FRICTION Vocabulary Rain Steam and Speed The Great

FRICTION Vocabulary: “Rain, Steam, and Speed – The Great Western Railway” (1844) J. M. W. Turner • friction • normal force • coefficient of friction (μ) • coefficient of kinetic friction (μk) • coefficient of static friction (μs) This topic can be found on pp. 135 – 139 of your textbook.

(p)re: think �What is friction? �What causes friction? �Draw it! �Let’s compare.

What is friction? Friction is a force that opposes motion. sliding friction fluid friction rolling friction

What is friction? We are going to only discuss sliding friction during this unit. sliding friction

What causes friction? glass aluminum foil �How do these highly magnified images of glass and aluminum foil fit in with your predictions of the cause of friction from earlier?

What causes friction? �When two surfaces slide across each other, microscopic imperfections between the surfaces cause the two surfaces to exert opposite forces on one another. movement

Friction is everywhere!! Watch this

What factors do you think affect friction (i. e. , what can increase or decrease the force of friction? )

Lab Time!!!! You will experimentally determine which factors affect the force of friction.

Factors Affecting Friction a. The normal force acting on an object, FN. The normal force is a function of the weight of the object experiencing the friction. b. The nature of the surfaces in contact. This is quantified using a coefficient of friction, µ. The higher the value of µ, the higher the friction force.

Factors Affecting Friction Which will have a larger value of ?

Factors Affecting Friction is the ratio between the friction force between two objects and the normal force. It must be experimentally determined.

Calculating the Force of Friction (Ff) �Ff is directly proportional to Fn (normal force) �Coefficient of friction ( ): Determined by the nature of the two surfaces s is for static friction (force is applied, but object does not move. ) k is for kinetic friction (force is applied and object is moving. ) s > k

Typical Coefficients of Friction � Values for have no units and are approximate (why? ) Why do you think μs is always greater than μk?

Static vs. Kinetic Friction � μs When two objects are stationary, the “bumps” on each object settle into one another. To get the objects moving, you have to get these bumps out from one another. � μk When two objects are moving, the “bumps” do not have time to settle into one another. Instead, they glide over one another offering less resistance.

What about surface area? Friction force does not depend on the area of contact. Each block has the same FN and µ!

Review: Weight and Mass �Mass is the amount of matter in an object unit = Kilograms �Weight is a measure of the gravitational force on an object unit = Newtons Depends on the acceleration of gravity �Weight = mass acceleration of gravity W = mag where ag = 9. 8 m/s 2 on Earth

Review: Normal Force � Force on an object perpendicular to the surface (Fn) � It may equal the weight (Fg), as it does in the picture of the TV. � It does not equal the weight (Fg) if the surface is angled, as in the second example Fn = mg cos

Classroom Practice Problem � A 24 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to just get it moving. Once it is moving, it only requires 50. N of horizontal force to keep it moving. Find the coefficient of static friction and coefficient of kinetic friction between the crate and the floor. Draw a free-body diagram and use it to find: ▪ the weight ▪ the normal force (Fn) ▪ the force of friction (Ff) Find the coefficients of friction (both static and kinetic). � Answer: s = 0. 32 k =. 21

Classroom Practice Problem What is the frictional force in this scenario? EXPLAIN Fnormal Ffriction Fapplied weight �Mass of the block is 500. g �Force applied is 3. 0 N. � = 0. 75 �The block is stationary What is the maximum force that can be applied before the block starts to move? Answer: 3. 7 N

Challenging problem A 1450 kg ‘ 64 Mustang travelling at 34 m/s hits brakes and skids to a stop in 75 m. What is the coefficient of kinetic friction (μk) between the road and the car’s tires? Fnormal Ffriction Answer: 0. 79 Fgravity (weight)