Friction

Note: These activities involve experimental design skills. The write ups are deliberately brief.

Activity 1

You have a track, a force sensor, several boxes with different bottom materials, and some masses.

Pick a box. Slide it along a level track using a force sensor to pull it. Use different masses. Plot F(m) and use your plot to find the coefficient of friction. Explain what is directly measured versus what you are inferring you measured. (Note: you never measured the friction force directly, so how did you find the coefficient of friction?) What assumptions have you made that you have not measured?

Activity 2

How can you demonstrate whether the coefficient of friction depends on the type of surface? Design an experiment, perform your experiment, and report your results. Remember that two experimental values can only be said to be different if their difference is much larger than their uncertainties. By convention, much larger in physics means at least 3 times larger.

Activity 3

Predict what angle you should set the ramp to so that the box will slide down at a constant speed once pushed. Your prediction should include uncertainties! Test your prediction.

Should the angle change if you change the mass in the box? Test your prediction.

Activity 4

Design your own experiment to determine the coefficient of static friction for one of your boxes. Design a second, independent experiment (which uses a different measuring device) and use it on the same box. Do your results agree with each other? Why is this important?