Everything Science Grade 11

12.3 CHAPTER 12. FORCE,MOMENTUM AND IMPULSE

Example 22: Coefficients of friction

QUESTION

A block of wood weighing 32 N is placed on arough, flat incline and arope is tied to it. The

tension in the rope canbe increased to 8 N before the block starts toslide. A force of 4 N

will keep the block moving at constant speed once it has been set in motion. Determine the

coefficients of static andkinetic friction.

SOLUTION

Step 1 : Analyse the question and determine what is asked

The weight of the blockis given (32 N) and twosituations are identified: One

where the block is not moving (applied force is 8N), and one where the block is

moving (applied force is4 N).

We are asked to find thecoefficient for static friction μsand kinetic friction μk.

Step 2 : Find the coefficient ofstatic friction

Ff = μsN

8 = μs(32)

μs = 0, 25

Note that the coefficient of friction does not have a unit as it shows a ratio. The

value for the coefficientof friction friction can have any value up to a maximum

of 0,25. When a force less than 8 N is applied,the coefficient of frictionwill be

less than 0,25.

Step 3 : Find the coefficient ofkinetic friction

The coefficient of kinetic friction is sometimes also called the coefficient of dy-

namic friction. Here welook at when the blockis moving:

Ff = μkN

4 = μk(32)

μk = 0, 125

Exercise 12 - 4

1. A 12 kg box is placed on a rough surface. Aforce of 20 N applied at an angle of 30◦to the
   horizontal cannot movethe box. Calculate themagnitude and direction of the normal and
   friction forces.


2. A 100 kg crate is placed on a slope that makes an angle of 45◦with the horizontal. The box
   does not slide down theslope. Calculate the magnitude and direction ofthe frictional force and
   the normal force present in this situation.