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Static Friction Revisited Renate Fiora What is static friction? • Static friction Is the force that keeps an object “stuck” on a surface and prevents its motion. – The direction of static friction is opposite the direction the object would move if there were no friction. • In other words, the force of friction points in the direction needed to prevent motion. Determining the magnitude is a little weirder… • Consider the situation where someone is pushing on an unmoving box. – If there is no velocity, then there is no acceleration. – No acceleration means no net force, so the force of static friction must equal to the applied force. • What is the person pushes harder but there’s still no movement? – Well, the force of static friction must still be equal to the applied force. So, the force of static friction increases as the applied force increases up to some maximum value above which the object finally moves. • What factors affect this maximum static friction? 1. The types of surfaces involved. • A smooth surface is more slippery (has less static friction) than a rough surface. 2. The force pushing the two surfaces together.