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Unit 1: Lesson 3 (Friction) Name_____________________________ Friction ‐ What a drag Most of us are aware that there is something that slows us down, because otherwise, anytime we ride our bikes or drive our cars, we would be going forever without stopping. Another thing most of us know is what is slowing us down is called friction. We know that friction causes us to lose speed and energy, gives off heat, and causes inefficiency in all things. As bad as this sounds, without friction, we wouldn’t be able to exist. It is the interactions between atoms that causes friction, allows us to stop when we’re going to fast, and without friction we wouldn’t be able to start moving in the first place. What most of us don’t know about friction is how it works, and that there are actually 2 types of friction that we deal with in our everyday lives. They reason friction exists is the interactions between atoms of 2 different objects. Some atoms are more interactive and cause higher amounts of friction, such as rubber, and different metals. Other materials have less interaction at the atomic level and have lower amounts of friction, such as smooth surfaces and ice. To have friction you always have to have 2 surfaces (you can’t have an interaction between one object and nothing). As I said, the types of surfaces determine the amount of friction between the objects. The 2 objects have a certain coefficient of friction between them. What the 2 surfaces are determine what the coefficient of friction is. If one of the 2 surfaces is an object with higher amounts of friction like from above (rubber, metals), the coefficient of friction will be higher. If they are both objects with higher friction, the coefficient will be even higher. The coefficient of friction is just a ratio of the normal force (force between the surface and the object) and the amount of force it takes to move it along the surface. Luckily, most of the coefficients of friction between common surfaces used, especially in industry, have already been determined and can be found in tables like this: You’ll notice there are two surfaces listed, and things like metal and rubber tend to be higher, where as things like ice and ice tends to be very low (which is why it’s easier to glide on ice skates). The other © 2013 Supercharged Science www.ScienceLearningSpace.com Page 1 of 9 Unit 1: Lesson 3 (Friction) Name_____________________________ thing you may notice is there are 2 different coefficients listed, and the one on the left is always higher than the one on the right. The one on the left is called static friction, while the one on the right is kinetic friction. The word static means still or stationary and it is the amount of force required to get an object to move from a stationary position. We know that kinetic means moving, and kinetic friction is the amount of force required to keep an object moving at a slow, constant speed. Again the mount of static friction is always higher than kinetic friction. Think about if you have ever had to push a heavy object like a car that has run out of gas. It is always harder to start it moving than it is to keep it moving. This is because you have to overcome the object’s own inertia. The activity below shows the difference between static and kinetic friction. © 2013 Supercharged Science www.ScienceLearningSpace.com Page 2 of 9 Unit 1: Lesson 3 (Friction) Name_____________________________ Static vs. Kinetic Friction Objective: Visually and numerically see the difference between static and kinetic friction Materials: textbook or large block of wood String Spring scale (easy to find at fishing store) or a rubber band and ruler if you can’t find a spring scale) 5 different surfaces (table top, carpet, rubber mat, etc.) Hypothesis: Rank your 5 surfaces in order of which you think will have the highest amount of friction with 1 being the highest and 5 being the lowest Procedure: 1) Attach the string to the wooden block or textbook, so that you can drag it along a surface. 2) Attach the string to the spring scale on the end you are going to pull the book. If you do not have a spring scale, tie the string to a rubber band instead. 3) Place the book on your first surface. And slowly start to pull it (make sure to pull the spring scale/rubber band flat, parallel to the surface) 4) You’ll notice that when you pull the book, as the book just begins to move, the scale goes from a maximum reading to one slightly less. You want to record the measurement of the spring scale just before the book begins to move. This will be your static friction measurement. If you are using the rubber band, hold the ruler next to the rubber band and record the length of the rubber band just before the book begins to move. 5) Continue to pull the book at a slow, constant speed and record the reading on the spring scale or the length of the rubber band. This will be your kinetic friction measurement. 6) Repeat 2 more time for a total of 3 trials on your first surface. 7) Repeat steps 3‐6 on the remaining 4 surfaces. Data: Surface #1 Trial 1 2 3 Static Friction Average Kinetic Friction Average © 2013 Supercharged Science www.ScienceLearningSpace.com Page 3 of 9 Unit 1: Lesson 3 (Friction) Name_____________________________ Surface #2 Trial 1 2 3 Static Friction Average Kinetic Friction Average Surface #3 Trial 1 2 3 Static Friction Average Kinetic Friction Average Surface #4 Trial 1 2 3 Static Friction Average Kinetic Friction Average Surface #5 Trial 1 2 3 Static Friction Average Kinetic Friction Average Analysis: 1) Rank the surfaces in order of which had the highest static and kinetic friction. Did this order match your hypothesis? © 2013 Supercharged Science www.ScienceLearningSpace.com Page 4 of 9 Unit 1: Lesson 3 (Friction) Name_____________________________ Friction problems As stated earlier, the coefficient of friction is a ratio of the force required to move the object along a surface and the normal force. It is represented by the equation below µ = FF/FN µ = coefficient of friction (No units) FF = Force of friction (N) FN = Normal Force (N) In most cases (when the object is moving along a flat surface). The normal force (FN) is the same magnitude as the weight of force of gravity (FG). Another trick that question writers like to use is to make the applied force equal to the force of friction. They will say something like “the object just started to move.” That means the applied force was just enough to overcome the force of friction keeping it still, so the applied force is equal to the force of static friction. Or they could say the force was required to pull the object at a constant velocity.” That means the applied force is just enough to keep it moving, but not accelerating, so the applied force is equal to the force of kinetic friction. These type of word problems is where it is very important to pick up on key words like the one above. 1) A wooden box weighing 150 N slides along a horizontal wooden floor. The coefficient of friction is 0.4. What force is needed to keep the box sliding at a constant speed? Given: Equation(s): Solve: Substitute: Unknown: 2) A wooden box weighing 50 N in placed on a flat wooden table. The horizontal force required to overcome static friction and just cause the box to slide is___? (μ = 0.70 for wood‐on‐wood) Given: Equation(s): Solve: Substitute: Unknown: © 2013 Supercharged Science www.ScienceLearningSpace.com Page 5 of 9 Unit 1: Lesson 3 (Friction) Name_____________________________ 3) The same 50 N box is now moving and the μ = 0.40 for moving wood‐on‐wood, what force is required to keep it moving at a constant speed? Given: Equation(s): Solve: Substitute: Unknown: 4) A horizontal force of 2250 N will cause a stationary 2500 N truck to slide (with wheels locked) when the truck is on a dry concrete surface. The tires are made of rubber. What is the coefficient of friction for the tires? Given: Equation(s): Solve: Substitute: Unknown: 5) A 1000 N refrigerator needs to be moved across the floor. The static coefficient of friction is 0.42. The kinetic coefficient of friction is 0.32. What is the difference in the force required to start the refrigerator moving and keeping it moving? Given: Equation(s): Solve: Substitute: Unknown: 6) Sports such as soccer involve running, stopping, jumping, and kicking. Discuss 3 ways that friction helps players. (Be specific, use COMPLETE SENTENCES) © 2013 Supercharged Science www.ScienceLearningSpace.com Page 6 of 9 Unit 1: Lesson 3 (Friction) Name_____________________________ Solutions: 1) A wooden box weighing 150 N slides along a horizontal wooden floor. The coefficient of friction is 0.4. What force is needed to keep the box sliding at a constant speed? Solve: 60 N Given: Equation(s): Substitute: Unknown: 2) A wooden box weighing 50 N in placed on a flat wooden table. The horizontal force required to overcome static friction and just cause the box to slide is___? (μ = 0.70 for wood‐on‐wood) Given: Equation(s): Solve: 35 N Substitute: Unknown: 3) The same 50 N box is now moving and the μ = 0.40 for moving wood‐on‐wood, what force is required to keep it moving at a constant speed? Given: Equation(s): Solve:20 N Substitute: Unknown: 4) A horizontal force of 2250 N will cause a stationary 2500 N truck to slide (with wheels locked) when the truck is on a dry concrete surface. The tires are made of rubber. What is the coefficient of friction for the tires? Given: Equation(s): Solve: 0.9 Substitute: Unknown: © 2013 Supercharged Science www.ScienceLearningSpace.com Page 7 of 9 Unit 1: Lesson 3 (Friction) Name_____________________________ 5) A 1000 N refrigerator needs to be moved across the floor. The static coefficient of friction is 0.42. The kinetic coefficient of friction is 0.32. What is the difference in the force required to start the refrigerator moving and keeping it moving? Given: Equation(s): Solve: 420 N – static 320 N kinetic 100 N difference Substitute: Unknown: 6) Sports such as soccer involve running, stopping, jumping, and kicking. Discuss 3 ways that friction helps players. (Be specific, use COMPLETE SENTENCES) Need friction to stop. You need friction to plant your foot. You need friction to make the ball curve (air resistance) © 2013 Supercharged Science www.ScienceLearningSpace.com Page 8 of 9