Friction 2011

Pre-Quiz Which of these is NOT a type of friction? – A. Rolling B. Kinetic C. Static D. Potential Which type of friction is the strongest? – A. Rolling B. Kinetic C. Static D. Potential Which type of friction is the weakest? – A. Rolling B. Kinetic C. Static D. Potential

Types of Friction “Dry” Frictions: Depend on Normal Force – Kinetic Friction-the friction of two surfaces sliding past one another (AKA Sliding Friction) – Static Friction-the friction of two surfaces in stationary contact with one another – Rolling Friction-the friction of one surface rolling over another All these types of friction follow the formula:

Types of Friction Notation for “Dry” frictions: – Let’s say you need to write about more than one type of friction while solving a problem. How can you notate the friction formula to differentiate between those different types of friction?

Types of Friction Fluid Frictions: Depend on Velocity, not Normal Force – Air Resistance – Drag – Viscosity Since these forces change with velocity, we will not be studying them in this course, as they require more complex mathematical methods to analyze.

Relative Strength of “Dry” Frictions Imagine you are trying to push a heavy box across a floor. Will you have to push harder to start the box moving or keep it moving, once it’s sliding? – Static Friction is stronger than Kinetic Friction, so it takes more force to start pushing the box than to keep pushing the box.

Relative Strength of “Dry” Frictions How would putting wheels under the heavy box change the amount of force needed to push the box? – Rolling Friction < Kinetic Friction < Static Friction

Effect of Normal Force on Friction Place your hand together gently and try to slide one hand past the other. Now place your hands together and push as hard as you can and try to slide one hand past the other. Which was easier? Why? Increasing the force applied on the surfaces increases the amount of friction between the surfaces.

Normal Force If a heavy box that weighs 30 N is resting on the floor, what is the normal force that the floor applies up on the box? If you push down on that box with an additional force of 20 N, what is the normal force that the floor applies up on the box?

Post Quiz Which of these is NOT a type of friction? – A. Rolling B. Kinetic C. Static D. Potential Which type of friction is the strongest? – A. Rolling B. Kinetic C. Static D. Potential Which type of friction is the weakest? – A. Rolling B. Kinetic C. Static D. Potential