5.1 Friction Intro and Statics SWBAT describe what factors affect the frictional force. SWBAT calculate the static friction between two surfaces.

Optional HW to do before this lesson Read and outline p. 135-139 If students want to get a head start on this topic, they can read about friction on these pages.

After answering this question in your notebook, answer the following: Do Now A phonebook is at rest on top of a table. If it’s mass is 2 kg, what is the normal force acting on the book? After answering this question in your notebook, answer the following: The pages of phone book A and B are interleaved (shuffled together). How much force does it take to pull them apart? Do you think you can do it??? Critical Thinking:

Phone Book Friction Demo

Friction 2 Things to Consider: Definition: Friction is the force exerted by a surface as an object moves across it or makes an effort to move across it. 2 Things to Consider: Do all surfaces exert the same frictional force? What other factor can increase frictional force?

Coefficient of Friction (μ) We measure a surfaces’ “roughness” using the coefficient of friction. μ (mu) is the symbol of the coefficient of friction. μ does not have units. μ is a scalar quantity. The larger the μ, the rougher the surface. (when μ=0, the surface is frictionless) Holt Ch. 4 Slide 37 Normally Not enough time to show the Holt Slide. Students normally get the fact that some materials are rougher than others, and don’t need to be shown the animation of the rough surfaces.

Where can we find the coefficients of friction? Reference Table Where can we find the coefficients of friction?

Polished Metal Surface Rough Surfaces Polished Metal Surface Typical Metal Surface http://academic.greensboroday.org/~regesterj/potl/Mechanics/Friction/FrictionA.htm Wood Surface Porcelain Surface

2 Things to Consider Do all surfaces exert the same frictional force? No, we use μ to differentiate between surfaces. What other factor can increase frictional force?

Normal Force and Friction Which will have a larger force of friction? (Which will be harder to move?) 5 kg 10 kg Fa= 5 N 10 kg will have a larger Normal Force, so it will have a larger force of friction

Normal Force and Friction Which will have a larger force of friction? (Which will be harder to move?) 5 kg Fa= 5 N Fa= 5 N Equal forces of friction

Normal Force and Friction Which will have a larger force of friction? (Which will be harder to move?) 5 kg Fa= 5 N Even though the weight is the same, the Normal Force increases, and so does the frictional force.

2 Things to Consider: Do all surfaces exert the same frictional force? No, we use μ to differentiate between surfaces. What factors can increase the force of friction? Changing the normal force acting on the object. Rougher surface More weight/ pushing down (larger normal force) How are these factors related to the frictional force? As the roughness increases, so does the friction As the normal force increases, so does the friction Therefore, they are both directly related to friction How are these factors related to the frictional force?

F f =μ F N Friction Formula Frictional Force = (coefficient of friction) (Normal Force)

Where is the formula for friction? Reference Table Where is the formula for friction?

Static Friction A force that resists the initiation of sliding motion between two surfaces that are in contact and at rest. Static Friction (Ffs) acts opposite the applied force (Fa) Ffs Fa Ffs Fa As Fa increases, Ffs increases until FfsMAX is reached and the block starts to move. Friction always acts parallel to the surface of contact

Example 1 A 5 kg wooden block is at rest on a wooden table. How much force is required to start the block moving? What would the frictional force be if the applied force is 10 N?

Example 2 Zip the monkey wants to move a stationary 20 kg crate of bananas across a horizontal floor. The material of the crate and the floor are unknown, but it takes 10 N to start the crate moving (the maximum static friction is 10 N). What is the coefficient of static friction between the crate and the floor?

Exit Ticket A 10 kg copper block is at rest on a steel table. How much force is required to start the block moving across the table?

5.1 Static Friction