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**Friction & Inclined Planes**

AP Physics

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TWO types of Friction Static – Friction that keeps an object at rest and prevents it from moving Kinetic – Friction that acts during motion

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Force of Friction The Force of Friction is directly related to the Force Normal. Mostly due to the fact that BOTH are surface forces The coefficient of friction is a unitless constant that is specific to the material type and usually less than one. Note: Does friction depend on surface area? Discuss.

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**Some Common Coefficients of Friction**

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**The Key to Force Problems**

Draw an accurate, labeled FBD Ask if the system is in equilibrium (∑F = 0 ) or accelerating Identify the required unknown Set up your equations, minding signs Solve for the required unknown NOW plug in values and evaluate. Check units and check for reality

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**Coefficient of Friction Calculation**

A 24-kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. A: Given Fs, max = 75 N m = 24 kg Unknown: μs = ? Solve: μs = Fs, max Fn μs = Fs, max mg = (75N)/(24 kg*9.8 m/s2) = (note: dimensionless)

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**Friction & Newton’s First Law**

If the coefficient of kinetic friction between a 35-kg crate and the floor is 0.30, what horizontal force is required to move the crate to the right at a constant speed across the floor? Fn Fa Ff mg 102.9 N

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**Friction & Newton’s Second Law**

Suppose the same 35 kg crate was not moving at a constant speed, but rather accelerating at 0.70 m/s/s. Calculate the applied force. The coefficient of kinetic friction is still 0.30. Fa= - Ff Fn Fa Ff mg 127.4 N

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**Warm Up: FBD Practice You take the elevator to the fourth floor.**

What forces are acting on you while in the elevator?

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Warm Up: FBD Practice 2 You push two boxes down the hall over a rough floor. What forces are acting on box B?

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Warm Up: FBD Practice 3 Box B accelerates constantly to the right while box A sits on top of it. What forces are acting on box A?

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**Stranger Than Friction**

Students in an introductory physics lab design an experiment to determine the coefficient of static friction between a 1.8 kg jack-o-lantern and the tabletop on which it rests. Their setup is that they attach a massless string to the jack-o-lantern, loop it over a pulley, and then attach the other end to a pail in which the students will add sand as the experiment progresses. The pail with sand was observed to weigh 11 N when the jack-o-lantern began to slide. A. Draw a free body diagram for this set up. B. What is known? What are you solving for? What equations can you use? C. Find the coefficient of static friction.

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**Stranger Than Friction Continued**

Key to solving: The maximum force of static friction is equal to the force exerted by the pail at the moment the jack-o-lantern begins to slide. Fs, max= μsN μs = Fs, max = W N mg After substituting values….. μs = 0.61 Reality Check: Coefficients of friction are always between 0 and about 1 and are dimensionless

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**Static and Kinetic Friction Lab**

Today: 1. Collect data 2. Analysis questions 1-3 Tomorrow: 1. Graphical analysis 2. Conclusions

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**Inclines q q Ff FN q q q Tips Rotate Axis Break weight into components**

Write equations of motion or equilibrium Solve mg q

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Friction & Inclines A person pushes a 30-kg shopping cart up a 10 degree incline with a force of 85 N. Calculate the coefficient of friction if the cart is pushed at a constant speed. Fa Fn q Ff mg 0.117 q

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Example A 5-kg block sits on a 30 degree incline. It is attached to string that is thread over a pulley mounted at the top of the incline. A 7.5-kg block hangs from the string. a) Calculate the tension in the string if the acceleration of the system is 1.2 m/s/s b) Calculate the coefficient of kinetic friction. T FN m2 m2gcos30 Ff 30 T m2g m1 30 m2gsin30 m1g

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Example 64.5 N 0.80 N

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**10/4/13 Exit Ticket On a half sheet of paper:**

Explain why pushing downward on a book as you push it across a table increases the force of friction between the table and the book. (Use vocabulary from this unit to answer this question!)

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