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Forces
Dynamics
FORCEMAN
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What causes things to move?
• Forces
• What is a force?
– A push or a pull
that one body
exerts on another.
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•Balanced
No change in motion
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Unbalanced
• If the forces acting on a body are
"unbalanced" this means that there is a "net"
or "resultant" force.
• The body will do one of the following:
1. Speed Up
2. Slow Down
3. Change the Direction of its Motion
• In other words
• The Body will Accelerate
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Newtons
• SI unit of force is the Newton
(N).
• Defined as the amount of force
that when acting on a 1 kg object
produces an acceleration of
1m/s2.
• Therefore, 1N = 1 kg • m
s2
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Newton’s First Law
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Newton’s First Law
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Newton’s First Law
• An object at rest stays at rest, and
an object in motion stays in motion
unless acted upon by a net external
force.
• What do you mean by “Net Force”?
• Net means: Final Sum, so net
external force is the sum of all the
forces acting on the object.
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Newton’s First Law
•Newton’s First Law is also called
the Law of Inertia.
•
Inertia is the tendency of
an object to resist any
change in its motion.
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Why does the ladder
continue to move?
Inertia!
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Why does the person continue to move?
Inertia!
Clip
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Second Law
• The acceleration of an object is
directly proportional to the net
external force acting on the object
and inversely proportional to the
object’s mass.
•F=ma
• Clip
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Newton's 2nd Law of Motion:
•Force = mass x acceleration
F = (ma)



Mass is in kg
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Acceleration is in m/s
2
Force is in kg x m/ s (or N)
Force Problems:
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What force is needed to give an
object with a mass of 25.0 kg an
acceleration of 15.0 m/s2 ?
Formula
Given:
m = 25.0 kg
a = 15.0 m/sec²
F = ma
Unknown:
F=?
F = (25 kg)(15 m/sec²)
F = 375 kg•m/sec² = 375 N
NewtoN’s
Third Law
34 Newton’s Third Law of Motion
…to
every action force
there is an equal and
opposite reaction force
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ActionReaction
forces act on
different
objects
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Review
Newton’s
Laws Clip
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Forces
that
oppose
motion
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Forces that
oppose motion
Friction
Song 1
Friction
Song 2
Forces that
oppose motion
-Air resistance is the force
air exerts on a moving object
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•
•
Friction
What Causes Friction?
Microscopic, electrostatic
interactions between contacting
surfaces
• Three types of friction:
1. Static Friction
2. Sliding Friction
3. Rolling Friction
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
Types of Friction
Types of friction:
1. Static – force required to
overcome inertia of a
stationary object
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Static Friction (FS)
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of Friction (cont’.)
2. Sliding – force required to keep an object
sliding at a constant speed
3. Rolling – force
required to keep an
object
rolling
at
a
To decrease friction, surfaces can be lubricated
with a liquid suchconstant
as oil.
speed
FRICTION
LAB
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CLIP
The amount of force
required to overcome
static friction is always
greater than the force
required to overcome
sliding or rolling
friction.
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GRAVITY!!
• The force of gravity is an attractive force
between objects.
• The amount of GRAVITATIONAL FORCE
between objects depends upon their:
• 1- Masses (greater mass means more gravity)
• 2- Distance apart (greater distance means
less gravity)
*All objects fall toward the earth at the same rate
of acceleration, regardless of their masses!
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Calculating the Gravitational Force
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Gravitation
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Weightlessness
Gravity in
Space
Moon
Moon Hammer & Feather
29Free fall and Terminal Velocity
• The opposing force of air resistance
increases with speed.
• Objects accelerate towards the Earth
until the force of gravity is cancelled or
balanced by this air resistance.
• This is the highest speed an object can
reach.
• Different objects have different terminal
velocities.
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Weight and Gravity
W =m x g
BECAUSE IT’S A FORCE!
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Weight
• When the mass of an
object and the
acceleration due to
gravity are known, the
weight of an object can
be calculated.
Ex. Mass = 100kg
Acc.= 9.81 m/s2
Weight = 981 N
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Example Problem
• What is your weight if your mass 4.52
kilograms?
Given:
Unknown:
m = 4.52 kg
W=?
g = a = 9.8 m/sec²
Formula: W = mg
W = (4.52kg)(9.8 m/sec²)
W = 44.296 kg•m/sec² = 44.296 N
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Projectile Motion
Projectiles follow a curved path because
gravitational pull.
of the Earth’s
 They have 2 types of motion that are
independent of each other:
1. Horizontal motion
2. Vertical motion
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Projectile Motion
Which will hit the ground first?
A ball that is dropped straight
down or a ball that is “shot”
out horizontally?
* An object launched horizontally
will land on the ground at the
same time as an object simply
dropped from the same height!!!!
Another
Example
of
Projectile
Motion
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40 Circular Motion
•
Roller Coaster Clip
An object moving in a
circular motion at the same
speed is accelerating toward
the center because its
direction is constantly
changing.
Centripetal acceleration is
acceleration toward the
center of a curved path.
'Centripetal' comes from the Latin word for 'center seeking'
Centrifugal 'force' is really a function of the
inertia of the object being pushed into a circle.
It is not really a force at all, it is simply the
tendency of an object to go in a straight line.
only
centripetal is
the real
force
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Circular Motion cont.’
• Centripetal
force is the
force that
causes a
moving object
to move in a
curved path
Without a
centripetal
force, an object
in motion
continues along
a straight-line
path.
With a
centripetal
force, an object
in motion will
be accelerated
and change its
direction.
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How do rollercoasters work!?!
Amusement Park Forces