Mr. Rogers' AP Physics C: Mechanics (With IB Physics Topics) Objectives

Syllabus 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter
Newton's Laws(4)
 Friction(5) Mech Energy(7)  Momentum Semester Exam

Chapter 5 (Continued): Friction 

AP Physics C Newtonian Mechanics: B. Newton's laws of motion (friction) approx 8%, cumulative 38%
 

Practice Test Study Guide

Objectives
Essential Question: Is friction a necessary ingredient in our world?

The 3 Models for Friction

  1. Describe static friction.

    • Prevents sliding between surfaces
    • Variable - adjusts to match the force which would otherwise cause sliding (the parallel force).

     

  2. Correctly use the model for calculating static friction.

Fs = Fp

static friction = parallel force

 

  1. Correctly use the model for calculating the transition point between static and dynamic friction.

Fsmax = msFn

  1. Describe the relationship of normal force to transition point between static and dynamic friction and describe how this knowledge is used with fasteners. Fasteners, such as screws, are a form of inclined plane and have very high mechanical advantages that can produce extremely high normal forces resulting in extremely high friction forces that resist "unscrewing".

  2. Describe dynamic or sliding  friction.

    • Resists sliding between surfaces
    • Constant for a give normal force

     

  3. Correctly use the model for calculating the dynamic friction.

Fd = mdFn

  1. State which form of friction tends to be lower, the maximum static friction or sliding friction. Dynamic is generally lower but never higher than the maximum static friction.

  2. Be aware that there are actually 3 different mathematical models for friction (see above).

  3. State the relationship between contact area and friction (Hint: contact area is not in any of the 3 equations).

  4. Solve flat bed truck problems.

 

Homefun (formative/summative assessment): Read section 5.8, Prob 37, 39 p.131

Relevance: Friction is a major material property that influences everything from tire performance to the performance of plastic trash bags.

Activities

Lesson 1

Key Concept: Static friction

Purpose: Understand the variable nature of static friction and the meaning of the static friction force.

Interactive Discussion: Objectives 1-4. Note that the mathematical model for static friction calculates the maximum value and that there is no effect from surface area.

Demo 5.1: Special pipe with rough and smooth surface - object: Friction is a highly complex phenomenon. Simple explanations like surface roughness do not explain it. 

Demo 5.2: Attach a large spring scale to a weight on a table and pull it horizontally. Do not allow the weight to move. For each reading of the scale ask what the static friction force is. - object: static friction force adjusts to match the pulling force.

Demo 5.3: C-clamp and wood block - object: increasing normal force increases the maximum static friction force.

In Class Problem Solving:  

  1. Jeb's flat bed truck accelerates with his dog Blue in the back.
  2. Jeb slams on the brakes with Blue on the roof.

Demo 5.4: Place a heavy object on a piece of paper and slowly pull on it. The weight moves with the paper. Jerk the paper quickly and the paper moves but the weight stays in virtually the same location. - object: An object resting on a second object will accelerate at the same rate as the lower one only if the friction force is sufficient.

Resources/Materials: Special pipe with rough and smooth surface, C-clamp and wood block, video Speed II.

Mini-Lab Physics Investigation (Requires only Purpose, data, and conclusion)
Title Speed II  (groups of three)
Purpose Determine if the heroes would fly through the window of the ship during its collision with the dock.
Overview A large ship crashes into a dock while the first mate calls out the ship's speed. The movies heroes crash through the windshield of the ship during the collision.  By measuring the time as the first mate calls out the speed, it's possible to plot a velocity vs. time curve.
Data, Calculations
  • Plot a velocity verses time curve for the ship and estimate the acceleration when the heroes fly through the windshield. Use linear regression.
  • Draw a free body diagram of the heroes just before they go through the windshield. 
  • Model them as though they can only experience linear motion and estimate the size of the friction force needed to restrain them. 
  • Calculate the maximum static friction force for the heroes assuming that the heroes have a mass = 80 kg and static ms =0 .5
Questions, Conclusions
  1. Is Hollywood movie time the same real time? Why or why not would it differ?
  2. Given the above calculations, were the people aboard the ship in danger? Explain.
Resources/Materials: Speed II video, stop watches
Essential Question: Can the friction force cause an object to accelerate?

Friction and Transportation

  1. State the force which moves people when they walk.

  2. State the key advantage in tug-of-war assuming COF for both teams is equal.

  3. State the force which accelerates a car.

  4. Calculate the maximum acceleration of a car given the coefficient of friction.

  5. Calculate the maximum slope a vehicle can climb given static COF

Homefun (formative/summative assessment): 40, 41, 43 p. 131-132 Serway

Metacognition Problem Solving Question: What is the practical limit on a vehicle's acceleration? In many cases maximum static friction provides the limit on a system's acceleration.

Lesson 2

Key Concept: The role of static friction in moving people and vehicles

Purpose: Illustrate the similarity of equations for the maximum acceleration of walking and vehicles on flat ground and on slopes

Interactive Discussion: Objectives 6-7. 
Demo 5.3: Tug of war between two people, one with friction, one without - object: Friction between the participants and the ground is the key factor in tug of wars.
Objectives 6-7. 
Demo 5.4: Wheel with marking pen attached.- object: show that the friction between a wheel and the ground is static friction

In Class Problem Solving: Objectives 9. Simultaneously list the following problems: 

  1. Old Blue sliding in Jeb's flatbed
  2. Max accel for the truck
  3. Max stopping accel for the truck
  4. Max centripetal force on the truck

Interactive Discussion: Objective 10. Derive an expression for the maximum slope a vehicle can climb. Is mass a factor?

Resources/Materials: Rope with handles on end and chair with wheels, tape and marker
Essential Question: At what angle does the parallel component of a object on a slope exceed its normal component?

Friction on Slopes

  1. Solve problems involving kinetic friction on slopes.

  2. Solve F=ma problems involving kinetic friction on horizontal surfaces with pushing or pulling forces at various angles.

  3. Solve stacked block friction problems on horizontal surfaces.

Homefun (formative/summative assessment) 45, 47, 53 p. 132-133 Serway

 
Essential Question: How can I make an "A" on the test?

Newton's Laws Review

  1. Work the practice test.
  2. Review the objectives.
  3. Correctly draw FBDs.
  4. Review the study guide

 

Summative Assessment : Unit Exam objectives 1- 18

 

Lesson 3

Key Concept: Changes in normal force interact with friction

Purpose: Illustrate how slopes and forces at differing angles affect kinetic friction and acceleration.

Group problem solving: Plot the acceleration, normal, friction, and parallel forces vs angle. Draw conclusions from the plots.
  1. Box on a slope with angle changing
  2. Box on horizontal ground being pushed with angle changing.
  3. Box on horizontal ground being pulled with angle changing.

In Class Problem Solving: Slope problems

  1. Fireman's rescue.
  2. Jackie slides down a slope
  3. Superman pulls Lois up a ramp.
  4. Stacked boxes.

Formal Lab Investigation
Title The Effects of Friction and Drive System on Vehicle Performance
Category Mechanics
Purpose Determine if the maximum slope a motorized Lego vehicle is able to climb can be determined from the static COF and the drive system type.
Models bmax = Tan-1(m) ;   m = Ff / Fn  Note: for tracked and four wheel drive vehicles Fn = mg . For two wheel drive vehicles Fn < mg . 
Overview Using the Lego Robotics kit construct a tracked vehicle. Measure the weight and static friction force of the vehicle using a spring scale. Next, measure the largest angle of the slope the vehicle can climb. Repeat the process twice, once for a four wheel drive vehicle and once for a two wheel drive vehicle. In each case, strive to  keep the gear ratios the same.
Safety Issues Lego Robots can be a tripping hazard if left on the ground.
Equipment Limitations When sending electric current to the motors, make sure their wheels are turning! If not, the motors will burn up. Do not allow the wheels and tracks to spin any longer than absolutely necessary. They will wear out.

 
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