

## Friction 6.2 Friction

### Learning Objectives

By the end of this section, you will be able to:

• Describe the general characteristics of friction
• List the various types of friction
• Calculate the magnitude of static and kinetic friction, and use these in problems involving Newton’s laws of motion

### Friction Discuss!

Consider how you would answer these questions. Then bring this to class for a group discussion.

### At-home experiment to determine the coefficient of static friction

The coefficient of static friction between two objects is determined experimentally. Below is a table that gives some friction coefficients for different materials:

To experimentally determine the coefficient of static friction between two objects, you will need the following materials:

• ruler or meter stick
• flat, somewhat rigid, and movable surface, like a book or a piece of cardboard
• some object that can slide on the flat surface

Place the flat surface on a table and place the object on the flat surface. When you lift one end of the flat surface, it makes an angle θ with the table. If you increase this angle enough, the object on it will slide down. You want to find the angle where the object is just about to slide but not actually sliding. This is the point of impending slip and where the static frictional force is a maximum. If you increase the angle any more, the object slides and the frictional force becomes kinetic friction.

With the flat surface at this maximum angle and the object not sliding (but is at the point of impending slip), measure the angle. You can use the meter stick to measure two sides of the right triangle formed with the flat surface and the table. Then use the necessary trig to find the angle.

Draw a free body diagram of the forces acting on the object when it is at this point of impending slip. Then use the free body diagram and Newton’s 2nd law to solve for μ. This will be a function of the angle. Plug in the value of the angle you measured to find a numerical value for the coefficient of static friction.

Consider a box that is placed on different surfaces. In which situation(s) is no friction forces acting on the box? In which situation(s) is a static friction force acting on the box? In which situation(s) is a kinetic force acting on the block?

• The box is at rest on a rough horizontal surface.
• The box is at rest on a rough tilted surface.
• The box is on the rough-surfaced flat bed of a truck that is moving at a constant velocity on a straight level road, and the box remains in place in the middle of the truck bed.
• The box is on the rough-surfaced flat bed of a truck that is speeding up on a straight level road, and the box remains in place in the middle of the truck bed.
• The box is on the rough-surfaced flat bed of a truck that is climbing a hill, and the box is sliding toward the back of the truck. Practice!