exploring frictional forces friction friction and newton’s laws static and kinetic friction...

Exploring Frictional Forces

• Friction• Friction and Newton’s

Laws• Static and Kinetic

Friction• Coefficient of Friction• Practice

Friction• The analysis of friction is very important to

practical situations since friction is present everywhere.

• Friction is a good and bad thing. – Good when our tires meet the pavement– Bad when reducing efficiency and/or

producing heat

Friction

• Friction always opposes motion (or Fnet)– Kinetic friction– Static friction– Air resistance

Friction and Newton’s Laws

• Friction is simply a force included and accounted for in a FBD.

• Applies to situations where:– Fnet = 0– Fnet = ma

Two types – Static and Kinetic

• Static friction – stationary scenarios• Kinetic friction – moving scenarios

• The magnitudes of friction depend on – the surfaces in contact with each other (ie.

shoe and floor)– the normal force

Static Friction

• The magnitude of the force needed to start a stationary object moving is the maximum static friction, FSmax

Kinetic friction

• Kinetic friction acts on any object that is in motion

• The magnitude of the force needed to keep the object moving at a constant velocity is the kinetic friction, FK.

Coefficients of Friction

• The coefficient of friction is a ratio of the magnitude of the force of friction between two surfaces to the normal force between those surfaces.

Coefficients of Friction

• Determining μS and μK for given substances is done through experimentation.

• i.e.

Practice

1. A crate of fish of mass 18.0 kg rests on the floor of a parked delivery truck. The coefficients of friction between the crate and the floor are μS = 0.450 and μK = 0.410. The local value of gravitational acceleration is 9.80 m/s2. What are the force of friction and the acceleration if:

a) a horizontal force of 75.0 N [E] is applied to the crate?

b) if a horizontal force of 95.0 N [E] is applied?

Practice

2. A musician applies a horizontal force of 17 N [W] to an instrument case of mass 5.1 kg. The case slides across a table with an acceleration of 0.39 m/s2 [W]. What is the coefficient of kinetic friction between the case and the table?

Practice Solutions

1. a) No movement because the Fapp < Fs

b) a = 1.26 m/s2

2. μK = 0.30