October 21 physics In: What do you think affects the force of friction? Objective: To build an understanding of the force of friction. Success criteria: Understand the variables that make up friction. Discovery Lab: Force of friction List the possible variables that make up the force of friction Using the force probe and friction block, test each variable. Data: 1. Variable 1: a) How are you testing it b) What was your result 2. Variable 2 : ….Etc… What did we find out: Force of friction Static Force of Friction – Force needed to start an object moving Kinetic Force of Friction – Force needed to keep an object moving Ff=µFn Force of static friction uses µstatic – The force it takes to get something moving Force of kinetic friction uses µkinetic – The force it takes to keep it going at a constant velocity Mass=32 kg µs = .7 µk = .3 Find the force to get the box moving. Find the force to keep the box moving at a constant velocity. A 700 kg polar bear slides across the ice with a force of friction of 25 N. What is the coefficient of friction between fur and ice? A 25 kg boy slides along a slip-n-slide. He slows down with an acceleration of .5 m/s2. What is µk between the slide and the boy? A group of people pull a 200,000 kg plane with a force of 40,000 N to keep it moving at a constant velocity. What is the kinetic force of friction? What is the coefficient of kinetic friction of the tires on the tarmack? Out: Draw a free body diagram of your friction block in constant velocity.