Friction & Applying Newton’s 2nd Law System Chapter 6.2

What is Friction? Friction is a that is parallel to the surfaces of two objects that are in contact with one another that the relative of the two objects. Ff

What causes friction? Friction is caused by the temporary created between two objects in contact with one another.

Friction How does friction affect the motion of objects? It can an object down like the friction between the tires and the road. It is responsible for the speed of an object like a car. It is also responsible for objects being able to .

Since the crate is not accelerating, Fnet = __ Friction ( Objects) Static Friction: The that keeps an object from moving. Since v = ___, a = ___. FApplied Ffriction Fground-on-crate Fgravity System Fnet = Since the crate is not accelerating, Fnet = __ = Note: As long as the crate does not move, _____ = _____

Friction (Objects in ) Kinetic Friction: The that opposes the relative motion of two contacting surfaces that are past one another. Fnet may or may be ___. Fapplied Ffriction Fground-on-crate Fgravity System Fnet Fnet = Note: If the crate moves at a , then _____ = _____ and Fnet = ___.

Determining the Frictional Force (The of Friction, __) The force of friction (Ff) is proportional to the normal force (FN) and a proportionality constant (___ - pronounced _____) called the of friction. For static friction: 0 < Ff, static < sFN For kinetic friction: Ff, kinetic =  kFN As per the formula, the greater ___, the greater the . Note: FN = the ( ) to the on the object. __ is dimensionless ( ) _____ > _____ Ff FN

The Force The force is a force that most often opposes the Earth’s gravitational attraction and is to the that an object rests or is moving on. For a surface, ____ = ____ = ____. For a surface that is as seen in the figure below, _____ = ________. FN 

The Normal Force  FN FN Fg Fg ____ = ____ = ____ cos = adj/hyp Fg ____ = ____ = ____ ____ = ________ = ________

Example 2: Determining Friction (Balanced Forces) Assume that the man in the figure is pushing a 25 kg wooden crate across a wooden floor at a constant speed of 1 m/s. How much force is exerted on the crate? Fapplied Ff FN Fg System

Diagram the Problem y-direction: ____ = ____ x-direction: Fnet = System y-direction: ____ = ____ x-direction: Fnet = Since the crate is moving with speed, a = ___, Fnet = ___, and ______ = ______

State the Known and Unknowns What is known? Mass (m) = Speed = Acceleration (a) = k = What is not known? Fapplied = ?

Perform Calculations y-direction: ___ = ___ = ___ (Note: surface) x-direction: Since a = ___, Fnet = ___ Fnet = –

Example 3: Determining Friction ( Forces) Assume that the man in the figure is pushing a 25 kg wooden crate across a wooden floor at a speed of 1 m/s with a force of 73.5 N. If he doubled the force on the crate, what would the acceleration be? Fapplied Ff FN Fg System

Diagram the Problem y-direction: ____ = ____ +x System y-direction: ____ = ____ x-direction: Since , Fnet =

State the Known and Unknowns What is known? Force = Mass (m) = Speed = k = What is not known? a ?

Perform Calculations y-direction: x-direction: Fnet = ma = a =

Key Ideas Friction is an that exists between bodies. Friction is to the and the of ; static or kinetic. The force required to overcome static friction is than that required to overcome kinetic friction.