Evans                                                                         SPH 4U1

Physics Grade 12

Unit 3: Dynamics(Causes of Motion)

Note 2: Friction (Chpt 2.4)

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Reference Text Section 2.4  (not Bernoulliís or fluid friction)

Static Friction: Force which opposes the tendency of a surface to slide.

Maximum Static Friction: The magnitude of the force needed to start a stationary object moving. Through experimentation, coefficient of max. static friction is  no units

Kinetic Friction: The magnitude of the force needed to keep an object moving at a constant velocity.  Through experimentation,           coefficient of kinetic friction is   no units

Note: friction is perpendicular to the normal force (hence only deal with magnitudes of forces when finding ).

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Example 1:

A 15 kg mass slides down an inclined plane which makes an angle of 37 degrees with the horizontal as shown.  The coefficient of kinetic friction is 0.40.  Find the acceleration of the mass.

 

 

 

Example 2:

In the diagram below:

a)      Find the acceleration of the 10 kg mass assuming that the inclined plane is frictionless.

b)      Find the acceleration of the 10 kg mass assuming that the inclined plane is not frictionless and has a coefficient of kinetic friction of 0.4.

c)      Find the tension in the string.

 

 

 

 

 

 

 

Example 3:

A man pushes a 25kg law mower eastward along horizontal ground.  He applies a force to the handle of N.  The handle is set at an angle of  above the horizontal.  The frictional force is 64 N [W]. Find

a)      The acceleration of the law mower.

b)      The normal reaction force between the lawn mower and the ground.

c)      The coefficient of kinetic friction.

 

 

 

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Air Resistance (Air Friction):

        Air friction is very different from static and kinetic surface friction.  For kinetic friction the force is independent of velocity and it is constant. Air friction increases as the velocity increases.

 

I.E. If you jump out of a plane your acceleration is 9.8 m/s2 and your air resistance is 0 at first.  As your velocity increases your air friction will increase.  Eventually, you will reach

Terminal velocity where Air resistance = g (hence net force is 0 and your acceleration will be 0)

 

        Shape, orientation, mass and density affect an objectís terminal velocity.

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