2. 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)
3. 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
4. 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
5. 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
144
