Friction

Friction is the force between two objects in contact that opposes the relative motion of those objects. Static friction occurs between two objects when there is no relative motion. The value of static friction ranges from 0N to a maximum value depending on the net force parallel to the surfaces that are in contact. Kinetic friction occurs when the two objects in contact with each other have a relative motion. Friction is proportional to the normal force on the object. The ratio of the friction to the normal force is given by the symbol μ. When solving problems involving friction the force should be parallel to the surface and pointed either opposite to the direction of motion or opposite to the sum of the other forces in the parallel axis. Friction between a fluid and a moving object is called a drag force. The drag force is proportional to either the velocity or the square of the velocity depending on the speed of the object. Please watch the following presentation that will introduce friction.

Friction Practice

The following presentation will give you some examples of problems that include friction when applying Newton's Laws of Motion.

Friction Self-Assessment

Complete the self-check questions to verify that you understand how to solve problems dealing with objects moving in one dimension under constant acceleration.

1. A 15kg block is pushed by man up the side of a wall at a constant velocity of 3.5 m/s. The man pushes with a force of 200N at an angle of 25^o to the vertical as indicated in the diagram.

Problem 1 Image

A. Draw the free body for the block.
B. What must be the coefficient of kinetic friction for this to occur?

SOLUTIONS

2. A 40kg box accelerates at 2 m/s² to the right across a horizontal table when a force of 425N @ 20^o is applied to it. This box is tied to a second box by a rope that passes over a massless frictionless pulley. That box hangs suspended in the air over the side of the table. The coefficient of friction between the table and the 40kg box is 0.35.

Problem 2 Image

A. Draw the free body diagram for the boxes.
B. What is the mass of the box hanging from the rope?
C. Determine the tension on the rope connecting the two boxes.

SOLUTIONS

3. A 2.5kg ball is dropped from a building and reaches a terminal velocity of 15m/s. Draw the free body diagram for the force on the ball? What is the value of the drag force? If the drag force is defined as bv, what is the value of b?

SOLUTIONS

Friction Practice Problems

Download Friction Practice Problems for more practice.

Download solutions to the practice problems.

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