PreClass Notes: Chapter 5, Sections 5.4,5.5 From Essential University Physics 3 rd Edition by Richard Wolfson, Middlebury College 2016 by Pearson Education, Inc. Narration and extra little notes by Jason Harlow, University of Toronto This video is meant for University of Toronto students taking PHY131. 2012 Pearson Education, Inc. Slide 1-1 Outline At the microscopic level, friction is complicated. The simple equations we ll develop here provide approximate descriptions of frictional forces. R.Wolfson 5.4 Static Friction, Kinetic Friction 5.5 Drag force 2012 Pearson Education, Inc. Slide 1-2 1
Image from http://www.which.co.uk/home-and-garden/leisure/reviews/walking-shoes/best-buy/about/ 2015-07-15 Static Friction A shoe pushes on a wooden floor but does not slip. On a microscopic scale, both surfaces are rough and high features on the two surfaces touch and adhere. This produces force parallel to the surface, called the static friction force. With increased normal force, the shapes locktogether better, there s more contact area, hence the maximum friction force increases. f s Shoe Wood 2012 Pearson Education, Inc. Slide 1-3 Static Friction The figure shows a person pushing on a box that, due to static friction, isn t moving. Looking at the free-body diagram, the x-component of Newton s first law requires that the static friction force must exactly balance the pushing force: points in the direction opposite to the way the object would move if there were no static friction. 2012 Pearson Education, Inc. Slide 1-4 2
Static Friction Static friction acts in response to an applied force. 2012 Pearson Education, Inc. Slide 1-5 Static Friction Static friction force has a maximum possible size f s max. An object remains at rest as long as f s < f s max. The object just begins to slip when f s = f s max. A static friction force f s > f s max is not physically possible. There is an approximate equation for predicting f s max : where the proportionality constant μ s is called the coefficient of static friction. 2012 Pearson Education, Inc. Slide 1-6 3
Image from http://www.which.co.uk/home-and-garden/leisure/reviews/walking-shoes/best-buy/about/ 2015-07-15 Got it? You try to push a 10-kg box across the floor. If the coefficient of static friction between the box and the floor is 0.20, what is the approximate minimum force that must be applied to start the box moving from rest? A. 50 N B. 40 N C. 20 N D. 10 N E. 2.0 N 2012 Pearson Education, Inc. Slide 1-7 Kinetic Friction A box slides to the right on a wooden floor. On a microscopic scale, both surfaces are rough and high features on the two surfaces collide and are deformed. This produces thermal energy, and produces force parallel to the surface, called the kinetic friction force. With increased normal force, more collisions occur, so the friction force increases. f k Box Floor 2012 Pearson Education, Inc. Slide 1-8 v 4
Kinetic Friction Measurements show the kinetic friction force is approximately proportional to the magnitude of the normal force: where the proportionality constant μ k is called the coefficient of kinetic friction. The kinetic friction direction is opposite to the velocity of the object relative to the surface. For any particular pair of surfaces, μ k < μ s. 2012 Pearson Education, Inc. Slide 1-9 μ μ 2012 Pearson Education, Inc. Slide 1-10 5
Typical Coefficients of Friction 2012 Pearson Education, Inc. Slide 1-11 Got it? You are pushing a 10-kg box across the floor with a pushing force of 20 N. If the coefficient of kinetic friction between the box and the floor is 0.10, what is the approximate acceleration of the box? A. 2 m/s 2 forward (speeding up) B. 1 m/s 2 forward (speeding up) C. 0 (not accelerating) D. 1 m/s 2 backward (slowing down) E. 2 m/s 2 backward (slowing down) 2012 Pearson Education, Inc. Slide 1-12 6
Rolling Without Slipping If you slam on the brakes so hard that the car tires slide against the road surface, this is kinetic friction. Under normal driving conditions, the portion of the rolling wheel that contacts the surface is stationary, not sliding. If your car is accelerating or decelerating or turning, it is the static friction of the road on the wheels that provides the net force which accelerates the car. 2012 Pearson Education, Inc. Slide 1-13 Section 5.5: Drag Objects moving through fluids like water or air experience a drag force. Faster objects experience a greater drag force than slower objects. The drag force on a high-speed car is significant. The drag force direction is opposite the object s velocity relative to the fluid. 2012 Pearson Education, Inc. Slide 1-14 7
Terminal Speed The drag force from the air increases as an object falls and gains speed. If the object falls far enough, it will eventually reach a speed at which F drag = F g. At this speed, the net force is zero, so the object falls at a constant speed, called the terminal speed. 2012 Pearson Education, Inc. Image from http://www.scientificamerican.com/article/bring-science-home-free-fall/ Slide 1-15 Terminal Speed 2012 Pearson Education, Inc. Slide 1-16 8