Evans SPH 3U1

Physics Grade 11

Unit 2: Dynamics

Note 12: Normal Force and Friction


Reference: Chapter 4.2, 4.3

Normal Force: The reaction force of a surface to an object. It is always perpendicular to the surface (show box on incline).

                        I.E,. Box on table (Action force is force of box on table, Reaction force is the normal force of table on box)

Example 1: For a 40 kg person sitting on a box, what is the value of the normal force? (hint: net force is zero)


Friction: (use overheads)

Friction is a contact force that resists motion.

Static Friction:

Static friction is the force that tends to prevent a stationary object from starting to move.

The maximum static friction (Fs) is called the starting friction. It is the amount of force that must be overcome to move a stationary object.

Kinetic Friction:

Kinetic friction is the force that acts against an object's motion in a direction opposite to the direction of motion. (types: sliding, rolling, fluid)

It is usually less than the starting friction between the surfaces.

For horizontal motion, if the applied force has the same magnitude as kinetic friction (Fk) the object will maintain a uniform velocity.

Coefficient of Friction:

Coefficient of friction,  

Since there are two types of frictional forces (static and kinetic), there are two different coefficients, and


Example 1: A force of 50 N is needed to pull a 6.0 kg object along a table at a constant speed. What is the coefficient of kinetic friction?

Example 2: A 60g tennis ball is dropped from the ceiling and undergoes an average acceleration of 8.2 m/s2 [down]. Calculate the force of kinetic friction (air resistance) acting on the ball?

Example 3: Two boxes are tied together with a rope. The first box has a mass of 5.0 kg and the second box has a mass of 2.0 kg. The coefficient of friction between the sleds and the snow is 0.10. You pull on the fist sled with a force of 100. N forward. 

a) What is the acceleration of the sleds?

b) What is the magnitude of tension in the rope?

Example 4: A string is tied to a 4.0 kg mass on a table and a 3.0 kg mass hanging over a pulley. The coefficient of kinetic friction between the 4.0 kg mass and the table is 0.30.

a) What is the acceleration of each mass?

b) Find the magnitude of the tension in the string?

c) How far will the masses move in 1.5 s if the initial velcoity of the 4.0 kg mass is 1.0 m/s forward?


Pg 171 #1.2

Pg 172 #3,5,6,7

Pg 174 # 1,2

Pg 177 #1,3,4

Pg 178 #3,5,6,8