“If I have seen farther than others, it is because
I have stood on the shoulders of giants.”
Sir Isaac Newton (1642 – 1727)
Physicist
Forces and the Laws of Motion
4-1 Changes in Motion
 Force – a push or pull applied to an object
 Unbalanced forces cause objects to accelerate.
Cause objects to start moving, stop moving or change
direction.
 SI Unit of Force is the Newton –
1 N = the amount of force acting on a 1 kg mass,
produces an acceleration of 1m/s2
1 N = 1kg x m/s2
Forces can act through contact or without physical contact
Contact force – force through touch (contact)
Field Force – electric and magnetic fields, gravity
FORCES COME IN PAIRS
 When a force is applied to an object, then the object
applies a counterforce.
 Force is a Vector – magnitude and direction.
Unit is N and direction (up down, south, north ect)
 Force Diagrams used to show all forces on object.
Isolate the object – only show forces acting on it.
Free Body Diagrams – FBD’s
FN – normal force
(always perpendicular to the surface
the object is touching)
Fg - force of gravity
(weight of object – always directed
straight down)
Fa – applied force
(forced applied to object by person or
another object)
Ff - force of friction
(usually opposes motion of object)
Review: Free Body
Diagrams
4-2 Newton’s First Law
“An object at rest remains at rest and an object in motion
remains in motion unless acted upon by an outside force”
Sometimes called “Law of Inertia” – the mass of an object
(not it weight) causes it to resist changes in motion.
Inertia – “laziness”
Newton’s 1st Law
 Mass is a measure of inertia – easier to accelerate
an object with less mass.
 Objects are in equilibrium when they are either at
rest or moving at constant velocity  The net external forces are zero.
1
• Mass and Weight are different! Mass never changes
• Weight depends on the gravitational pull.
• Weight = mass of an object times the force of gravity (g)
Formula:
W= m x g
Weight is a force so the units of weight are in Newton's (N)
Fg (force due to gravity) = m x g
http://www.exploratoriu
m.edu/ronh/weight/
Astronauts on the orbiting space station are “weightless” because...
a. there is no gravity in space and they do not weigh anything.
b. space is a vacuum and there is no gravity in a vacuum.
c. space is a vacuum and there is no air resistance in a vacuum.
d. the astronauts are far from Earth's surface at a location where
gravitation has a minimal affect.
Astronauts on the orbiting space station are “weightless” because...
a. there is no gravity in space and they do not weigh anything.
b. space is a vacuum and there is no gravity in a vacuum.
c. space is a vacuum and there is no air resistance in a vacuum.
d. the astronauts are far from Earth's surface at a location where
gravity has a minimal affect- but still there just less
Find your mass in kilograms using the
conversion:
___lbs = 1 kg
2.2 lbs
Then Determine how much you weigh in Newton's.
W = mass x gravity (m x g) =
______________ N
Newton’s 2nd Law
 A Net Force on an object will cause it to accelerate.
Formula:
F = mxa
Units of Force: = kg x m/s2 = Newton (N)
Space-shuttle astronauts
experience accelerations of
about 35 m/s2 during takeoff.
What force does a 75 kg
astronaut experience during
an acceleration of this
magnitude?
2625 N Or 2600 N (2 sig figs)
Use equation for weight (force of gravity):
Fg  mg
Rearrange and get
or
Fg  ma
a
Fg
m
Falling and Air Resistance
A. Air resistance decreases the net forces
acting on a falling object
1. When air resistance equals downward
force on falling object (force of gravity–
also called weight) then net force is zero
and no further acceleration occurs.
terminal speed– when acceleration terminates
we call this maximum speed terminal velocity
Fdrag
Fg
The Fg (force in Newton’s due to gravity) can be shown
as two useful vectors
FII and F
FII
F
ϴ
Fg = mg
F
perpendicular
= FN (normal)
Together these 2 forces replace the force of gravity
Using the law of similar triangles and your trigonometric functions
we can solve for FII and F
Sin
= opp / hyp
= FII / m x g
Cos
= adj / hyp
= F /mxg
Hypotenuse = Fg = m x g
ϴ
ϴ
F = m x g cos
FII = m x g sin
Fg = mg
FII = m x g sin - parallel to the plane (x axis)
F = m x g cos
- the normal force that acts
perpendicular to the plane (y axis)
The net force is the vector sum of all the forces.
2 Types of Friction: Static and Kinetic
• Static friction = friction between surfaces Force required to get something moving
• Each surface has its own “coefficient of friction”
• Coefficient of Static Friction ratio of the
static friction to the normal force (perpendicular force)
μs = Fs / F
Kinetic Friction (μk) = friction between moving surfaces.
Examples: Sliding, rolling, fluid friction (air or liquid)
 Coefficient of Kinetic Friction (μk) = ratio of kinetic friction
to the normal force (perpendicular force)
μk = Fk / F
Coefficient of Friction – it’s a Decimal – percent of the
weight required to move an object
Example:
How much force does it take to move a 100 N
object that has a μk = .33
Example:
How much force does it take to move a 100 N
object that has a μk = .33
Given:
Weight: 100 N (FN) = F
µk = .33
Answer:
.33
x 100N = 33 N
Newton’s 3rd Law
For every action there
is an equal but
opposite reaction
Forces always exist in PAIRS – or Action – Reaction Pairs.
 Occur at same time and are equal but opposite in
magnitude.
 The forces can act on different objects
http://www.nbclearn.com/nhl/cuecard/56615
Review of Newton’s 3 Laws of Motion
Discrepant event
This is a discrepant event demonstration
that can be used to help students begin to
understand some applications of
Newton's Third law of motion. In this
activity, students are asked to predict
what would happen if you were sitting on
a low friction cart and threw a heavy
medicine ball off the back. Students will
apply knowledge learned from this
demonstration to help them understand
basic rocket propulsion.