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Forces and
Newton’s Laws
(Dynamics)
Dynamics
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Dynamics – study of the effects of forces
on matter.
Isaac Newton (1642 – 1727) studied the
ways in which forces interact and their
influence on motion.
Types of Forces
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Contact Force – Acts on an object by touching it.
Long Range (field) Forces – Exerted without contact.
Forces in Nature
Weak
Gravity – mass pulls on other mass
Electromagnetic - + pulls weak - found in radioactive decay
Nuclear Strong
strong – holds quarks and nucleus together
Newton’s 1st Law
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“An object at rest will remain at rest and an object in
motion will remain in motion until acted on by an outside
force.”
Also known as the law of INERTIA
Inertia is the property of an object that causes it to resist
changing motion.
The greater the mass, the more inertia an object has.
Ex. Riding in a car.
Example
FBD (Free Body Diagram)
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A diagram that represents all of the forces acting on an
object or a system.
FN = Normal force
always ┴ to the
surface. = in
magnitude to weight on
horizontal surfaces
Fw = W = mg
Net Force (Fnet)
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The net force acting on the object is the
sum of all the forces.
Forces are vectors, treat them as such
Equilibrium
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If the sum of the forces in the y – direction
(ΣFy = 0) and the sum of the forces in the x
– direction (ΣFx = 0) equal zero, the system
is in equilibrium. The motion of the object is
not changing. (at rest or constant velocity)
A change in velocity or acceleration is due
to a net force Fnet ≠ 0.
Newton’s 2nd Law
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The acceleration of an object is directly
proportional to the Fnet on an object and
inversely proportional to the mass of the
object.
Fnet = ma
If m a , if F a
Unit of force is a Newton (N)
Force to accelerate 1kg, 1m/s2
2nd Law Practice
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Draw an FBD of you sitting in your chair.
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If your mass is 100kg, what is your W (Fw or
Fg)? What is the FN?
More Practice
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Draw an FBD of a skydiver before they
open their chute and are being accelerated
by gravity.
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Draw an FBD for after the chute is open
and they are falling at a constant rate.
One More
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A 5 kg block is being accelerated at 2m/s2 across
a flat horizontal surface.(assume there is no
friction) Draw an FBD of this system.
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What is ΣFy and ΣFx? Is the object in equilibrium?
Friction Forces
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Static Friction - Friction acting on an object
that is not moving.
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Responds to the applied force and is proportional
to the FN of the object.
Kinetic Friction - Friction acting when two
surfaces slide past each other.
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Proportional to the FN and independent of speed,
velocity, or acceleration of the object.
Causes of Friction
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When two surfaces are in contact, the
highpoints of the surface bond together.
To move the object the bonds must break
(static friction ----> kinetic friction)
As the surfaces slide past each other,
these valleys and peaks interact.
Calculating Friction
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Ff = µFN
µ (mu) is the coefficient of friction. This
coefficient changes based on the surfaces
that are interacting.
The front page of your reference tables
contain a chart listing coefficients of friction
between many surfaces.
µ is different for static surfaces compared
to kinetic surfaces
Sample Problems
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You push a 10.0kg rubber block across a dry
concrete floor at a constant velocity of 1m/s. How
much pushing force is exerted on the box? (Start
with an FBD)
FN
Ff
Fp
Fg
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Because it is at a constant speed it is in
equilibrium therefore Fp = Ff
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Ff = µFN
FN = mg = (10.0kg) x (9.8m/s2) = 98N
µ = 0.68
Ff = µFN = (0.68) x (98N) = 66.64N
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One More Sample
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What would the acceleration of the object
be if the pulling force from the last question
was doubled?
FN
Ff
Fp
Fg
y - direction FN + Fg = 0
 x - direction Fp + Ff = Fnet = ma
Ff = µFN = µmg
ma = Fnet = Fp - µmg = 133.2N - 66.6N
Fnet = 66.6N = ma
66.6N = 10.0kg a
a = 6.7m/s2
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Practice
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A sled of mass 50.0kg is pulled along flat, snow
covered ground. The static coefficient friction is
.30 and the kinetic friction coefficient is .10.
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What does the sled weigh?
What is force is needed to start the sled moving?
What force is needed to keep the sled moving at a
constant velocity?
Once moving, what total force must be applied to cause
the sled to accelerate 3.0m/s2?
Periodic Motion
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When an object is pulled away from its
equilibrium position and a forces tries to
restore that object to its equilibrium
position.
Also called simple harmonic motion
Examples: Mass on a spring, pendulum
Pendulums
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A simple pendulum consists of a bob
attached to a fixed point by a string or rod of
length (l).
Period (T) - the time it takes to complete one
cycle of motion.
Amplitude - maximum distance the object
moves from equilibrium.
Resonance
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Applying a force at regular intervals to a vibrating
or oscillating object will increase the amplitude of
the object. This is called mechanical resonance.
Examples:
#1
#2
Newton’s 3rd Law
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“For every action, there is an equal and opposite
reaction.”
These are interaction pairs.
Ex. A ball hits a bat. The ball exerts a force on the bat. The
bat exerts a forces on the ball equal in magnitude and
opposite in direction.
Outcome: The ball changed direction and accelerates. The
bat is slowed down by the ball.
We are now looking at a system instead of an object.
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A ball falls toward Earth. It accelerates
toward Earth. The Earth also accelerates
toward the ball.
Rockets
Land Speed Record