Warm up What do you think causes friction? What are some high friction surfaces? Some low friction surfaces?

Friction

friction Friction is a force that opposes motion. Symbol: 𝑭 𝒇 Units: Newtons (it’s a force!) The Causes of Friction: Interactions of “bumps”—how rough or smooth surfaces are. Creation and Destruction of welds Intermolecular Forces

Types of Friction Static Friction: the force that resists the initiation of sliding motion between two surfaces that are in contact and at rest.  Kinetic Friction: the force that opposes the movement of two surfaces that are in contact and are sliding over each other. *Static Friction is always greater than Kinetic Friction When pushing a heavy box, is it harder to get the box moving or to keep it in motion?

Friction relationships Is it easier to pull an empty wagon across a field or the same wagon full of puppies? This is because the force of friction is proportional to the normal force (The force perpendicular to the surface)

Friction Is it easier to push a chair across a tile floor or carpet? The force of friction is dependent on the surfaces involved. The interaction of two surfaces have a coefficient of friction

Coefficient of Friction (μ) Coefficient of Friction Coefficient of Friction (μ): the ratio between the force necessary to move one surface horizontally over another (force of friction) and the pressure between the two surfaces (normal force) μ = 𝐹 𝑓 𝐹 𝑁 or 𝑭 𝒇 = μ 𝑭 𝑵 μ = coefficient of friction (values usually between 0 and 1); NO UNITS Low μ =slippery High μ =sticky/rough 𝐹 𝑓 = Force of Friction (N) 𝐹 𝑁 =Normal Force (N)

μ Examples of

Coefficient of friction The coefficient of kinetic friction is a ratio of the force of kinetic friction and the normal force μk = Ff,k/FN   OR Ff,k= μk FN The coefficient of static friction is a ratio of the force of static friction and the normal force μs = Ff,s(max)/FN OR Ff,s ≤ μs FN

Example 1 A 24 kg crate initially at rest on a horizontal floor requires a 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. 0.32

Example 2 Once the crate is moving it only requires 63 N of horizontal force to keep it moving at a constant velocity. What is the kinetic coefficient of friction between the crate and the floor? 0.27

Example 3 The same crate is now pulled with a rope that is 30° above the horizontal. If the coefficients of friction are the same, then a) what must the tension in the rope be to get the crate to start moving? b) what must the tension in the rope be to keep the crate moving? a) 73.35 N b) 63.44 N