Question: you are trying to determine the coefficient of static friction...
You are trying to determine the coefficient of static friction between your shoe and the floor. To do so, you place your shoe on the ground, attach a spring scale, then pull horizontally on the shoe until it just barely starts to move. You take note of the spring scale reading at that moment. Then you put some known extra masses into your shoe and repeat the process several times.
(a) What is the purpose of adding extra masses and repeating the experiment?
(b) Since the only two vertical forces are the gravitational force and normal force, you can directly calculate the normal force by multiplying mass by the gravitational field strength. You then create a plot with the normal force values and their corresponding spring scale readings. If you want the slope of the line to represent the coefficient of static friction, should you place normal force on the y-axis and spring scale reading on the x-axis, or vice versa? Explain.
(c) In the plot described in the previous part, what does the y-intercept represent, and what should you expect its value to be?
(d) Why is it especially important that you pull exactly horizontally?