Study Guide 3.2—Friction

Answer the following questions using complete sentences.

1.  What is friction? (page 85)  Friction is a force that resists the motion between two surfaces in contact. 


2.  When does friction occur? (85)  When you try to slide two surfaces across each other friction occurs.


3.  List previous experiences that you have had walking on these surfaces:

a)     ice-   When I walk on ice it is hard to exert a backward force on the ground so the reaction force that moves you forward is less.  It’s hard to walk!


b)    gravel or rocks-  When I walk on small stones the reaction force is also less.  Some stones are kicked back making it hard to move forward. 


c)     concrete-  Concrete is solid, so the action force of my foot pushing off of the ground is followed by an equal reaction force of the ground pushing back.  It is much easier to walk on concrete surfaces. 


4.  What are the action/reaction forces that allow you to walk?  Friction between your feet and the ground are the action/reaction forces that allow you to walk.


5.  Read the first paragraph on page 86, directly underneath where it says Forces and Surfaces.  Summarize this paragraph in your own words using complete sentences.  Flat surfaces might look and feel smooth.  If you looked at them under a microscope, though, you would see that there are tiny bumps and ridges.  These stick to the bumps and ridges on other surfaces.  This causes friction when things slide. 


6.  Several factors determine the friction between two surfaces.  Describe these factors in the chart below. (pages 86-87)


Types of Surfaces

Friction between two surfaces depends on the materials that make up the surfaces.  Different surfaces produce different frictional forces.  Rubber hockey pucks on ice produce less friction than rubber pucks on wood.


Motion of the Surfaces

Frictional forces can only be so large.  It is hard to push a chair at first, but when you get it moving it pushes more easily. 


Force Pressing the Surfaces Together

The harder two surfaces are pushed together, the harder it is for the surfaces to slide over each other.  If someone is sitting in a chair it presses it to the floor.  This increases friction and makes the chair harder to move. 


7.  When you rub your hands together really fast heat is produced.  What do we use in car engines to reduce friction? (page 88)  In car engines we use oil to reduce friction. 


8.  A fluid is a substance that can flow easily.  Examples of fluids would include

a)     solids and liquids

b)    solids and gases

c)     gases and liquids

d)    none of the above


9.  Explain what happens to a skydiver jumping out of a plane using complete sentences.  Use what you know about air resistance to describe this event.  (page 89)  When a skydiver jumps out of a plane gravity causes the skydiver to accelerate toward the ground.  As the skydiver falls, his body pushes against the air.  The air pushes back (air resistance).  As the skydiver’s speed increases, his air resistance increases.  Eventually the air resistance balances gravity, and the skydiver reaches terminal velocity.  When he opens his parachute air resistance increases further and speed slows.  This enables him to land safely.