FRICTION Static Friction (Fs) = Force exerted on a stationery object by a surface that prevents the object from starting to move. Fa < Fs If the applied force becomes large enough, the body will start to move (Fa > Fs) Kinetic Friction (Fk) = Force exerted on a moving object by a surface, acting opposite the of motion of object 𝐹 Coefficient of friction μ = 𝐹 𝑓 𝑭𝒇 = μ𝑭𝑵 𝑁 𝐹 Coefficient of kinetic friction 𝜇𝑘 = 𝐹 𝑘 𝑁 Coefficient of static friction 𝜇𝑠 = 𝐹𝑠𝑚𝑎𝑥 𝐹𝑁 𝑭𝒌 = 𝝁 𝒌 𝑭𝑵 𝑭𝒔𝒎𝒂𝒙 = 𝝁𝒔 𝑭𝑵 >>>>>>>All depend on type of surface Example: A 3.2 kg block of wood requires a largest force of 15 N before it will start moving on a horizontal wooden floor. Once it starts moving, it only takes 9 N to keep it moving at constant velocity. a) Calculate the coefficient of static friction b) Calculate the force of friction acting on the block if a horizontal force of 7 N [E] acts on the block c) Calculate maximum magnitude of static friction acting on the block if a 2.1 kg object is placed on top of it d) Determine the coefficient of kinetic friction Practice: P171 # 1, 2 P172 # 2, 3, 5, 6, 7