Friction Page 1 Lab: Friction William Morris Leo Hayes High School Friction Page 2 Purpose: To investigate the coefficient of friction for a given surface and the effects that factors such as weight, surface area and changes to the surface have on the coefficient of friction. Hypothesis: The smaller the amount of normal force (weight), the less friction created and the least surface area and the greaser the surface the less friction is created. With the increase in normal force corresponding with the increase in friction should balance out “mu”. Materials: Spring scale, string, 4 textbooks, Pledge

Procedure: 1. Using a spring scale and some string we hooked through the pages of the text book to find the weight to find the weight of each text book we used and then recorded it. 2. We pulled the first text book across the table then estimated the amount of force we needed to apply to the scale attached to the text book to move it at a constant speed and then recorded our guess. 3. Using the string and spring scale we pulled the text book across the table at a constant speed keeping the string parallel to the surface of the table and then recorded the data. . From this measured force below, we determined the frictional force that had acted upon the book as we pulled it across the table. 5. Then we pulled the book across the table once more at a different constant speed to ensure accurate friction readings. 6. From the weights of the text book we found earlier we determined the normal force acting on the text book as we pulled it across the table and then recorded it below. 7. We added the second text book, (B), to the first one and pulled them across the table again recording their force of friction. 8.

We then repeated this procedure twice more adding a text book each time as recorded below. 9. We then pulled text book, A, on its spine across the table at a constant speed recording the force from the scale. 10. We pledged the table and rubbed it thoroughly with a cloth and then repeated only step 3 on this pledged surface and recorded the force required. Friction Page 3 Questions: 1. Compare the books weight with the force required to pull it horizontally at a constant speed. Which one is larger? In all cases the normal force is larger than the force of friction. 2.

It always takes more force to start an object than to keep it moving at a constant speed. Why? Friction is caused by tiny scratches in the smoothness of a surface. The rougher a surface is, the rougher it feels. And the rougher a pair of surfaces are, the harder it is for them to slide past each other. The tiny rough edges grab hold of each other and lock the surfaces into place, so that it takes a great deal of force to make them move. That's the static friction you're trying to overcome. Once you get the surfaces moving the rough edges are already dislodged and as long as you don't stop, they won't have a chance to lock again.

Therefore, it's easier to keep something moving once you've started than it would be to stop and start again. 3. Does it take more force to move an object at a higher constant speed than a slower constant speed? In our lab the force seemed to stay the same. This could have something to do with our sources of error. The lab did not reveal this. 4. The force of friction was found in part 3. Would it be more or less if you pulled the text book across: (a) Ice(b) Sand(c) Rough Concrete(d) Waxed Floor How is the force of friction affected by changing the surface from smooth to rough? a) Less friction (b) More friction (c) More friction (d) More friction A rough surface has more grooves to grab than a smooth surface therefore creating more friction. Friction Page 4 5. Does the force of friction depend on the speed of motion? In our lab it was impossible to figure this out due to our sources of error. 6. Write an equation showing the relationship between the force of friction and the normal force. Use this equation to calculate the coefficient of friction for each set of readings. 7. What are the units for “mu” , the coefficient of friction?

There is no unit measurement for the coefficient of friction. 8. Plot a full page graph of the force of friction verse the normal force. See graph on following page for the force of friction verse the normal force. 9. Based on your graph what is the relationship between the coefficient of friction verse the normal force. The relationship between the coefficient of friction verse the normal force is such that the coefficient is the same because the increase in normal force corresponds with the then increase in friction creating the same “mu”. 10.

How do the coefficients of friction compare for each of your five trials in the data table? Is this how they should be? With a smoother surface the friction is lowered therefore lowering “mu”. This is how it should be. Friction Page 5 Data/Results: Books| Normal Force| Friction| µ| A| 10. 5 N| 2. 5 N| 0. 24| A + B| 20. 5 N| 5. 0 N| 0. 24| A + B + C| 31. 5 N| 7. 5 N| 0. 24| A + B + C + D| 42. 5 N| 10. 0 N| 0. 24| Book A on spine| 10. 5 N| 2. 0 N| 0. 20| Books A on Pledged Surface| 10. 5 N| 2. 0 N| 0. 20| Books| Normal Force (weight)| A| 10. 5 N| B| 10. 0 N|

C| 11. 0 N| D| 11. 0 N| Discussion: There are several sources of error in this lab. One being the scales not being accurate and there is no way to fix this except to purchase new scales each time. Another could be the angle we are pulling the book from. There is no way for us to be exactly sure if this force is exactly parallel with the table. When finding our normal force we do not take into consideration the weight of the string and this cannot be fixed as our scales are not this precise. It is unknown if our hand was completely steady while finding our normal force.

This could result in our normal force being inaccurate. There is no way to fix this as we are not robots. The same is true for finding the force of friction as we are not sure we are pulling the book(s) at the same constant speed. Conclusion: After the lab I found my hypothesis to be true in that the smaller the amount of normal force (weight), the less friction created and the least surface area and the greaser the surface the less friction was created. With the increase in normal force corresponding with the increase in friction it balanced out “mu”.