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Static and Kinetic Friction

Normal Force N Applied Force F Friction Force ff Gravity Force W W = mg F = ma W= N F=f N = mg F – f =ma

Static Friction The Force of Static FN Friction keeps a F stationary object at rest! fs fs = sN Fg s = coefficient of static friction

Kinetic Friction Once the Force of Static Motion Friction is overcome, the Force of Kinetic Friction FN is what slows down a F moving object! fk Fg fk = k N k = coefficient of kinetic friction

Static & Kinetic Friction Coefficients Material Coefficient of Coefficient of Static Friction S Kinetic Friction S Rubber on Glass Rubber on Concrete 2.0+ 2.0 Steel on Steel Wood on Wood 1.0 0.8 Metal on Metal Ice on Ice 0.74 0.57 Synovial Joints in 0.25 – 0.5 0.2 Humans 0.06 0.15 0.03 0.003 0.1 0.01

Ex 1. If the coefficient of kinetic friction (µk) between a crate and the floor is 0.40, how much force is needed to slide a 100 kg crate uniformly across the floor?

Ex 2. A box of mass 10 kg requires 40 N to slide it across a surface. What is the coefficient of friction between the box and surface?

Ex 3. The tires on a 60 kg race car have a co‐efficient of Static Friction (μs) of 1.0. What is the maximum braking force (Static Friction) that can be applied to the car?

Ex 4. A box weighing 70 N is on a horizontal floor. The coefficient of kinetic friction is 0.22. Find the frictional force under the following conditions?

Ex 5. A 50 N force is applied to a 5 kg mass to slide it across a frictionless surface. What is the acceleration of the mass?

Ex 6. A 30 N force is used to push a box across a floor against a frictional force of 20 N. The box accelerates is 4 m/s2. What is the mass of the box?

Ex 7. A box weighing 30 N is on a horizontal floor. The coefficient of static friction is 0.30 and that of kinetic friction is 0.25. Find the frictional force under the following conditions: a) no additional forced is added b) a force of 5 N is applied to the east c) a force of 9 N is applied to the west