Friction

Biblical Reference And they pulled him up with the ropes and lifted him out of the cistern. Jeremiah 38:13

Free Body Diagrams A pictorial representation of forces complete with labels. Gravity (F g ) – Always drawn from the center, straight down (perpendicular to the surface) Force Normal(F N ) – A surface force always drawn perpendicular to a surface. Tension(T or F T ) – force in ropes and always drawn AWAY from object. Friction(F f ) – Always drawn opposing the motion. Applied Force (F a ) – Something pushing or pulling the object

Free Body Diagrams – Strategy: These are the only things you have to account for in a free body diagram 1.Is there gravity? (F g ) 2.Is it sitting on a surface? (F N ) 3.Is something pushing or pulling it? (F a ) 4.Is there friction? (F f ) 5.Is it accelerating? FgFg FNFN FaFa FfFf

Free Body Diagrams – Inclined Plane mg Gravity “pulls” the object down the inclined plane. Friction resists the downward motion.

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

Force of Friction The Force of Friction is directly related to the Normal Force. Mostly due to the fact that BOTH are surface forces Note: Friction only depends on the materials sliding against each other, not on surface area. (a unitless constant that is specific to the material type and usually less than one)

Since the F net = 0, a system moving at a constant speed or at rest MUST be at “Equilibrium”. TIPS for solving problems Draw an FBD Define x and y Resolve all forces into the x and y components Apply 2 nd law in x and y directions Newton’s First Law – The Law of “EQUILIBRIUM”

A 10-kg box is being pulled across the table to the right at a constant speed with a force of 50N. a)Calculate the Force of Friction b)Calculate the Normal Force mg FNFN FaFa FfFf Example

Suppose the 10 kg box is now pulled with a force of 50 N at an angle of 30  above the horizontal and the coefficient of kinetic friction is a) Calculate the Normal Force b) Calculate the Force of Kinetic Friction c) Calculate the acceleration mg FNFN 50 N FfFf 30  F ax F ay Example

a) Calculate the Normal Force mg FNFN 50 N FfFf 30  43.3 N 25 N Example Vector Sums in the y-direction:

b) Calculate the Force of Kinetic Friction c) Calculate the acceleration mg 73 N 50 N FfFf 30  43.3 N 25 N Example Vector Sums in the x-direction:

A dog with a mass of 5 kg is sliding down a ramp. The ramp has an angle to the ground of 30 . The force of friction is 15 N. a)What is the Normal Force? b)What is the Coefficient of Friction between the dog and the ramp? c)What is the dog’s acceleration? Example mg

a)What is the Normal Force? b)What is the Coefficient of Friction between the dog and the ramp? Example mg

c)What is the dog’s acceleration? Example mg Vector Sums in the x-direction:

A boy slides down the banister of a set of stairs. The angle of the stair banister is 35 . The Friction Force is 15 N, and the boy’s acceleration is 4.7 m/s 2. a)What is the boy’s mass? b)What is the Normal Force? c)What is the Coefficient of Friction between the boy and the bannister? Example mg

a)What is the boy’s mass? Example mg

b)What is the Normal Force? c)What is the Coefficient of Friction between the boy and the bannister? Example mg

A 45 kg panda slides down a slide with an angle of 45 . The Coefficient of Kinetic Friction between the panda and the slide is a)What is the panda’s acceleration? Example mg

a)What is the panda’s acceleration? Example mg

a)What is the panda’s acceleration? Example 441.5N

Newton’s “4 th Law” Inertia: objects want to move in a straight line at constant speed until acted upon by an outside force Gravity: force that pulls them out of a straight line

Calculate the force between two objects that have masses of 70 kg and 2,000 kg separated by a distance of 1 m. Given: m 1 = 70 kg, m 2 = 2,000 kg, r = 1 m Find: F Example

1 st Law: Objects in motion stay in motion, and objects at rest stay at rest, unless acted upon by an outside force. 2 nd Law: The acceleration of an object is equal to the net force exerted on the object divided by the object’s mass. F = ma 3 rd Law: When an object exerts a force on a second object, the second object exerts a force of the same size, but in the opposite direction, on the first object. Review – Newton’s Three LawsNewton’s Three Laws