1. 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

2. 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
The coefficients of friction are nearly independent of the area of contact

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  µ n

4. Kinetic Friction, ƒk
The force of kinetic friction acts when the object is in motion
ƒk = µ n
Variations of the coefficient with speed will be ignored

6. Example 4. 12 The hockey puck in Figure 4
Example 4.12 The hockey puck in Figure 4.22, struck by a hockey stick, is given an initial speed of 20.0 m/s on a frozen pond. The puck remains on the ice and slides 1.20x102 m, slowing down steadily until it comes to rest. Determine the coefficient of kinetic friction between the puck and the ice.

7. 40. A woman at an airport is towing her 20
40. A woman at an airport is towing her 20.0-kg suitcase at constant speed by pulling on a strap at an angle θ above the horizontal. She pulls on the strap with a 35.0-N force, and the friction force on the suitcase is 20.0 N. Draw a free-body diagram of the suitcase. (a) What angle does the strap make with the horizontal? (b) What normal force does the ground exert on the suitcase?

8. Example 4. 13 (a) A block with mass m1 4
Example 4.13 (a) A block with mass m kg and a ball with mass m kg are connected by a light string that passes over a frictionless pulley, as shown in Figure 4.23a. The coefficient of kinetic friction between the block and the surface is Find the acceleration of the two objects and the tension in the string. (b) Check the answer for the acceleration by using the system approach.

9. Block on a Ramp, Example Axes are rotated as usual on an incline
The direction of impending motion would be down the plane
Friction acts up the plane
Opposes the motion
Apply Newton’s Laws and solve equations

10. Example 4. 11 Suppose a block with a mass of 2
Example 4.11 Suppose a block with a mass of 2.50 kg is resting on a ramp. If the coefficient of static friction between the block and ramp is 0.350, what maximum angle can the ramp make with the horizontal before the block starts to slip down?