4-4 Review

Normal Force

• Equal in magnitude to Fg, but opposite in direction• Perpendicular to the surface• Fn = mg(cos of angle)–Angle is between normal force and a

vertical line, and also contact surface horizontal line–See Figure 4-20 (b)

Static Friction

• Static friction (Fs) – resistance force that keeps an object from moving– Small push on a heavy object doesn’t cause

it to move– Equal to the applied force as long as the

object doesn’t move (Fs = -Fapplied)

– Fs,max – applied force is a great as it can be without causing the object to move

Friction (cont.)

• Kinetic friction (Fk) – force that opposes an object’s motion–Always less than Fs,max

–Net external force acting on an object is equal to difference between applied force and force of kinetic friction (F - Fk)–Depends on surfaces in contact

Friction (cont.)

• Coefficient of friction – expresses the dependence of frictional forces on particular surfaces in contact• Represented by lower case greek

letter,• See formulas and table 4.2 on page

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