Newton’s Laws
of Motion
(and force!!)
A
force
is a push or pull on an object
control over
movement:
desired motion
or preventing
undesirable
movement
Apply Force:
Contact Forces
muscles
machines/mechanical
ropes and wires
Friction
adhesion
EXAMPLES of forces:
EXAMPLES of FORCES:
Non

contact forces:
Nuclear
Electrical/magnetic
Gravity
weight
Non

contact
Forces
such as
gravity,
magnetic,
and electric
forces.
Like Velocity and Acceleration…..
FORCE has both magnitude
AND
Direction!!!
NET FORCE (unbalanced force)
results in motion
for an object at rest)
or acceleration (a change in velocity)
For an object already in motion
Equilibrant Force
Net Force = 0 …..means,
an object at rest remains at rest
OR
an object moving with constant motion
will continue at that constant
motion.
FORCE IS MEASURED IN
NEWTONS
One NEWTON (N)
1 kg x 1m
s
2
Think about it….
Force = mass X acceleration
that gives a 1 kg object, an acceleration of 1m/s2
Inertia
tendency of an object to
RESIST a change in motion
Example: velocity of an object remains
constant unless a force changes it
the greater the mass, the
greater the inertia
Newton’s First Law of Motion
An object continues in a state of rest or in
a state of motion at constant speed along
a straight line…
unless compelled to change that
state by a
net force
.
an object
moving at a
constant velocity
remains at that
velocity unless a
NEW FORCE
(>0) acts upon it
Inertia plays
a central role
in one type of
seat belt
mechanism.
Newton's First Law of Motion (inertia):
Two mousetrap cars with different masses.
which one has more inertia?
Construction Tip: Wherever possible, remove mass = less energy
required to overcome inertia and get your car moving. So More
energy available to propel your car forward.
Less mass = less inertia!
friction
Friction
force of adhesion
depends on the surface and
force acting between surfaces
EXAMPLES OF FRICTION: air,
fluid, solids
Examples of
contact forces
Describe what
role Newton’s
first law plays
a role in what
is occurring in
this picture
The Normal Force & Friction
The Normal Force & Friction
Newton’s Second Law of Motion
When a net force
F
acts on an object
of mass
m
, the acceleration
a
that
results is directly proportional to the
net force and is inversely proportional
to the mass.
If: net F =
m
X a then….
direction
of acceleration = direction of the net force
SI Unit of Force: kg • m/s
2
= newton (N)
m
F
a
Newton’s second law and mousetrap cars:
mass resists acceleration, and force causes
acceleration.
to double the acceleration of an object, double the
force or remove mass.
•
Force causes acceleration!
•
Need more pulling force?
•
Shorten the lever arm and move the
mousetrap closer to the drive wheels.
•
Shortening the lever arm implies
moving the string closer to the
hinge on the mousetrap.
ONCE AGAIN!!!
A NEWTON
….is
the amount of force
required to accelerate a
1 kg object 1 m/s
2
2
m/s
059
.
0
kg
1850
N
110
m
F
a
N
+110
=
N
560
N
395
+
N
275
+
F
m
= 1850 kg
With two guys pushing…….what are
The net forces???
If the airplane’s mass is 13 300 kg, what is the
magnitude of the net force that the catapult and
jet engine exert on the plane?
N
10
4.1
=
)
m/s
kg)(31
300
(13
5
2
ma
F
The Normal Force
GRAVITY (gravitational force)
the pull an object exerts on another object
the amount of gravitational force is
dependent on:
1) Mass of the two objects
2) distance between objects
the greater the mass, the greater the
gravitational pull on that object
Weight
measure of gravitational force
varies dependent on proximity to EARTH
Weight vs Mass
Mass is the measure of…
the matter of an object and
an object’s resistance to change in
motion
Newton’s Third Law of Motion
Whenever a
body exerts a
force on a
second body, the
second body
exerts an
oppositely
directed force of
equal magnitude
on the first body.
Newton’s 3
rd
Law……
action force = reaction force
for every action, there is an equal and
opposite reaction!!
tires of a car push against the road,
and the road in turn pushes back on the
tires,
a swimmer pushes the water backward,
and the water pushes swimmer forward
"A mousetrap car is propelled because the drive wheels push
on the floorand the floor pushes back on the car [wheels]”
The wheels can only push on the floor as hard as the floor can
push back. If the floor cannot push back with the same force
as the wheels push, then the wheels will spin in place and the
car will not accelerate to its fullest potential.
Newton’s Third Law
For every action there is an equal and
opposite reaction.
Fortunately for
sparky,
Zeke knew the
famous
“Rex Maneuver”!
Free Body Diagrams
A Free Body Diagram distills the
problem’s complexity down to only
those actions that are interacting with
the object of interest.
Consider the following situation:
You are standing in the middle
of the room.
Are there any forces acting on
you?
Is there net force acting on you?
Draw the forces acting on you.
Justify your answer by referring to
Newton’s Laws.
Now remove the floor and draw
the forces acting on you.
What is your state of motion
now?
Characterize it by stating
something that we can
calculate.
Distance Car Tips:
•
longer lever arms
•
larger drive wheel(s), ideally 1

2 feet in diameter
•
small drive axle
•
small power output
•
decrease mass and rotational inertia
•
remove ALL friction
Speed Cars
•
shorter lever arms
•
smaller drive wheel(s), ideally around 2

5" in
diameter but no larger than a compact disc. If the
wheel

to

axle ratio is too small, your vehicle will
[spin out and] waste energy.
•
Increase traction
•
Larger diameter drive axle. You can get more torque
with a thick axle

more torque means greater
acceleration!
•
Larger power output. For speed, the objective is [to
quickly convert potential energy into kinetic energy.]
•
Decrease mass and rotational inertia
•
Remove ALL friction
•
Friction slows and stops the mousetrap car
•
Energy moves the vehicle.
•
too much friction = energy consumed too quickly
and vehicle won’t travel far or accelerate fast.
•
Check moving components and decrease the
friction at each point.
•
The more moving components, the greater the
force of friction will be (usually.
•
The smaller the frictional force, the farther your
supply of energy will propel your vehicle.
•
Slow

moving cars have a smaller force of air
resistance and travel farther than faster

moving
vehicles
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