Friction & Applying Newton’s 2 nd Law
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Transcript of Friction & Applying Newton’s 2 nd Law
Friction & Applying Newton’s 2nd Law
Chapter 6.2
System
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 betweenthe 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 .
Friction ( Objects)
System
FAppliedFfriction
Fground-on-crate
Fgravity
Fnet =
Since the crate is not accelerating, Fnet = __
=
Note: As long as the crate does not move, _____ = _____
• Static Friction:– The that keeps an object from moving.– Since v = ___, a = ___.
Friction (Objects in )
System
FappliedFfriction
Fground-on-crate
Fgravity
• Kinetic Friction:– The that opposes the relative
motion of two contacting surfaces that are past one another.
– Fnet may or may be ___.
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
fF
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
Fg
FN
Fg
____ = ____ = ____
cos = adj/hyp
____ = ________ = ________
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?
System
FappliedFf
FN
Fg
Diagram the Problem
System
y-direction: ____ = ____ x-direction: Fnet =
Since the crate is moving with speed, a = ___, Fnet = ___, and ______ = ______
+y
+x
State the Known and Unknowns
• What is known?o Mass (m) = o Speed = o Acceleration (a) = k =
• What is not known?o Fapplied = ?
Perform Calculations• y-direction:
o ___ = ___ = ___ (Note: surface)
• x-direction: Since a = ___, Fnet = ___
o 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?
System
FappliedFf
FN
Fg
Diagram the Problem
System
y-direction: ____ = ____ x-direction: Since , Fnet =
+y
+x
State the Known and Unknowns
• What is known?o Force = o Mass (m) = o Speed = k =
• What is not known?o a ?
Perform Calculations
• y-direction:
• x-direction:o Fnet = o ma =o 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.