PHY131H1F - Class 9. Today, finishing Chapter 5: Kinetic Friction Static Friction Rolling without slipping (intro) Drag

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1 PHY131H1F - Class 9 Today, finishing Chapter 5: Kinetic Friction Static Friction Rolling without slipping (intro) Drag Microscopic bumps and holes crash into each other, causing a frictional force. Kinetic Friction creates internal energy (thermal). Clicker Question A box is being pulled to the right at steady speed by a rope that angles upward. In this situation: A. n > mg. B. n = mg. C. n < mg. D. n = 0. E. Not enough information to judge the size of the normal force. 1

2 You should now see your mark out of 32 under My Grades on portal. The raw average was 19/32 = 59%. The instructors in this course have decided to adjust the marks by adding 2 points to every student s mark, but holding the maximum mark to 32/32 = 100%. Test 1 Marks Posted The mark posted on portal is your adjusted mark. The adjusted average and median were 21/32 = 66%. After the adjustment, 263 students (23% of the class) got less than 16/32 (fail). After the adjustment, 72 students (6% of the class) got 32/32 = 100%. Test 1 Marks Posted You will receive the long-answer part of your test in Practicals between today and Oct.20. The multiple choice bubble sheet is stored in MP129 if you would like to review it, please stop by and ask April. Check over the marking and compare it to the Test 1 Marking Scheme posted on the portal. If there has been a mistake in the marking or computation of your mark, please bring the test to me or April Seeley in MP129 and we will fix the mistake. The deadline to report mistakes in the marking of Test 1 is Oct

3 Class 9 Preclass Quiz on MasteringPhysics This was due this morning at 8:00am 84% of students answered correctly: A packing crate is sitting at rest on an inclined loading ramp. The magnitude of the force of static friction is equal to the magnitude of the component of the weight of the crate parallel to the inclined ramp. 74% of students answered correctly: The coefficient of static friction is greater than the coefficient of kinetic friction. 55% of students answered correctly: You are driving your car along a horizontal road and you see the light turn yellow, and then red. You apply the brakes and the car slows to a stop. While the car is slowing down, the main external force on the car which causes it to slow down is static friction. 95% of students answered correctly: Terminal speed is the speed a falling object reaches at which the drag force and gravity have equal magnitudes. Last day I asked at the end of class: Does friction always slow things down? ANSWER: No! Kinetic friction does oppose the relative motion of two surfaces. If the one of these surfaces is stationary, then it will tend to slow down the moving object. Can friction ever speed things up? ANSWER: Yes! Static friction between your feet and the floor is what allows you to walk! Walking certainly involves speeding up, and this would not be possible if the floor were frictionless or covered in marbles! 3

4 Class 9 Preclass Quiz Student Comments Friction can be a real drag sometimes How a rolling wheel could have static friction. (But I could just read chapter 10 in advance too.) Harlow response: Right. In order to fully understand rolling without slipping, you need to study rotation, which is not until chapter 10. For now you should trust me that for cars and bikes, it is static friction between the road and the wheel, not kinetic. I am confused with static friction and kinetic friction being related to the normal force. Why? Harlow answer: I m not really sure!! The short answer is that it s an experimental result! (ie it is an empirical law, not a theoretical law). But it makes sense: as the normal force increases, the surfaces will have more microscopic contact points, which allows the parallel forces to also increase. Class 9 Preclass Quiz Student Comments it made me feel good about myself knowing that physicists too, have bad hair days my goal in this course is not to pass physics but to get one of my comments posted on your lecture slides Harlow comment: why not both? Dream big! why don't we get a reading week, even two or three days would help. Harlow response: You do get two days actually: November 9 and 10 Harlow comment: wait did someone give me ASCII roses on MasteringPhysics? Thanks! 4

5 Class 9 Preclass Quiz Student Comments New rule: No swearing (ie f***, sh**, etc) or sexual references in the comments section, please. It s unprofessional. You will get a zero on that particular preclass quiz if I catch this. Thank you. Why does friction exist? Because at the microscopic level, nothing is smooth! 5

6 Kinetic Friction Also called sliding friction When two flat surfaces are in contact and sliding relative to one another, heat is created, so it slows down the motion (kinetic energy is being converted to thermal energy). Many experiments have shown the following approximate relation usually holds for the magnitude of f k : f k f k f k k n where n is the magnitude of the normal force. The direction of f k is opposite the direction of motion. Clicker Question A wooden block weighs 100 N, and is sliding to the right on a smooth horizontal concrete surface at a speed of 5 m/s. The coefficient of kinetic friction between wood and concrete is 0.1. A 5 N horizontal force is applied to the block, pushing toward the right. What is the force of kinetic friction of the concrete on the block? v A. 100 N, to the left B. 10 N, to the left C. 5 N, to the left D. 10 N, to the right E. 5 N, to the right 6

7 Example A sled of mass 5.0 kg is pulled at a constant velocity by a rope which makes an angle of 20.0 above the horizontal. The coefficient of kinetic friction between the sled and the surface snow is What is the tension in the rope? T 20 Static Friction When two flat surfaces are in contact but are not moving relative to one another, they tend to resist slipping. They have locked together. This creates a force perpendicular to the normal force, called static friction. f s There is no general equation for f s. The direction of f s is whatever is required to prevent slipping. 7

8 Multiple Forces on a Single Object A car is parked on flat, horizontal pavement. Which of the following forces are acting on the car? A.Gravity B.Normal C.Static friction D. All of the above E. A and B, but not C Maximum Static Friction There s a limit to how big f s can get. If you push hard enough, the object slips and starts to move. In other words, the static friction force has a maximum possible size f s max. The two surfaces don t slip against each other as long as f s f s max. A static friction force f s > f s max is not physically possible. Many experiments have shown the following approximate relation usually holds: where n is the magnitude of the normal force, and the proportionality constant μ s is called the coefficient of static friction. 8

9 Clicker Question A wooden block weighs 100 N, and is sitting stationary on a smooth horizontal concrete surface. The coefficient of static friction between wood and concrete is 0.2. A 5 N horizontal force is applied to the block, pushing toward the right, but the block does not move. What is the force of static friction of the concrete on the block? F A. 100 N, to the left B. 20 N, to the left C. 5 N, to the left D. 20 N, to the right E. 5 N, to the right Clicker Question A wooden block weighs 100 N, and is sitting stationary on a smooth horizontal concrete surface. The coefficient of static friction between wood and concrete is 0.2. A horizontal force is applied to the block, pushing toward the right. What is the magnitude of the maximum pushing force you can apply and have the block remain stationary? F A. 200 N B. 100 N C. 20 N D. 10 N E. 5 N 9

10 F What happens if you steadily increase the applied force on an object which is resting on a surface? μ μ Accelerating Here the kinetic friction is constant as the applied force continues to increase. As the magnitude of the net force increases, so does the magnitude of the object s acceleration. Typical Coefficients of Friction Student comment from this morning s preclass quiz: Right after I finished the reading, I gave my mom a lecture about the static friction in driving. Harlow comment: Good! If her tires are slipping, she is not maximizing the friction! 10

11 Rolling Without Slipping Under normal driving conditions, the portion of the rolling wheel that contacts the surface is stationary, not sliding In this case the speed of the centre of the wheel is: v = C T where C = circumference [m] and T = Period [s] If your car is accelerating or decelerating or turning, it is static friction of the road on the wheels that provides the net force which accelerates the car. (More about this coming in Chapter 10!!) Clicker Question When an object moves through the air, the magnitude of the drag force due to air resistance A. increases as the object s speed increases. B. decreases as the object s speed increases. C. does not depend on the object s speed. 11

12 Class 9 Preclass Quiz Student Comments what will happen if an object travels faster than its terminal speed? Harlow answer: Interesting question.. If an object is thrown or propelled downward at faster than its terminal speed, then the upward drag force will be greater than the downward force of gravity, so the net force will be up. It will slow down to terminal speed as it falls. I found the paragraph about terminal speed interesting because it reminded me of the time I went skydiving with my dad. 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. 12

13 From on Exterior Ballistics. Non-Free Fall When an object falls downward through the air it experiences: force of gravity pulling it downward. F drag air drag force acting upward. 13

14 Drag force in a fluid, such as air Air resistance, or drag, is complex and involves fluid dynamics. For objects on Earth, with speeds between 1 and 100 m/s and size between 1 cm and 2 m, there is an approximate equation which predicts the magnitude of air resistance F drag 1 2 C Av 2 where A is the cross-sectional area of the object, ρ is the density of the air, C is the drag coefficient, and v is the speed. The direction of air resistance, or Drag Force, is opposite to the direction of motion. It depends on size and shape, but not mass. Drag force in a fluid, such as air Air resistance, or drag, is complex and involves fluid dynamics. For objects on Earth, with speeds between 1 and 100 m/s and size between 1 cm and 2 m, there is an approximate equation which predicts the magnitude of air resistance F drag 1 2 C Av Which falls faster, a piece of paper that is uncrumpled, or crumpled? A. Uncrumpled (flat) B. Crumpled Why??? What is different? 2 14

15 Terminal Speed F drag increases with speed Net force goes to zero when the object is moving fast enough so that F drag = mg (air resistance = weight) Then no net force No acceleration Velocity does not change Non-Free Fall Example A skydiver jumps from plane. Weight is the only force until air resistance acts. As falling speed increases, air resistance on diver builds up, net force is reduced, and acceleration becomes less. When air resistance equals the diver s weight, net force is zero and acceleration terminates. Diver reaches terminal velocity, then continues the fall at constant speed. 15

16 Clicker Question Consider a heavy and light person jumping together with same-size parachutes from the same altitude. Who will reach the ground first? A. The light person. B. The heavy person. C. Both will reach at the same time. D. Not enough information. Free Fall vs. Non-Free Fall Coin and feather fall with air present Feather reaches terminal velocity very quickly and falls slowly at constant speed, reaching the bottom after the coin does. Coin falls very quickly and air resistance doesn t build up to its weight over short-falling distances, which is why the coin hits the bottom much sooner than the falling feather. 16

17 Free Fall vs. Non-Free Fall Coin and feather fall in vacuum There is no air, because it is vacuum. So, no air resistance. Coin and feather fall together. Watch Brian Cox do this experiment at Before Class 10 on Monday Note that MasteringPhysics Problem Set 4 is due Monday evening (not Fridays any more)! Please read Wolfson Chapter 6. Something to think about: A pendulum bob swings back and forth on the end of a string, describing a circular arc. Does the tension force in the string to any work? 17