Measuring the Coefficient of Friction

I developed this lab for my physical science classes, but I wish I had thought of it for my physics classes.  Since we do more with the coefficient of friction in physics, I will be upgrading and adding this lab to my course for next year.

In this lab, the students pull various objects across different surfaces.  The objects pulled were what I could get my hands on in a very short amount of time.  I had wooden blocks, plastic coated weights, steel weights, aluminum ringstand rings, and I asked the students to also use their sneakers.

They pulled these objects across a whole range of surfaces.  They used the top of the desk, the floor tiles, cardboard, plywood, tile board (white board), a rubber coated lab apron, cork board, and Styrofoam sheets.  I asked them to get creative and find objects and surfaces in the room.  I didn’t have sandpaper out, but many of them asked for it in their write-up.  The plan was to pick an object, pull it across as many surfaces as possible, then move on to another object.

They were surprised at the stickiness of the aluminum ring on the shop apron. I was too, it had a coefficient of friction greater than 0.5.

When I do this with my physics students, I will probably add a component where they have to predict the maximum angle of incline for an object on a surface before it starts slipping.  I like that.

Here’s my lab paper:  measuring-the-coefficient-of-friction1

Finding the Normal Force on an Incline

When we discuss normal forces, I drum into the students’ heads that the force is perpendicular to the surface.  They get that eventually.  Where they get into trouble is on an incline when the angled component of the weight of an object F(i)=mg*cos(Θ).  I called this force F(i) meaning the force exerted on the incline (and the (i) is really a subscript, but I can’t make that work in this blog).

This is sometimes the normal force, but not always.  If there are any other forces, like F(x), in the angled Y-axis, then the normal force is not the same as F(i).

Here is what I tell them:  imagine there is a scale under the object in question, what would it read?  If it is only the object and no other forces, then F(n) is F(i).  From the drawing above, it’s fairly clear that there are two forces down that combine to create our normal force up.  The scale under the block would read the value of F(i) + F(x), so that is our F(n).

Conservation of Energy

This is an experiment I did in my Conceptual Physics class to demonstrate Conservation of Energy.  I take a clean plastic peanut butter jar and put in about an inch of copper BB’s.  We then wrap the jar in a towel and have a student start shaking the jar as hard as they can.  The towel serves two purposes; it stops the transfer of heat from the students’ hands to the jar, and it helps deaden the deafening sound this little activity generates.  Use the non-contact thermometer to take a reading of the BB’s before beginning and then after every two minutes of shaking.  I usually set up two or three jars and have the students pass the jar to someone else after the temperature is taken.  Chart the temperature on the board. Don’t expect to get anything else done while this is happening.  There is too much noise and everybody wants to see how vigorously they can shake the jars.

The students are putting energy into the system because they are shaking the jar.  They are actually taking the chemical energy from the food they ate and turning that into kinetic energy.  The kinetic energy is transfered into themal energy (heat) because of the friction of the BB’s banging into each other.  I just thought of this, but it would be a good idea to have another jar set up as a control.