2 TWO types of FrictionStatic – Friction that keeps an object at rest and prevents it from movingKinetic – Friction that acts during motion
3 Force of FrictionThe Force of Friction is directly related to the Force Normal.Mostly due to the fact that BOTH are surface forcesThe 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.
5 The Key to Force Problems Draw an accurate, labeled FBDAsk if the system is in equilibrium (∑F = 0 ) or acceleratingIdentify the required unknownSet up your equations, minding signsSolve for the required unknownNOW plug in values and evaluate.Check units and check for reality
6 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 kgUnknown: μs = ?Solve: μs = Fs, maxFnμs = Fs, maxmg= (75N)/(24 kg*9.8 m/s2) = (note: dimensionless)
7 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?FnFaFfmg102.9 N
8 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= - FfFnFaFfmg127.4 N
9 Warm Up: FBD Practice You take the elevator to the fourth floor. What forces are acting on you while in the elevator?
10 Warm Up: FBD Practice 2You push two boxes down the hall over a rough floor. What forces are acting on box B?
11 Warm Up: FBD Practice 3Box B accelerates constantly to the right while box A sits on top of it. What forces are acting on box A?
12 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.
13 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
15 Inclines q q Ff FN q q q Tips Rotate Axis Break weight into components Write equations of motion or equilibriumSolvemgq
16 Friction & InclinesA 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.FaFnqFfmg0.117q
17 ExampleA 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/sb) Calculate the coefficient of kinetic friction.TFNm2m2gcos30Ff30Tm2gm130m2gsin30m1g
19 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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