* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Free Body Diagrams
Classical mechanics wikipedia , lookup
Newton's theorem of revolving orbits wikipedia , lookup
Coriolis force wikipedia , lookup
Hunting oscillation wikipedia , lookup
Rolling resistance wikipedia , lookup
Jerk (physics) wikipedia , lookup
Minkowski diagram wikipedia , lookup
Fundamental interaction wikipedia , lookup
Nuclear force wikipedia , lookup
Work (thermodynamics) wikipedia , lookup
Mass versus weight wikipedia , lookup
Fictitious force wikipedia , lookup
Rigid body dynamics wikipedia , lookup
Centrifugal force wikipedia , lookup
Newton's laws of motion wikipedia , lookup
Frictional contact mechanics wikipedia , lookup
Classical central-force problem wikipedia , lookup
Day 51 Friction Aim: What are the different types of Friction? LO: Relate friction to the normal force LO: Calculate friction for different surface combinations LO: AGENDA Do Now - Worksheet Notes Worksheet HW# Due Friction Friction Friction is a special force that is caused by the surface roughness of an object. It always acts in the opposite direction of the motion of the object. There are two types of friction – Static, and kinetic Coefficient of Friction All surfaces exhibit friction, some more than others. It depends on the roughness of the surface of the object. It is represented by the symbol m. – For static friction: ms – For kinetic friction: mk Sliding Friction – Microscopic model Depends on microscopic (electrostatic) bonding forces Depends on roughness of the surface Kinetic Friction Kinetic friction is the force of friction on an object when it is moving The formula is: Ff = mkFN Static Friction Static Friction is the force of friciton on an object when it stands still. We find that it is harder to start an object moving than it is to keep it moving. The formula is: Fs msFN Graph of the behavior of sliding friction f s ms N f k mk N A Table of coefficients of sliding friction Example A boy exerts a 36N horizontal force as he pulls a 52N sled across a cement sidewalk at a constant speed. What is the coefficient of friction between the sidewalk and the sled (ignoring air resistance)? 36N 52N Solution Known: FN = Fg = 52 N Fpull = Ffriction = 36N because the sled is moving at constant velocity Ffriction = FNmk Therefore mk = Ff/FN mk= 36N/52N = ? Example 2 Suppose the sled runs on packed snow. The coefficient of friction is now only 0.12. If a person weighing 650N sits on the sled what is the force needed to pull the sled across the snow at a constant speed? m= 0.12 Fw = mg= 650N What force to pull sled? Inclined Plane A common free body diagram used is often the inclined plane. Another name for an inclined plane is a ramp. Look at the diagram to the right showing the usual forces on an inclined plane FN Ff W Vector Diagram FN Ff W If we look at just the vector diagram we see some interesting things We usually know the weight of the object, so we can find the normal force. The normal force is perpendicular to the friction force and the force of the inclined plane Example 3 A skier (Ki) has just begun to descend a 30o slope. Assuming the coefficient of kinetic friction is 0.10 calculate: (i) his acceleration and (ii) his speed after 4 s Example 3 (i) A skier (Ki) m = 7 kg has just begun to descend a 30o slope. Assuming the coefficient of kinetic friction is 0.10 calculate: his acceleration and (ii) his speed after 4 s Approach: (i) Resolve forces | | and to slope (ii) Calculate frictional force (iii) Find net force down the slope => acceleration (iv) Use vf = vi + at => vf Solution Force of gravity down the slope is: Fgpara = FgSin() Fgpara = 7kg*10 m/s/s*0.5 = 35 N Calculate Normal = Fgperp Fgperp = FgCos() Fgperp = 7kg * 10m/s/s *0.866 = 60.62 N Solution Continued Calculate Frictional Force: Ff = mkFnormal Ff = 0.1 * 60.62 N = 0.6062 N Caluculate Net force down the slope Fnet = Fgperp – Ff Fnet = 35 N – 0.6062 N = 34.4 N Solution last page Calculate Acceleration down the slope: Fnet = ma a = 34.4 N/7kg = 4.9 m/s/s Calculate velocity at t = 4 seconds Vf = Vi + at Vf = 0 m/s + 4.9 m/s/s * 4 s = 19.7 m/s Inclined Plane A common free body diagram used is often the inclined plane. Another name for an inclined plane is a ramp. Look at the diagram to the right showing the usual forces on an inclined plane FN Ff Fp W Fp = the force caused by the ramp Vector Diagram FN Ff Fp W If we look at just the vector diagram we see some interesting things We usually know the weight of the object, so we can find the normal force. The normal force is perpendicular to the friction force and the force of the inclined plane