FRICTION Introduction/Review o Review the definition of a force (Q & A) 1. As you learned from us from the last class about force. Anybody who can tell me what is the meaning of force ? 2. In daily lives we can find many types of forces. Who can give us any types of force you have ever learned before? friction gravity magnetic force etc. applied force So, today we are going to focus more about friction Development o Divide students into smaller groups. o Let them think about the following questions. 1. Why we can walk on the ground without slipping, sliding or falling down ? 2. Why the car move and stop smoothly? 3. When you use a pen or pencil to write on the paper. What factors can help us hold a pen or pencil ? o Students express their idea about questions. o Teachers let them think about the meaning of friction. How friction produce ? What effect does friction have on the speed of a rolling object? And what types of surfaces will produce the most friction? Friction Lab 1) Use a 5 N spring scale to measure the weight of the wooden box, and record your result. Then, using the spring scale, apply a HORIZONTAL force to the box to pull the box at a constant, slow velocity along the HORIZONTAL friction plane (NOT along the classroom table !). Try to make the box's velocity as constant as you can - start with a gentle pull and increase your pull gradually until you can move the box with constant speed. Read the force of pull for five values of mass (0 to 1 kg in increments of 0.2 kg) placed in the box 2) As you were carrying out the measurements above, what did you notice about the reading on the scale just as the box began to move? Make a conclusion about the difference between the friction force on an object at rest and that on the object while moving. 3) Plot and fit graph of FRICTION FORCE vs NORMAL FORCE. (Where will the data come from?) Sketch the graph in your Data Lab 4) If 0.700 kg of mass were placed in the box, what would you expect the force of friction to be? Use your equation of fit to calculate the answer. Then check your answer by making the measurement. Quantitatively compare the measured value to the calculated value. Show all work for predictions, and record all measurements in your lab journal. Compare the prediction with measurement (% difference?). Table Added Mass Total Weight Pull Force (kg) (N) (N) 0.200 0.400 0.600 0.800 1.000 Summary Their values depend on certain properties of the moving bodies and on the quality of surfaces which touch each other. This is obvious to every child. We cannot slide on our shoes on asphalt, but can easily do it on ice. The term surface means much more than the surface of a table, floor, road or any other surface from our daily life. Later on we will discuss the frictional forces between molecules in liquid or gas and in such cases the surface of molecules plays a role in the formulas defining these frictional forces. A very important field related to friction is motion of solid objects in the air or in water. In this case the frictional force is called drag force. In spite of a quite different name the drag force is also a frictional force and only the mechanism which creates this type of friction is very different from the one creating the friction between two solid objects. Independent Practice o Search information about the use of friction and how to reduce friction from any websites? Evaluation o From students’ answering. o From the information that they searched. o Quiz and test. t.