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Lecture PowerPoints
Chapters 2, 3.1, 4, 5.10
Physics: Principles with
Applications, 6th edition
Giancoli
© 2005 Pearson Prentice Hall
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Constant Force
Apply Newton’s Second Law
• Force and Acceleration
• Types of Forces
• Equilibrium
• Analyze Friction
Constant Force
Newton’s Second Law
• In an inertial reference frame, the sum of
the forces F on an object is equal to
the mass m of that object multiplied by
the acceleration a of the object
F = ma.
Where:
F is force in newtons
m is mass in kilograms
a is acceleration in m/s2
Constant Force
Newton’s Second Law
• Another way to state this is that if you apply
an unbalanced force to an object with mass,
it will accelerate in the direction of the
applied force
Constant Force
Newton’s Second Law
• Relationships between acceleration, force
and mass
• Rewrite F = ma to solve for a
• F/m = a
• So force is directly proportional to
acceleration (for the same mass,
more force = more acceleration)
• And mass is inversely proportional
to acceleration (for the same force,
more mass means less acceleration)
Constant Force
Newton’s Second Law
• Types of forces (review)
–Field force
• Includes gravitational, electromagnetic
–Contact force
• Includes normal, tension, friction
Constant Force
Newton’s Second Law
• Equilibrium
–When net force on an object is zero
–Σ =0
• Nonequilibrium
•Σ =ma
Constant Force
Friction
Exists between two solid surfaces because
even the smoothest surface is quite rough on
a microscopic scale.
The vector friction is always opposite the
direction of the applied force or motion
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Types of friction:
If no motion occurs, referred to as static
friction
Example: You push on a refrigerator, and it
finally starts to move.
Static friction kept it from moving
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Types of friction:
If motion occurs, referred to as kinetic (sliding)
friction
Example: You let go of the moving refrigerator,
and it starts to slow down then stops due to
sliding friction
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Types of friction:
When object rolls on surface, referred to as
rolling resistance
Example: One reason your bicycle slows if you
stop pedaling
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Types of friction:
Air resistance opposes the motion of objects moving
in air (examples: open parachute, sail on sailboat)
Terminal velocity* is highest velocity attainable by an
object as it falls through a fluid (gas or liquid). At
terminal velocity, resistance = weight of object.
Terminal velocity in air for a skydiver in freefall is
195 km/h (122 mph or 54 m/s).
*Source: https://en.wikipedia.org/wiki/Terminal_velocity
Constant Force
What causes friction?
• Surfaces are not smooth even if they
appear to be (microscope)
• Dry friction (static friction and sliding
friction) arises from a combination of
–
–
–
–
Inter-surface adhesion
Surface roughness
Surface deformation
Surface contamination
Constant Force
Friction
Coefficient of static friction (μs)
If you know Fs, you know only one of the infinitely many
possible static frictional forces that could be exerted
between the two bodies.
Which force do you know? The maximum static frictional
force.
Constant Force
Friction
Coefficient of kinetic (sliding) friction (μk)
The amount of kinetic friction Fk a body experiences is
proportional to the size of the normal force N exerted on
the body by the structure it slides against.
Constant Force
Friction
Coefficient of kinetic (sliding) friction (μk)
Note: The statement “kinetic friction is a function of
normal force only-- surface area has nothing to do with it”
is true ONLY as long as you are dealing with two rigid
bodies that are sliding relative to one another.