October 4, 2017 Please take out the friction lab & copy the daily objective and homework assignment Today I will build a consensus underst developing and using a mathematical model for friction forces

Drill If an object moves with constant velocity we know that: The forces are balanced and the net force is zero There are not any forces acting on it There must be an unbalanced force in the direction of motion. A student uses a force sensor to pull a 2.4 kg block across a surface at a constant speed. The sensor indicates a force of 8.2 N. Draw a free body diagram for the block. Find the magnitude of: Gravitational force, flex force, friction force.

FRICTIONAL FORCES

What are they? Forces acting parallel to surfaces that oppose the relative motion of the surfaces. (Forces that resist sliding)

Two types Static friction – tries to prevent two surfaces from sliding past one another Kinetic friction – force exerted in the opposite direction when two surfaces ARE sliding past one another

What factors determine the size of frictional forces. How much the surfaces are pressing into each other. (Fnormal/flex) Which surfaces are in contact. (m)

Normal Force Friction force is directly proportional to normal force. In our investigation Fnormal/flex = Fgravity. Doubling the normal force causes frictional force to double. The ratio Ffriction/Fnormal,flex=constant=slope of graph This ratio is called the coefficient of fricition

Coefficient of friction m=Ffriction/Fnormal Dimensionless A property of the surfaces in contact. Does NOT depend on Ffriction or Fnormal Examples: mdry pavement = 1.0 mwet pavement = 0.20 mice = .05

Static friction forces Are adjustable – they are as big as they need to be to keep surfaces from sliding (up to a point) Coefficient of static friction is determined by maximum static friction force. Ffriction(static) ≤ms FNormal

Kinetic Friction Forces Typically smaller than static friction Less force needed to balance friction once it is moving then is needed to overcome static friction to start it moving. Important example: antilock brakes

Friction Examples (plug & chug) Find the frictional force: µ = 0.200 Fflex = 142 N Ffriction = ? Ffriction= µ Fflex = 0.200 (142 N) = 28.4 N Your turn: µ = 0.300 Fflex = 268 N Ffriction = ? µ = 0.400 Fflex = 64.7 N Ffriction = ?

Finding the coefficient µ = ??? Fflex = 1230 N Ffriction = 352 N µ = Ffriction / Fflex = 352 N / 1230 N = 0.286 Your turn: µ = ??? Fflex = 865 N Ffriction = 117 N How does this surface compare to the one in the previous example?

Practice Problems How much horizontal force is needed to move a 72 kg crate across the floor at a constant speed if the coefficient is 0.30? Mark Watney (‘The Martian’) applies a 51 N horizontal force to pull a 55 kg bucket of potatoes across the floor of his habitat at a constant speed of 0.48 m/s. What is the coefficient of friction? (gMars = 3.7 N/kg)