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Everyday Forces •Weight- Fg •Normal Force-Fn •Friction-Ff Normal Force-Fn • Force exerted by one object on another in a direction perpendicular to the surface of contact. The normal force opposes gravity, typically. Fn Fn=mg Fg Fg=mg Force of Friction • The amount of frictional force between two objects is proportional to the normal force. Normal force Friction Force Applied Force Force of gravity • The amount of force required to move an object starting from rest is usually greater than the force required to keep it moving at constant velocity once it is started. • Friction is also dependent on the type of surface (tile vs. carpet), and the weight of the object (heavier objects, more friction). Coefficient of Friction • MU or is the coefficient of friction • = Force required to slide object on surface at constant speed normal force (or weight) • • • • Since we are at constant speed, Ff=Fapplied = Sliding Force (Ff)/Fn No unit, just a ratio Should be between 0 and 1. Coefficient of Friction Calculations • = Force required to slide object on surface at constant speed normal force (or weight) • = Ff/Fn • Example: Find the coefficient of friction that is present when a 600. N ice skater uses 30.0 N of force to push off with each of his strides to maintain a constant velocity. • Answer: =30.0 N / 600. N = 0.0500 Coefficient of Friction Calculations • = Force required to slide object on surface at constant speed normal force (or weight) • = Ff/Fn • Example: A 90. kg crate can be pushed along the floor at a constant speed with a force of 78 N. What is the coefficient of friction between the two? = Ff/Fn= 78 N/ (90 kg)(9.8m/s2)= .088 Coefficient of Friction Calculations • = Force required to slide object on surface at constant speed normal force (or weight) • = Ff/Fn • Example: How much force is needed to keep a 88.0 kg sofa moving across a floor at a constant speed if the coefficient of friction between the two surfaces is 0.220? 0.220=F/(88.0kg)(9.8m/s2) F=190 N