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Forces
What is a Force?
A force is a push or pull acting on an
object that changes the motion of the
object.
Types of Forces
Contact Force – Forces that act through
direct contact between two objects
 Applied Forces, Friction
 Long Range Forces – Forces that can
act over distances
 Gravity, Electromagnetic Force (EMF)

Newton’s Second Law of Motion
Newton discovered the idea of a Force
 He found the Force is proportional to
the Acceleration of an object (more
Force = more Acceleration)
 He found the Force is proportional to
the mass of the object (more mass =
more force needed).

Newton’s Second Law of Motion
Moving objects accelerate when an
unbalanced force (F) acts on them. The
stronger the force, the greater the
acceleration (a). Also, the greater the
mass (m) the greater the force required
to change the motion.
 Force = mass x acceleration
 F = ma

Newton’s Second Law of Motion
Force is a Vector Quantity and
therefore has magnitude and
direction.
 The direction of the force is the same
as the direction of the acceleration.

S.I. Unit For Force

The Unit for Force is a Newton (N)
 1N = 1kg m/s2
 A Newton (N) is defined as the
amount of force required to
accelerate 1 kg of mass at a rate of
1 m/s2.
Net Force

When Multiple forces are acting on an
object. The Net Force is the amount of
force that is left after adding all the forces
on the object.

Net force (FNET) = Resultant Force (FR)
Balanced Forces
Balanced Forces are forces that are
equal and opposite so that they
cancel out.
10 N East
10 N West


Net Force = +10 N + -10 N = 0
Unbalanced Forces
Unbalanced Forces are forces that when
added do not cancel out and cause a
change in the motion of the object.
30 N East
10 N West


Fnet = +30 N + -10 N = +20 N East
Inertia
Inertia measures the tendency of an object
to resist changes in motion.
 Galileo came up with the idea of inertia
 Objects do not want their motion to
change
 Mass measures how much inertia an
object has (More mass = More inertia)
Newton’s First Law of Motion
(Law of Inertia)

If no unbalanced forces act on a moving
object, then the object will continue to
move with a constant velocity (constant
speed in a straight line). If an object is
at rest it will stay at rest.

Newton took his concept of forces and
combined it with Galileo’s idea of inertia
Equilibrium

First Condition for Equilibrium
 If the Net Force acting on the object is
zero
FNET = 0 a = 0
 The object is either stationary (v = 0) or
traveling with a constant velocity
(v = constant)
Free Body Diagram (FBD)

A diagram that looks at all the forces
acting on a single object. A FBD has all
the forces labeled with their magnitude
and direction as well as the motion of the
object.
Mass and Weight
Mass is amount of matter that an object
possess. Mass does not change with
location.
Weight is the gravitational force that a large
body (such as a planet) exerts on another
object.
Weight




Weight is a Force! It is measured in
Newtons.
Weight = Mass x Acceleration due to
Gravity (Newton’s 2nd law)
W = mg
Weight does change with location!
(“g” will change with location)
Solving Net Force Problems
1.
2.
Make a Free Body Diagram (Label all
forces and the acceleration of the object)
Use the following Equations to solve for
the unknown



FNET (x-direction) = max
FNET (y-direction) = may
f = m Fn
Microgravity


Microgravity is the illusion of
weightlessness experienced in freefall.
(All objects are falling at the same rate)
Apparent Weight is the weight a scale
gives you but may change if are not in
equilibrium.
Friction
Friction resists motion.
 Friction causes moving objects to
slow down.
 Friction produces heat.

Friction
1.
2.
3.
Static Friction – occurs between
stationary surfaces in contact.
Sliding Friction – occurs when one
surface slides over another.
Rolling Friction – occurs when a
rounded surface rolls over another.
Ways to Reduce Friction
Use wheels or rollers
 Lubricants - produce a smooth layer
between the surfaces
 Sanding the Surfaces – smooth the
surfaces

Friction and Newton’s Laws
Friction is a force (Newton’s laws apply)
Friction causes an acceleration (slowing
down the motion of an object)
Friction always acts parallel to the surface
in the opposite direction of motion
Calculating the Frictional Force
Friction depends on:
1. Forces acting between the
surfaces (Normal Force)
2. Nature of the surfaces
(Coefficient of Friction)
Calculating the Frictional Force

Normal Force (FN) – Force that acts
perpendicular to the surface and
away from the surface
 The Normal Force (FN) is usually
found by summing the forces in the
y-direction
Calculating the Frictional Force
Coefficient of Friction (m) – describes
how rough/smooth the surfaces are
 Rough Surfaces – High value of m
m > 0.5
 Smooth Surface – Low value of m
m < 0.5

Calculating the Frictional Force
The frictional force is the product of
the coefficient of friction and the
normal force
 Frictional force = (Coefficient of
Friction) x (Normal Force)
 f = m FN

Air Resistance
Free Fall – The situation where
gravity is the only force acting on an
object (Assume No Air Resistance)
 Air Resistance – The force the air
applies on a moving object. It
attempts to slow down falling objects
(similar to friction)

Air Resistance and
Terminal Velocity
Terminal Velocity – The maximum
speed a falling object reaches when
dropped from rest
 An object reaches terminal velocity
when the force of gravity is balanced
with the force of air resistance

Air Resistance and
Terminal Velocity
1. An object is dropped and at first the force
of gravity (W) is much greater than the
force of Air Resistance (FAR)
W >> FAR
2. The force of Air Resistance (FAR)
increases as the speed of the falling
object increases
Air Resistance and
Terminal Velocity
3. Eventually the force of Air
Resistance is equal and opposite to
the force of gravity and the Net Force
acting on the falling object is zero.
W = FAR Therefore FNET = 0
Air Resistance and
Terminal Velocity
4. Since the Net Force acting on the
object is zero, the object continues to
fall but falls the rest of the way at a
constant velocity (Newton’s First
Law). This velocity is called the
Terminal Velocity!
Air Resistance and
Terminal Velocity
FNET = 0 and F = ma
therefore ay = 0
Object falls with a constant velocity
(Terminal Velocity)
Note: Terminal Velocity can be a
Maximum or a Minimum!
Periodic Motion and
Simple Harmonic Motion


Periodic Motion - motion that repeats back
and forth through a central position
Simple Harmonic Motion (SHM) – periodic
motion where the Force is proportional to
the displacement from the equilibrium
position (F a d)
Simple Harmonic Motion (SHM)



In Simple Harmonic Motion the Net Force
at the equilibrium position is zero. When
the object moves away from the
equilibrium position a Restoring Force
pulls the object back.
As “d” h then “F” h
Examples; Spring, Pendulum
Simple Harmonic Motion (SHM)

Objects in SHM can be described with two
quantities.
Period (T) – the time needed to complete
one cycle of motion
 Amplitude (A) – is the maximum distance
the object moves from the equilibrium
position

Springs


Hooke’s Law – the Force of a Spring is
proportional to the distance from the
equilibrium position
FS = kd



FS = Restoring Force of the Spring
d = displacement from the equilibrium position
k = spring constant for that particular spring
Period of a Spring

To find the period of a spring (TS) in simple
harmonic motion:
 TS = 2p (m/k)1/2 (Square Root)
 m = mass of the spring
 k = spring constant
Pendulums


A Pendulum is a mass attached to a string
or wire of length (L) that swings back and
forth through the equilibrium position
A pendulum swinging through small
angles is an example of SHM
Pendulums


Gravity is the restoring force and is
proportional to the distance away from the
equilibrium position
The components of gravity change as the
pendulum moves back and forth resulting
in Fg a d
Period of a Pendulum

The Period of a Pendulum (TP) can be
found with;
 TP = 2p (L/g)1/2 Square Root
 The Period of the Pendulum depends
only on the length and the acceleration
due to gravity (Not the mass)
Resonance


Resonance is increasing the amplitude of
vibration by adding a small force at regular
time intervals
Examples: Swing (Pendulum), and Sound
Newton’s Third Law of Motion
States: Every Action has an equal and
opposite Reaction.
 Forces always come in pairs!
 Action-Reaction Forces always occur
between two objects.
Four Fundamental Forces of
Nature
1.
2.
Gravity – Force of attraction between
any two masses. Weakest of the
fundamental forces but acts over the
largest distances.
Electromagnetic Force (EMF) – force
between charged particles. Stronger
than gravity but does not reach as far.
(Like charges repel, opposite charges
attract)
Four Fundamental Forces of
Nature
3. Strong Nuclear Force – Force of
attraction between subatomic particles
inside the nucleus. Strongest force in
Nature but only acts inside the nucleus
(shortest distance). Holds the atom
together.
Four Fundamental Forces of
Nature
4.
Weak Nuclear Force – is the force
observed in the radioactive decay of
some elements
Unification Theory – This theory looks to
unify all the fundamental forces to a
single unified force.
Forces on Ropes and Strings




Tension (T) – is the force acting along a
rope or wire.
Tension always acts away from the object
along the rope.
Newton’s Third Law applies to Tension
when looking at two objects.
Only One Tension needs to be drawn on a
Free Body Diagram