Download Forces & Newton’s Laws of Motion

Forces
A force can be defined in numerous ways. However,
the simplest would be a push or pull on an object.
 The unit is the Newton (N).
 A force is a vector quantity, which means it has a
direction.
 You need to memorize one important conversion:
1 lb = 4.448 N
 Examples of Forces: Hitting a ball, pulling a box,
Weight of an object
 Non-Examples: Heat, Light, Mass, Temperature

Fundamental Forces
There are four fundamental
interactions/forces in the universe:
These are forces that act over distances
without touching one another.
 They are considered Field Forces.
 Field Forces do not need a medium to
travel through.

Gravitational Force
Any object with a mass has a
gravitational field and will attract any
other object with mass. This is done
through the distortion of space.
 Weight is a measurement of the force of
gravity on a mass.
W  Fg  mg
 Symbol = Fg
 What is your weight?
m = mass kg

g = 9.81m/s2

Electromagnetic Force
This rises from the interaction of
charged particles.
 It is responsible for holding electrons to
nuclei and molecules together
 Charged particles include the proton and
electron

Strong Force
These forces arise from the interaction
of gluons.
 (Theoretically) This force holds together
the protons and neutrons in a nucleus of
an atom and the quarks within the
protons and neutrons.

Weak Force
This force works over small distance
and helps to keep the nucleus of atoms
together.
 This force also mediates the nuclear
decay of neutrons and protons.

Strength Order of Forces
Strongest
1.
2.
3.
4.
Strong Force
Electromagnetic Force
Weak Force
Gravitational Force
Weakest
Contact forces
These are really specific
electromagnetic forces in nature
 These are forces that arise during
physical contact between objects.

 The two main contact Forces that we will talk
about are the Normal Force (FN) and Friction
(Fμ)
Normal Force
A contact force that exists when two
objects are pressed together and always
acts perpendicular to the surface of the
contact. Symbol: FN
 FN=mgcosθ

Friction Force
This force opposes the displace of an object
(matter) as it moves across the surface of
another object.
 The amount of Friction depends of the shape of
the surface.

F s / k  s / k  FN

Types of Friction
 Static Friction – the force that resists the
initiation of sliding motion between two
surfaces that are in contact and at rest. (Fs)
○ Equal and opposite forces are present until
object moves
○ Fs,max – the amount of static friction force at
the moment right before motion occurs
 Kinetic Friction – the force that opposes the
movement of two surfaces that are in
contact AND sliding over each other. (FK)
Forces on Ropes or Strings
Tension - the force exerted by a string, rope
or chain
 This is a contact force
 The Force of the rope is equal to the force
exerted by the object the rope is attached to.

Newton’s Laws of Motion
Newton’s First Law of Motion (Law of
Inertia)
 An object will continue its motion until it
is acted upon by an outside force.
 In other words an object has a tendency
to keep doing whatever it is presently
doing. This is called its’ INERTIA. The
greater the mass of an object, the
greater its inertia.
Newton’s Laws of Motion (2)
Newton’s Second Law of Motion
 The acceleration of an object is directly
proportional to the NET external force
acting on the object and inversely
proportional to its mass.
Acceleration = Net Force
F

ma
Net
Net
Mass
Newton’s Laws of Motion (3)
Newton’s Third Law of Motion
 If two bodies interact the magnitude of the
force exerted on object 1 by object 2 is equal
to the magnitude of force simultaneously
exerted on object 2 by object 1.
 In other words, for every action, there is an
equal and opposite reaction.
 These are called Action-Reaction Pairs.
Forces will always come in pairs.
 Remember that action-reaction pairs DO NOT
result in equilibrium because they act on
OPPOSITE objects.
Force Analysis
Free-Body Diagrams
 Forces are vector quantities. When analyzing
different scenarios it is common practice to draw
a FREE-BODY DIAGRAM.
 This helps an individual visualize what and how
the forces are acting on the object.
 When you are drawing a free-body diagram you
should:
1. Draw picture.
2. Identify forces acting on object
3. Place forces on picture.
Translational Equilibrium
When analyzing different scenarios you
must first decide if the object is in
equilibrium.
 An object is said to be in translational
equilibrium when there is no net force on
an object.
 How can you tell: If it has a constant
velocity (Not accelerating) it IS in
equilibrium. Otherwise it is NOT in
equilibrium.
