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Friction is a Force Friction slows down all moving objects. • Friction is a force that resists motion. • One reason Aristotle and lots of other people didn’t understand inertia…didn’t believe that a moving object would/will move forever unless some force slowed or stopped it. • Friction is that force! Everything we know slows down and stops eventually because of friction. • Friction is a net, external force that negatively accelerates most moving objects we witness. • …it is due to microscopic roughness on all surfaces. • … it slows down all moving objects. Examples of Forces ► Ff - - Friction (opposes sliding) - - Applied Force (an external push or pull) ► FN - - Normal (perpendicular to a surface) ► FW - - Weight (force due to gravity) ► Fsp - - Spring (push or pull of a spring) ► FT - - Tension (spring, rope, cable) ► …many more… more… ► FA All Forces have an Agent and an Object Agent: the cause of a force (what does the pushing or pulling) Object: the ‘victim’ victim’ of a force (what gets pushed or pulled) Agents and Objects of Forces Example: A car is towed by a tow truck: AGENT: The tow truck OBJECT: The car Forces have magnitude and direction. We can represent forces by drawing arrows Example: This diagram shows all of the forces acting on an object: object: Conventional Notation: FTYPE (AGENT, OBJECT) Forces are Vectors FA (T,C) 1 Drawing Force Vectors ► The direction of the arrow represents the direction of the force. ► The length of the arrow represents the magnitude of the force. Free Body Diagrams A picture of a ‘body’ with all the force vectors acting upon it represented graphically is called a FREE BODY DIAGRAM ► We always draw force vectors as ‘pulls’ pulls’ on objects, not pushes (arrow starts at object) Free Body Diagrams Rules for drawing free body diagrams: 1. 2. 3. 4. 5. Draw the object as a dot Only draw forces acting on that object Draw all forces as ‘pulls’ (arrow point away from the dot) Draw and label every force acting on the object. Length of each arrow must reflect magnitude (stronger forces longer arrows!) (equal forces same arrow length.) Free Body Diagrams... FN (normal force) Book FW (weight force) Normal means perpendicular; not ordinary or regular! Try it... Draw a free body diagram of your book sitting on your desk. Fnet and Free Body Diagrams Remember, the sum of all the forces is called the net force. (F (Fnet or ΣF) A free body diagram makes it easy to see whether Fnet = 0 or not. Fnet = ma, soooooooooooo… soooooooooooo…. If Fnet equals 0, If Fnet is upwards, If Fnet is sideways, then a equals 0 then a is upwards then a is sideways 2 Free Body Diagrams... The Answer... FN What forces are acting on a skier as she races down a hill? Ff W or FW A Free Body Diagram shows all of the forces acting on an object: The Answer... FN FN Ff Ff surface Hmmm… What is Fnet ? W or Fw W or FW Fnet and Free Body Diagrams A free body diagram makes it easy to see whether Fnet = 0 or not. then a equals 0 then a is upwards then a is sideways Fw or W = Weight Force FN = Normal Force FA = Applied Force Ff = Friction Force A Free Body Diagram shows all of the forces acting on an object: Remember, the sum of all the forces is called the net force. (F (Fnet or ΣF) If Fnet equals 0, If Fnet is upwards, If Fnet is sideways, FA Physics FN Ff FA Physics surface W or Fw Fw or W = Weight Force FN = Normal Force FA = Applied Force Ff = Friction Force 3 Uniform Circular motion Lab: Centripetal Force and Circular Motion Remember Newtons First Law? So what is going on here? F Constant Speed …not Constant Velocity Velocity is Changing …so acceleration exists …therefore a force exists. Uniform Circular motion A centripetal force causes circular motion Circular motion A B Where there is an unbalanced force, there will be an acceleration. a In the case of uniform circular motion, that unbalanced force is centripetal (centerpointing) Circular motion A If the ball is moving at a constant speed, can it be accelerating at all? B The instantaneous velocity is always tangential (sideways), but the centripetal force causes the ball to change direction. Circular motion A B Acceleration means change in velocity, not speed. Speed is constant, velocity is not. 4