# Day 39: Shoe Friction Lab

College-Prep Physics:  Continuing our discussion of tug-of-war and the importance of friction…

ME: “So who wins tug-of-war?”

CLASS: “The team with the most friction.”

“So how could we measure the friction force between a person and the floor?”

“…”

“Well, how could we measure the friction force between this block and the table?”

“Pull on it with a spring scale until it moves. The scale reading is the same as friction in this case.”

“OK, so how could be measure the friction between a person and the floor?”

“Pull on the person with a spring scale until the move???”

“OK, so I need a volunteer.”

So I loop a large thick rope around the volunteer’s waist and connect the other end to a large 20 newton spring scale. And I begin to slowly pull on the student as we watch the reading on the scale approach 20 N, and then go past it.

ME: “Uh oh.”

CLASS: “You maxed out the scale. Guess we can’t measure the friction.”

I ask the volunteer for his/her shoe, then loop string around it and start to pull the string with the spring scale.

ME: “Looks like the shoe starts to slide at about 2 newtons.”

CLASS: “Yeah, but that’s just the shoe by itself. There’s a lot more friction when someone is wearing the shoe.”

So then we have a discussion about how if we could find a relationship between shoe weight (by adding weights to the shoe) and friction (within the range of our spring scales) then we could use the relationship to extrapolate all the way to the weight of the person. This is a great way to use experiments to determine the value of a quantity we can’t directly measure.

[PS: Tomorrow is a staff PD day, so we’ll be back on Wednesday!]

##BFPM