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Friction Objectives • After the lesson, you should be able to: – What is friction – Describe the effects of friction on motion Frictional Force How does friction affect motion? • Friction always opposes motion between two surfaces in contact. • The diagram below shows a microscopic view of two surfaces in contact. Frictional Force • Friction is a force that opposes motion. SI unit N • Friction makes our daily life possible. • The presence of friction reduces the net force acting on an object. 3N 5N 2N 2N 5N 3N 5N Frictional Force • A girl pushes a box of mass 5kg along a frictionless surface with constant force of 10N. – What is the acceleration of the box? • F = ma, a = F/m = 10N/5kg = 2ms-2 – If the surface has a friction of 4N, what will be the acceleration of box? • Net force on box = 10 – 4 = 6N • a = F/m = 6N/5kg = 1.2ms-2 Constant velocity 0.2ms-1 Frictional Force 50N 20kg Frictional force • A boy pushes a box of mass 20kg across the floor at a constant velocity of 0.2ms-1 by exerting a horizontal force of 50N. – What is the resultant force acting on the box? Since velocity is constant, zero acceleration. Resultant force is zero. – What is the frictional force acting of the box? For resultant to be zero, frictional force on box is equal and opposite to applied force, i.e. 50N Constant velocity 0.2ms-1 Frictional Force 50N 20kg Frictional force • Now, instead of 50N, the boy exerts 80N on the box. – What is the new resultant force acting on the box? The frictional force on box is still 50N. New resultant force = 80 – 50 = 30N – What is the acceleration of the box? a = F/m = 30N/20kg = 1.5ms-2 Acceleration a 80N 20kg Frictional force Which Direction is Friction? Frictional Force • • • • Without friction life would not be possible Daily activities involve friction Contact force Space has no friction Frictional Force • How tyres work? Frictional Force • Advantages – Make daily activities possible – Earth’ atmosphere use friction to protect us from meteorites force of foot pushes the ground backwards force of ground pushes the foot forward Frictional Force Friction can be a problem. Three reasons for friction being a nuisance: • rubbing of surfaces wears away materials. • friction slows down moving objects. • friction produces unwanted heat. friction slows down this cyclist friction has worn out the rubber on the top tyre Frictional Force • But what if the meteorites are bigger? Summary • Friction is a force that opposes motion. • The presence of friction reduces the net force acting on an object. • Frictional force is fixed for a particular object. Frictional Force • Ways to reduce friction include: – Wheels – Ball Bearings – Lubricants and polishing surfaces – Air cushion e.g. hovercraft and magnetic levitation in trains – Make body streamlined (reduced air resistance) methods to reduce friction Methods to reduce friction Examples Moving parts are made as smooth as possible The piston and cylinders of an engine made of highly polished steel or aluminium Materials with very low frictional resistance are used Slippery graphite used for contact points in motors and dynamos Ball and roller bearings are placed between the moving parts In the hubs of bicycle wheels Surfaces are separated by a lubricant Engine oil and grease used to lubricate engines Surfaces are separated by air cushion A hovercraft travel over rough land, swamp or sea friction Friction and Its Effects Identifying forces and free body diagrams • Block diagrams with arrows used to represent forces acting on a body are called free body diagrams. • These diagrams help you to solve problems on forces in Physics. Friction and Its Effects Key Ideas • Friction opposes motion between two surfaces in contact. • A non-zero resultant frictional force will cause a moving object to slow down to a complete stop. • There are both positive and negative effects of friction in our daily lives. • Free body diagrams are drawn to help us identify forces acting on a body or system of bodies, in order to solve problems. Friction and Its Effects Test Yourself 3.4 1. Can you lean against a wall if frictional forces are absent? Answer: No. When you lean against the wall, the frictional force between your body and the wall, as well as the frictional force between your feet and the floor, keep you from slipping. 2. How can you reduce the effect of friction on the motion of a body? Answer: Using wheels, ball bearings and lubrication. Friction and Its Effects Test Yourself 3.4 3. A force F of 50 N is needed to keep a trolley of mass 60 kg moving at a uniform velocity of 2 m s-1. What is the frictional force f on the trolley? a=0 F = 50 N friction f = ? Answer: Since the trolley is moving at uniform velocity, this means that acceleration a = 0. The resultant force is thus zero. Hence the pushing force F is equal to the friction force f. f = F = 50 N Friction and Its Effects Test Yourself 3.4 4. A feather and a stone are released simultaneously from the same height. Explain why the feather will fall more slowly to the ground than the stone even though the acceleration due to free fall is the same for both f the feather and the stone. air stone W Answer: The air resistance fair experienced by the stone is relatively much smaller compared to its weight W. Hence the stone has an acceleration close to g, the acceleration due to gravity. fair W Answer: The feather experienced an air resistance fair that is quite comparable to its own weight W. Hence the feather’s acceleration is very low.