C H A P T E R 4 Forces and Newton's Laws of Motion

4.9 Static and Kinetic Frictional Forces

4.9 Static and Kinetic Frictional Forces There are situations where friction helps us and at other times it is a hindrance.

4.9 Static and Kinetic Frictional Forces There are situations where friction helps us and at other times it is a hindrance. Advantages of friction:

4.9 Static and Kinetic Frictional Forces There are situations where friction helps us and at other times it is a hindrance. Advantages of friction: Disadvantages of friction: wear and tear & energy loss.

Microscopic view of Frictional Forces

Frictional force VERSUS Applied force

Frictional force VERSUS Applied force

Static Frictional Force The magnitude fs of the static frictional force can have any value from zero up to a maximum value of fsMAX, depending on the applied force. fs ≤ fsMAX fsMAX = m sFN

Kinetic Frictional Force The magnitude fk of the kinetic frictional force is given by,

Coefficients of Friction

EXAMPLE 10 Sled Riding A sled is traveling at 4.00 m/s along a horizontal stretch of snow, as Figure 4.23a illustrates. The coefficient of kinetic friction is mk = 0.0500. How far does the sled go before stopping?

4.10 The Tension Force

Free-body Diagram

Object on an Inclined Plane

Object on an Inclined Plane

Problem-73 A cable is lifting a construction worker and a crate, as the drawing shows. The weights of the worker and crate are 965 and 1510 N, respectively. The acceleration of the cable is 0.620 m/s2, upward. What is the tension in the cable (a) below the worker and (b) above the worker?

Free-Body Diagram