Friction :Friction


Force :Force A force can most easily be described as a push or a pull. When a force is applied to an object, the velocity of that object changes. Newton’s 3rd Law For every action (force), there is an equal and opposite reaction (opposing force).


Forces are Vectors so Directions are Important :Forces are Vectors so Directions are Important Total Force = 0 Forces Add Forces Cancel! In physics a vector is a quantity that has two aspects. It has a size, or magnitude, and a direction.


Friction is the resistive force :Friction is the resistive force the result of sliding or attempting to slide two surfaces that are pressed against each other.


Friction is a Force :Friction is a Force It’s the sum of all the forces that determines the acceleration. Every force has an equal & opposite partner. What’s missing in this picture?


What Causes Friction? :What Causes Friction? Irregularities in the surfaces and the force between the two objects causes friction. Even if the surfaces of the objects appear smooth, they are microscopically very jagged!


Friction Mechanism :Friction Mechanism Irregularities in the surfaces grind when things slide. Lubricants fill in the gaps and let things slide more easily.


Rules of Friction :Rules of Friction Friction always resists motion. Friction is always directed opposite that of motion. Friction cannot exceed applied force, unless the object is already moving. An applied force is a force which is applied to an object by a person or another object. The applied force is the force exerted on the car by the person.


Velocity :Velocity Velocity is a measure of the speed in a given direction. Speed is how fast an object is going with respect to an object. Speed is measured in the same physical units as velocity, but does not contain the element of direction that velocity has


What is acceleration? :What is acceleration? Rate of change in velocity. An object accelerates if it speeds up, slows down or changes direction.


Two types of friction: :Two types of friction: Kinetic Friction - friction due to two surfaces moving against one another and Static Friction - friction without motion. static friction can prevent an object from sliding down a sloped surface.


Static Friction :Static Friction Static friction results when the surfaces of two objects are at rest relative to one another and a force is applied on one of the objects to set it into motion. After a maximum static friction point is reached, the object will begin to move and the friction will become kinetic friction.


Kinetic friction :Kinetic friction If force F is greater than friction Fr (written as F > Fr), then the object will slide or move. This friction is considered kinetic friction, which means moving friction. Kinetic friction remains the same as long as the object continues moving, the surfaces remain the same, and the force between the objects stays the same.


Slide 15:The maximum force needed to overcome the static friction and begin the sled moving is noted


Kinetic friction is typically slightly lower than static friction. :Kinetic friction is typically slightly lower than static friction. Note the constant slope until the object begins to move, at which point there is zero slope and the frictional force remains constant even as the applied force continues to rise.


What caused the resistance to motion that you felt as you pulled the block? :What caused the resistance to motion that you felt as you pulled the block? Resistance was caused by frictional force


What was the acceleration of the block pulled between the two lines? :What was the acceleration of the block pulled between the two lines? F=MA - the second law of motion proposed by Sir Isaac Newton. Most of the answers stated the acceleration of the wood block was Zero. Why?


Reason - Acceleration was zero :Reason - Acceleration was zero The object remained at a constant speed in a constant direction. It did not speed up, slow down or change direction. So because the velocity of the block did not change the block’s acceleration was Zero.


What type of energy was present as the wood block was moved across the table? :What type of energy was present as the wood block was moved across the table? Kinetic energy


Slide 21:Kinetic energy is the extra energy which it possesses due to its motion.


As the block slowed down what happened? :As the block slowed down what happened? Kinetic energy decreased. If you remember the Law of Conservation of Matter and Energy, which states that energy and matter are never produced nor destroyed. What happened to the kinetic energy?