Where the Rubber Meets the Road

2.1.3 Friction

A force that OPPOSES MOTION.What causes friction?

Friction

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1. Attraction between protons and electrons in the object and surface

What causes friction?

Friction

2. Small ridges and bumps at the microscopic level cause objects and surfaces to catch

on one another.

• Friction force strength– How “sticky” are the surfaces involved?

• Coefficient of friction (μ) – NO UNITS

– How hard are the surfaces pushing together?• NORMAL FORCE

– Are the surfaces sliding against each other?• KINETIC vs. STATIC

Friction

Nf FF

• Kinetic – Used ONLY for SLIDING objects

• Pulled along a surface• Skidding

• Static– Used for any NON-SLIDING objects

• Not moving• Rolling

Which Coefficient?

• A 20 newton wooden box is sliding across a wooden surface. What is the friction force acting on the box?

Ff = μFN

Ff = (0.3)(20 N)

Ff = 6.0 N

Example #1

• A 40 newton copper box is placed on a flat, steel table. What is the minimum force needed to make the box move?

Example #2

Ff = μFN

Ff = (0.53)(40 N)

Ff = 21 N

End of 2.1.3 - PRACTICE