SCIENCE FRICTION “Hard to live with, can’t live without” A • discover and describe factors affecting friction • gather and analyze data for verifying assumptions about friction ASSUMPTIONS • Factors affecting friction: A. Weight B. Type of material C. Surface Area EXPERIMENT A • Weight and Friction Analysis: Weight influences friction. The greater is the weight, the higher is the friction force. Trial Weight Friction 1 4.45 N 2.00 N 2 6.41 N 3.20 N 3 8.42 N 4.50 N EXPERIMENT B • Surfaces in contact and Friction Analysis: Type of material influences friction. Rubber on vinyl has greater friction than wood on vinyl. Trial Surfaces Friction 1 Vinyl on wood 1.90 N 2 Vinyl on rubber 2.20 N EXPERIMENT C • Size of Contact Area and Friction Analysis: Surface area does not influence friction. There is no difference in the magnitude of friction force. Trial Surfaces Friction 1 large 2.00 N 2 small 2.00 N CONCLUSIONS • Factors affecting friction: A. Weight B. Type of material • Independent of: A. Surface area B. Speed LAWS OF FRICTION Law 1: Friction is parallel to the surface of contact but opposite the direction of the object’s motion FN FA Ff Fg LAWS OF FRICTION Law 2: If the applied force does not cause motion, it is STATIC FRICTION. Likewise if the applied force causes motion, it is KINETIC FRICTION. FA=50N FA=100N FA=150N FA=200N FA=250N Ff=50N Ff=100N Ff=150N Ff=125N Ff=125N LAWS OF FRICTION Law 3: The ratio of friction to the normal force is equal to coefficient of friction, “mu” (µ) µ = Ff / FN Materials µs µk rubber on dry concrete 0.80 0.65 rubber on wet concrete 0.60 0.40 wood on wood 0.50 0.20 steel on steel (dry) 0.78 0.58 steel on steel (wet) 0.15 0.06 static friction kinetic friction PHYSICS BUSTER! A girl exerts a 36-N horizontal force as she pulls a 52-N sled across a cement sidewalk at a constant speed. What is the coefficient of kinetic friction between the sidewalk and the sled? PHYSICS BUSTER! A small child is dragging a heavy rubber soled shoe by its laces across a sidewalk at a constant speed of 0.35 m/s. If the shoe has a mass of 1.56 kg, what is the horizontal component of the force exerted by the child? PHYSICS BUSTER! You push a 25.0 kg wooden box across a wooden floor at a constant speed of 1.0 m/s. How much force do you exert on the box? If the force applied is doubled, what is the resulting acceleration? PHYSICS BUSTER! 1. You need to move a 105 kg sofa to a different location in the room. It takes a force of 102 N to start it moving. What is the coefficient of static friction between the sofa and the carpet? 2. Ms. Anne Liwanag is dragging a box full of books from her office to her car. The box and books together have a combined weight of 134 N. If the coefficient of static friction between the pavement and the box is 0.55, how hard must Ms. Liwanag push the box in order to start it moving?
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