Lesson 2 - Force, Friction

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1 Lesson 2 - Force, Friction Background Students learn about two types of friction static and kinetic and the equation that governs them. They also measure the coefficient of static friction and the coefficient of kinetic friction experimentally. Engineers who really understand friction designed a braking system to improve our safety. Have you been in a car when the driver stopped abruptly by slamming on the brakes, but, instead of stopping or skidding, the car started to chatter? That vibration was caused by the anti-lock brake system (ABS). Since engineers know that a nonskidding wheel has more traction than a skidding wheel, they designed a braking system that prevents the brakes from locking up, which can cause a vehicle to slide. By not skidding, the static friction is maximized and the driver can stop the car quickly without loss of control. Clearly, engineers really need to know their friction facts! Concepts 1. Static Friction and Kinetic Friction 2. Experimental data acquisition 3. Calculation of the coefficients of friction 4. Understand different materials will have different coefficients of friction Student Learning Objectives NYS Standards Know the difference between static and kinetic friction Measure coefficient of static friction from experimental data collected Measure coefficient of kinetic friction from experimental data collected Define expected trends in the different frictional coefficients for different types of surfaces Key Terms Friction Work Static Friction Kinetic Friction Motion Coefficient of Static Friction Coefficient of Kinetic Friction

3 F F = μ x N Where, F F is the frictional force, μ is the coefficient of friction, N is the Normal force μ s - coefficient of static friction, used when an object is not sliding μ k coefficient of kinetic friction, used when object is sliding (see ) Approximate values of some frictional coefficients Coefficient of kinetic friction Coefficient of static friction Rubber on dry concrete Rubber on wet concrete 0.58 Rubber on dry asphalt Rubber on wet asphalt 0.53 Rubber on ice 0.15 Waxed ski on snow Wood on wood Steel on steel Copper on steel Teflon on Teflon 0.04 Data from NYSED science regents examinations ( ) Goal of class today is to observe and measure static and kinetic friction. The values for μ are usually found experimentally. In this activity, a method of calculating μ s and μ k from measured gravitational forces and application of free body diagrams. Demonstrate activity and distribute activity sheets. Explain that when the object begins to move the static frictional force equals the gravitational force on the basket. Draw free body diagram. Group Question: During the activity, ask the groups: Why are we using fishing line instead of rope or string? (The fishing line is very thin and smooth, resulting in any friction between the table and the line being extremely small so small that it can be ruled out. If we used rope or string, we might have to take into account its frictional effects.) How could we make the experiment even more accurate? Why is static friction stronger than kinetic? Which surface material will have greatest coefficient of friction? Why?

4 Supply list Each group needs: 3 ft. fishing line Block of wood ~ 10 cm on each side with different materials glued on 3 of sides, small eye screw on 5 th side (materials could include felt, sandpaper, plastic etc.) Extra block of wood (approximately same size) Weights (e.g., total weights required will be greater than the weight of the two blocks of wood) Small wicker or plastic basket with a handle Desk or table copies of the activity sheets Scale Tape Reflective Notes Factor in settle time, worksheet may need to be adapted for each classroom s experience level, remind students to bring the worksheet for the next day to complete analysis and discuss as necessary. The measurement and analysis might take two days Depending on total class time, the teacher can assign different groups to complete experiment with one or two of the materials if there is not time to complete the experiment with all 4 materials. There are more precise ways to determine coefficients of kinetic friction, but they really require precise measurement of the block velocity and are limited to laboratories that have suitable data acquisition equipment. Taping ¾ of a PVC pipe over the corner of the table edge would reduce frictional losses on the string Table string

5 Activity Force, Friction Background One safety element of roller coaster design is the braking system. Brakes are a good example of work, force, and friction working to our advantage. Engineers who really understand friction designed a braking system to improve our safety. Have you been in a car when the driver stopped abruptly by slamming on the brakes, but, instead of stopping or skidding, the car started to chatter? That vibration was caused by the anti-lock brake system (ABS). A non-skidding wheel has more traction than a skidding wheel; they designed a braking system that prevents the brakes from locking up, which can cause a vehicle to slide. By not skidding, the static friction is maximized and the driver can stop the car quickly without loss of control. Introduction/Motivation Friction can waste energy, wear down parts and cause things to heat up. But we also depend on friction to keep our shoes/feet from sliding out from under us and to keep our cars on the road (from friction between road and our car's tires). Two types of friction: static friction and kinetic friction Static friction - resists an object to start moving or sliding Kinetic friction - resists an object that is already moving or sliding and always acts in a direction opposite of the motion (the reason that anything sliding freely will eventually come to a stop) Note: static friction is always stronger than kinetic friction more force is needed to start to move an object than to keep it sliding. Static and kinetic friction between an object and the ground can be calculated using the following equation: F F = μ x N (1) Where, F F is the frictional force, μ is the coefficient of friction, N is the normal force μ s - coefficient of static friction, used when an object is not sliding μ k coefficient of kinetic friction, used when object is sliding The values for μ are usually found experimentally. In this activity, a method of measuring μ s and μ k will be introduced.

7 13. Tap the block with your finger. The goal is to initiate the block moving across the table at a constant velocity (you are the judge!). If the block does move at a constant velocity, record the mass in the basket. If it does not, add some more weight to the basket and repeat until the constant velocity movement is achieved. 14. Repeat steps with the same set up. 15. Repeat steps with the extra block on top of the sliding block. 16. Calculate the coefficient of kinetic friction

8 Data and Discussion Questions Force, Friction Name: Static Friction Data: Mass of block with eye screw: kg Mass of extra block: kg Mass of empty basket: kg Material 1 Mass in Block(s) (kg) Normal force of block(s) (N =m g) Grav. Force Basket (F =m g) μ s (from eqn. 1) Average μ s Material 2 Mass in Block(s) (kg) Normal force of block(s) (N =m g) Grav. Force Basket (F =m g) μ s (from eqn. 1) Average μ s

9 Material 3 Mass in Block(s) (kg) Normal force of block(s) (N =m g) Grav. Force Basket (F =m g) μ s (from eqn. 1) Average μ s Material 4 Mass in Block(s) (kg) Normal force of block(s) (N =m g) Grav. Force Basket (F =m g) μ s (from eqn. 1) Average μ s Discussion Question 1. How does μ s vary with mass of the blocks? With the surface material? Are these results consistent with what you expected based on theory (equation 1) and your knowledge of friction? Explain.

10 Kinetic Friction Data: Record the mass in basket when you were able to get blocks to slide at a constant velocity after tapping with your finger. Material: Mass in basket required to initiate sliding: kg (from results of static friction experiment. You should need less mass for this experiment) Mass in Block(s) (kg) Normal force of block(s) (N =m g) Grav. Force Basket (F =m g) μ k (from eqn. 1) Average μ s Discussion questions: 2. How does μ k for compare to μ s for the same material? Is this the trend that you expected? Explain. 3. Why are anti-lock breaks in cars more effective on slick roads than regular breaks? (Anti-lock breaks are used in cars so that when someone slams on their breaks, the breaks lock for a split second and then release, then lock for a quick second again, and so on.) 4. As an engineer, how would you build a solid structure that is extremely difficult to move and easy to move at the same time. Use the information about friction you just learned. Can you apply these ideas to the design of a roller coaster?

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