Objectives: The student will be able to: Distinguish between the different types of friction. Use free body diagrams and Newton's laws of motion to solve word problems. Calculate and compare the coefficients of friction of various surfaces.

Phone Book and Friction Separate the phone books JE JE

TWO types of Friction Static – Friction that keeps an object at rest and prevents it from moving Kinetic – Friction that acts during motion

Kinetic friction occurs when force is applied to an object and the object moves. Examples: Sliding Friction: pushing an object across a surface Rolling Friction: between wheels and a surface Fluid Friction: opposes the motion of objects traveling through a fluid (air or water) Types of Friction

To reduce the amount of friction, apply a lubricant between two surfaces. Motor oil, wax, and grease are examples. Friction can also be reduced by rolling, rather than pushing, an object. Affecting Friction

Friction increases as surfaces are made rougher. Friction increases when the force between two objects is increased. Affecting Friction

Friction Forces When two surfaces are in contact, friction forces oppose relative motion or impending motion. P Friction forces are parallel to the surfaces in contact and oppose motion or impending motion. Static Friction: No relative motion. Kinetic Friction: Relative motion Kinetic Friction: Relative motion.

2 N2 N2 N2 N Friction and the Normal Force 4 N The force required to overcome static or kinetic friction is proportional to the normal force, n The force required to overcome static or kinetic friction is proportional to the normal force, n. f k =  k n f s =  s n n 12 N 6 N n 8 N 4 N n

Friction forces are independent of area. 4 N4 N4 N4 N 4 N4 N4 N4 N If the total mass pulled is constant, the same force (4 N) is required to overcome friction even with twice the area of contact. For this to be true, it is essential that ALL other variables be rigidly controlled.

Friction forces are independent of temperature, provided no chemical or structural variations occur. 4 N4 N4 N4 N 4 N Heat can sometimes cause surfaces to become deformed or sticky. In such cases, temperature can be a factor.

Friction forces are independent of speed. 2 N The force of kinetic friction is the same at 5 m/s as it is for 20 m/s. Again, we must assume that there are no chemical or mechanical changes due to speed. 5 m/s 20 m/s

The Static Friction Force When we use the following equation, we refer only to the maximum value of static friction and simply write: f s =  s n When an attempt is made to move an object on a surface, static friction slowly increases to a MAXIMUM value When an attempt is made to move an object on a surface, static friction slowly increases to a MAXIMUM value. n fsfs P W

Constant or Impending Motion For motion that is impending and for motion at constant speed, the resultant force is zero and  F = 0. (Equilibrium) P fsfs P – f s = 0 Rest P fkfk P – f k = 0 Constant Speed Here the weight and normal forces are balanced and do not affect motion.

Friction and Acceleration When P is greater than the maximum f s the resultant force produces acceleration. Note that the kinetic friction force remains constant even as the velocity increases. P fkfk Constant Speed f k =  k n a

4-8 Applications Involving Friction, Inclines On a microscopic scale, most surfaces are rough. The exact details are not yet known, but the force can be modeled in a simple way. For kinetic – sliding – friction, we write: is the coefficient of kinetic friction, and is different for every pair of surfaces.

4-8 Applications Involving Friction, Inclines

Friction If the coefficient of kinetic friction between a 35-kg crate and the floor is 0.30, what horizontal force is required to move the crate to the right at a constant speed across the floor? mg Fn Fa Ff N

Friction Suppose the same 35 kg crate was not moving at a constant speed, but rather accelerating at 0.70 m/s/s. Calculate the applied force. The coefficient of kinetic friction is still mg Fn Fa Ff N

4-8 Applications Involving Friction, Inclines Static friction is the frictional force between two surfaces that are not moving along each other. Static friction keeps objects on inclines from sliding, and keeps objects from moving when a force is first applied.

The static frictional force increases as the applied force increases, until it reaches its maximum. Then the object starts to move, and the kinetic frictional force takes over. 4-8 Applications Involving Friction, Inclines

Friction Lab

4-9 Problem Solving – A General Approach 1. Read the problem carefully; then read it again. 2. Draw a sketch, and then a free-body diagram. 3. Choose a convenient coordinate system. 4. List the known and unknown quantities; find relationships between the knowns and the unknowns. 5. Estimate the answer. 6. Solve the problem without putting in any numbers (algebraically); once you are satisfied, put the numbers in. 7. Keep track of dimensions. 8. Make sure your answer is reasonable.

Homework Chapter 4 Friction activity due in 2 days Questions #8, 12, 21 Problems #s 36, 37

Homework Chapter 4 Problems Friction activity due next class. #s 38, 39, 44, 47