Transcript A Force is - Humble ISD

Newton’s Laws of Motion
We have studied “kinematics”, or the description
of motion.
Now, we look at “dynamics”, the causes of motion
A little history –
Galileo was the first to realize that
objects in “uniform motion” require no
“cause” for their motion.
Only “changes” in motion
------ accelerations -----require a cause: a force
Isaac Newton’s three simple laws are
recognized as the foundation for all of
physics.
In the early 20th century, however, it was
discovered that Newton’s laws must be
modified for objects moving near the speed
of light (relativistic physics) or for objects on
the atomic level (quantum physics).
But for us, Newton’s laws are supreme !!!
A Force is 
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A “push” or a “pull” that acts on an object
Something that is caused by an “agent”
A vector - when you pull a cart the force you exert
has both a magnitude (the amount of force) and a
direction
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Forces are represented by vector arrows
Measured in units of newtons (N) in the metric
system and pounds (lbs) in the English system
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A newton (N) is the amount of force needed to accelerate
a 1.0 kg mass at a rate of 1.0 m/s2.
So a N = kg * m/s2
Types of Forces
There are two basic types of forces –
Contact force - the forces existing between
two or more objects in contact with
each other. Example – tension, friction,
applied force, normal force
Field force - a force exerted through space not
requiring contact. Example – gravity,
magnetic forces, electrostatic forces
Types of Forces, cont.

Weight
–
the force of gravity on a mass
W = Fg = mg
(mass * “g” acceleration)

Normal force –
when a surface pushes back
This normal force is always perpendicular
to the contact surface. (FN)
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Tension
–
force applied through a rope or chain (T)
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Spring
-
the force exerted by a stretched or
compressed spring (Fk)
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Friction
–
force resistant to motion acting between
two surfaces (f or Ff)
Force Diagrams
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Also called FBD – “free body diagrams”
A diagram which
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Uses a dot to represent the center of mass of an object
Places the tail of the vector on the object and points in the
same direction that the force acts
A book on a table –
FBD –
FN
( normal force)
The vectors are
forces acting only
on the book.
W (weight force)
Newton’s First Law
Law of Inertia
Inertia – the tendency of a body at rest to remain at rest or, if in
motion, to remain in constant motion (no acceleration)
Review - acceleration is a change in velocity – either in
magnitude or direction. So if an object maintains constant
velocity, its motion never changes, it does not accelerate. It
does not slow down or speed up nor does it change direction.
Sometimes inertia is referred to as “laziness” – and the mass of
an object is a direct measure of its inertia or laziness. The
more massive something is, it has a greater tendency to be
lazy – to not want to change. So a larger accelerating force is
required to get it to change its motion.
This first law is also called the law of balanced forces.
Newton’s Second Law
Fnet = m * a
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Law of unbalanced forces
There is a net force which causes the object to
accelerate in the direction of the Fnet.
Newton expressed this relationship as
a = F/m
What does this tell us:
For a given force, the acceleration is inversely proportional
to the mass.
For a given mass, the acceleration is directly proportional to the
force.
If the forces on an object are balanced, then there is
no net force and the object does not accelerate – it
does not change its motion. Mathematically, we see
that the vector sum of the forces acting on the body
in both the horizontal and vertical directions is zero.
A car traveling with constant velocityS Fx = F1 + (-)F2 = F1 – F2 = 0
FN
F2
F1
W
S Fy = FN + (-)W = FN – W = 0
The sum of all the forces is zero. The
forces balance each other and the object
is in a state of equilibrium.
Fnet = m * a
but accel = 0, so
Fnet = m * 0 = 0
Just hangin’ around - Investigating tension forces
Absolutely, fundamental and most important concept:
In equilibrium, the horizontal forces must sum to zero
and the vertical forces must sum to zero.
The
up
force
comes
from the
wall.
These
two
must be
equal and
opposite.
Find all the horizontal and vertical
forces. If the weight is 200 N
down, there MUST be a 200 N
tension up.
200 N - weight and up tension
200 N
Using right triangle geometry,
the tension force forms the
hypoteneuse.
Use this information to find the
opposite and adjacent sides.
Forces on surfaces - tryin’ to be normal
Be careful – the normal force is ALWAYS perpendicular to the surface
Fn
Now, raise the surface to create an angled ramp
N
Fg = mg
This component
balances the
normal force.
Find it as:
mg cos θ
This is the component
of the weight acting
downramp.
Find it as: mg sin θ
By the geometry, these two
angles are equal.
Fg – you need to resolve this
weight vector. Make it
the hypoteneuse of a right
triangle.
Another situation 
Draw the FBD
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Determine both horizontal and vertical forces
FN = 100N
F = 25 N
f=5N
W = mg
= 10 kg * 10m/s2
= 100 N
SFy = Fn – W = 100 – 100 N = 0
(no motion in the vertical direction)
SFx = F – f
= 25 – 5 N = 20 N net
force
a = F / m so 2 m/s2 = 20 N / 10 kg
May the
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Net Force be with you
Total force acting on an object
Vector sum of all the forces
The unbalanced force referred to in Newton’s Law of Motion
Net force is equal to the mass of an object times the
acceleration of that object.
SF = Fnet = m * a
(Remember, in equilibrium an object is at rest or moving with a
constant velocity. Either way, acceleration = 0 and there is no
net force.)
Force Diagrams (FBD)
A Review
First case - (1) is in equilibrium (2) is accelerating
N
N
F
f
F
f
W
SFy = N – W = 20 N – 20 N = 0
SFx = F – f = 10 N – 10 N = 0
F net = 0
no acceleration
W
SFy = N – W = 20 N – 20 N = 0
SFx = F-f = 25N – 10 N = 5 N
Fnet = 5 N
Fnet = ma
5 N = 2 kg (a)
a = 2.5 m/s2
Forces at an angle
Any vector at some angle q must be resolved into its x
and y components.
F
N
f
W
F
f
N
Notice - now the Fy works WITH
the normal force.
SFy = (Fy + N) – W
This means that the normal force
actually decreases. Some of the
weight is balanced by the upward
lift of the pulling force.
Notce – in this diagram, the Fy works
WITH the weight.
SFy = N – (Fy + W)
This means that the normal force
actually increases. There is more
downward force and so the normal must
respond to remain in equilibrium.
W
The forward force comes from the x component and is opposite friction.
Newton’s Third Law
Third Law deals with action-reaction force pairs.
If you push on an object, the object pushes back. The two forces
are equal but opposite in direction. AND – the two forces work
on two DIFFERENT objects.