The Force of Friction

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Lesson 6

• The Force of Friction

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Friction

• Force that opposes two touching surfaces

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Types of Friction

• Rolling- allows a wheel to roll
• Sliding- opposes the motion of two surfaces
• Static- keeps an object at rest

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Sliding Friction

• Force that opposes the motion of two surfaces
  sliding past each other
• Microwelds constantly break and reform as the
  object slides. Motion ceases if the force cannot
  break the microwelds

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Static Friction

• Force that prevents two surfaces from sliding
  past each other
• The greater the force pushing the surfaces
  together, the stronger the microwelds

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Rolling Friction

• Force between a rolling object and the surface
  its rolling on
• The surfaces are deformed increasing static
  friction, causing the object to roll

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• What methods can you use to get a stuck car out
  of the snow, ice, or mud?
• Which type of friction would you increase?

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How does this game reduce friction?
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Question

• How do different surfaces affect the force of
  friction needed to move a block?

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Hypothesis

• (answer the question based on your own
  experiences with friction)

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Materials

• Waxed paper, paper towel, sandpaper, spring
  scale, block of wood

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Procedure

• See pages 50-51 of lab manual.

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TASK

• Follow procedures on pages 50-51
• Complete data table in your notes (test each
  surface 4 times to get an average)
• Create a bar graph to display your results
• Answer reflection questions on page 51

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Analysis

• See student data sheet and graph

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Conclusion

• (answer the question based on your test results
  even if its different than your hypothesis)

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New Questions

• Is the force of friction greater for two like
  surfaces or two different surfaces?

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Lesson 6.2

• Changing the Load

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Inertia

• Tendency of an object to keep moving
• (resist a change in motion)
• Inertia is a property of matter related to the
  mass of an object

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Newtons First Law of Motion

• An object at rest will stay at rest, and an
  object in motion will stay in motion, unless
  acted on by an outside force.

Law of Inertia
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Question

• What happens to the effort force needed to pull
  the block if you change the weight of the block?

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Hypothesis

• The greater the weight of the block the more
  effort force is needed to move it.

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Materials

• See material list from lesson 6

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Procedure

• Test the force needed to pull each block over 1
  surface type.
• Repeat, adding one block at a time.
• Show your results on a data table and bar graph.

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Analysis

• See student data sheet and graph

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Lesson 6.3

• Changing the Surface Area

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Question

• How is the force of friction affected by surface
  area?

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Conclusion

• (answer your question using the class data as
  evidence)

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Magnitude size and direction of a force

• Net Force
• Combination of forces acting on an object
• Balanced Forces
• Forces cancel each other and do not cause a
  change in motion
• Unbalanced Forces
• Forces do not cancel resulting in a change of
  motion

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Air Resistance Demonstration

• Test 1
• Single Filter vs. Seven Filter Stack
• Test 2
• Seven Filter Stack vs. Seven Filter Spread

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Activity Summary

• What can we prove about mass and air resistance
  from test 1?
• What can we prove about surface area and air
  resistance from test 2?

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Air Resistance

• The force of air resistance increases as an
  object speeds up. (wave your hands)
• Once this force equals the weight of the object,
  the forces are balanced.
• A net force of zero means there is no
  acceleration.

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Terminal Velocity

• If there is no acceleration the object falls at a
  constant speed
• 320 km/h

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The Whirligig Challenge

• Cereal companies like to attract attention to
  their products by including toys for children.
  Kellogg wants a new whirligig that will fall more
  slowly than the old ones.

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What is a Whirligig?
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The Whirligig Contest

• Activity Summary
• (answer the following questions in paragraph
  form)
• What did we do?
• What did we use?
• What did we learn?