Friction

1
Chapter 4

• Friction

2
Forces of Friction

• When an object is in motion on a surface or
  through a viscous medium, there will be a
  resistance to the motion
• This is due to the interactions between the
  object and its environment
• This is resistance is called friction
• Surfaces
• Air friction bullet, jet
• Fluids swim

3
Static Friction, ƒs

• Static friction acts to keep the object from
  moving
• If F increases, so does ƒs
• If F decreases, so does ƒs
• ƒs ? µs FN

4
Kinetic Friction, ƒk

• The force of kinetic friction acts when the
  object is in motion
• ƒk µk FN
• Variations of the coefficient with speed will be
  ignored

5
Friction Demo

• Friction force always opposes motion
• Book will not slide unless pushing force is
  larger than certain value
• Once book starts to slide, there is less friction

6
More About Friction

• Friction is proportional to the normal force
• The force of static friction is generally greater
  than the force of kinetic friction
• The coefficient of friction (µ) depends on the
  surfaces in contact
• The direction of the frictional force is opposite
  the direction of motion, tangent to sliding
  surface
• The coefficients of friction are nearly
  independent of the area of contact

7
Example

• How large a force needed to start a 4kg wood
  block moving? Keep it moving?
• Table 4.1 in the book gives
• for wood on wood

8
Example

• A car hits another car in an accident. Damage to
  the cars show that the speed of impact is 55 mph.
  There is also 25 feet of skid marks. If the speed
  limit was 65 mph, was the first car speeding?
  (?k0.8)

9
Example

• Woman pulls sled at an angle 42 with constant
  velocity along a flat road. (?k0.1)
• What is the tension in the pulling rope?

10
Inclined Planes

• Fig. 4.4 is very important
• Modify Fig. 4.4 for friction
• Choose the coordinate system with x along the
  incline and y perpendicular to the incline
• Replace the force of gravity with its components

11
Example

• Block being pulled up a hill. What tension must
  rope have before block moves? (m10kg, ?25,
  ?s0.6)

12
Example (prob. 29)

• m13kg
• m22kg
• ?k 0.3
• Find acc. and tension.

13
Terminal Velocity

• Object in free fall will actually come to
  constant velocity due to friction forces from
  air.
• Air friction forces increase with velocity
• Depends on size shape of object

14
Terminal Velocity