Transcript Newton`s third law of motion and friction

Forces and
Newton’s
Laws of
Motion
Newton’s Third
Law and
Friction
What We Want to Know…
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What is Newton’s Third Law of Motion?
What does action-reaction pairs mean for
forces?
Where does Newton’s Third Law affect
everyday life?
How does friction affect motion?
Newton’s Third Law of Motion
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The law of action and reaction.
“For every action, there is an equal and
opposite reaction.”
Forces always exist in pairs called “actionreaction pairs.”
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When two objects interact they exert forces on
one another that are equal and opposite.
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Action-reaction pairs do not cancel each
other out.
Each force acts on a different object.
Examples?
Friction
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Frictional force resists applied forces.
Friction always resists motion (parallel to the
surface).
Two types of friction:
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Static: the resistive force of a non-moving object.
Kinetic Friction: the resistive force of a moving
object. Weaker than static.
Coefficient of Friction
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Coefficient of friction (μ): friction’s
dependence on type of surface. It is a ratio of
frictional force over normal force.
Ffriction  Fnormal
or,

Ffriction
Fnormal
Coefficients of Friction
Surface
steel on steel (dry)
steel on steel (greasy)
ice on ice
rubber on wood
rubber on concrete (dry)
rubber on concrete (wet)
μs
.74
.1
.1
.7
1.0
.3
μk
.57
.05
.03
.8
.25
Coefficient of Friction
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What force is required to push a 25 kg box
across a floor at a constant rate if μk= .6?
A person pulls a 40 kg bag across a floor with
μk= .3. What is the net force if the bag is
pulled with a force of 150 N straight to the
side?
Practice
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A 35 kg bed is pulled by some unknown force
across the floor. If the bed accelerates at 4.0
m/s², with only friction opposing it, what is the
magnitude of the pulling force? (µ= .14)
A Little Review
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What are the two types of forces?
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What is weight and how is it different from
mass?
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Contact and Field Forces
Weight is the force of gravity which changes on
location. Mass is the matter in an object which
stays the same.
What is inertia?
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An object’s resistance to a change in motion.
A Little Review
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What is Newton’s 1st Law of Motion?
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What is ΣF?
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Objects keep the same motion unless acted upon
by an unbalanced force.
Net force = the sum of your forces in a certain
direction (x or y)
When is an object in equilibrium?
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ΣF = 0, object stays at rest, object has constant
velocity (no acceleration).
A Little Review
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What is Newton’s 2nd Law of Motion?
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What is Newton’s 3rd Law of Motion?
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ΣF = ma. Acceleration is proportional to force and
inversely proportional to mass
Every action has an equal and opposite reaction.
What are the two types of friction?
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Static and kinetic friction.
S.T.A.M.P.
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A 12 kg sled is pulled across the snow with a
force of 30 N. If the force is applied at an
angle of 30˚ above the +x-axis (µ = .13):
a)
b)
c)
d)
e)
Draw a free-body diagram
Find the x and y components of the applied force
Calculate the normal force
Calculate the frictional force
Calculate the acceleration of the sled
Physics Test Practice
1.
A student pulls on a 5 kg desk with a force of
40 N at an angle of 20˚ above the horizontal. If
the desk does not move, what is the force of
friction? What is the normal force?
2.
A 140 kg car accelerates straight along the
positive x-direction. If the car accelerates at 12
m/s², what is the force of the car’s acceleration.
(µ= .4)