Newton’s Laws of Motion
Newton’s Three Laws of Motion
• Sir Isaac Newton first presented his
three laws of motion in the Principia
Mathematica Philosophiae Naturalis
in 1686.
• To understand Newton’s laws of
motion, you must remember that on
Earth we always have GRAVITY
pulling down on us and AIR around
us.
• Newton’s laws are used in the design
of aircraft and spacecraft.
First Law of Motion
Objects in motion
tend to stay in
motion
AND
Objects at rest tend
to stay at rest
UNLESS
Something pushes
or pulls on the
object.
First Law of Motion
Example
Think of a block sitting still, or “at rest”.
If the block is to move, it must receive
a push. Otherwise it will stay at rest.
According to Newton’s First Law,
When Will the Block Stop?
Push
Push
First Law of Motion
Example
Think of a baseball thrown in space.
Without GRAVITY to pull it down, the ball
would just keep moving until something got
in its way and stopped it.
Aerospace Examples of the First
Law of Motion
The movement of an airplane changes
when the pilot changes the throttle setting
(or force) of the engine.
More force moves the airplane at a
greater speed.
Aerospace Examples of the First
Law of Motion
The movement of a model rocket
being launched into the atmosphere
The rocket was at rest (not moving)
until the force of the engine pushed it
upward.
Second Law of Motion
The acceleration of
an object depends
on:
– The size of the force
on the object
The relationship:
– The direction of the
F=m*a
force on the object Force = mass * acceleration
Or
– The mass of the
a=F/m
object
acceleration = Force/mass
Examples of Second Law of Motion
The direction of acceleration is the same
as the direction of the external force.
Remember the box?
If you want to move the box to the
right, you push toward the right.
Push
Push
If you want to move the box to the
left, you push toward the left.
Examples of Second Law of Motion
The
bigger
the
object
(the
more
mass),
the
The magnitude (size) of the acceleration is
more
force
is needed
accelerate
equal
to the
externaltoforce
dividedthe
by object.
the
mass of the object.
Back to the box (but let’s add a car).
50 lb
50 lb
If you push the car and
the eraser with the same
force,
50 lbs
The eraser will
accelerate much faster
than the car.
50 lbs
Third Law of Motion
For every action, there is an equal and
opposite reaction.
You, on roller blades, push against a wall.
Object A: You
Object B: Wall
Action: You apply
a force on the wall.
Reaction: The wall
applies the same
force back on you.
Examples of the Third Law of
Motion
You, on roller blades, push against a wall.
Object A: You
Object B: Wall
Your backward movement is the result
of the wall pushing back against you.
Examples of the Third Law of
Motion
Flaming hot gases are pushed out the bottom of
a rocket.
Object B: Rocket
Reaction: The rocket
moves upward
Object A:
Flaming hot gas
Action: Gas is pushed
downward out of the
rocket
Third Law of Motion
Examples
The upward
movement of
the rocket is
the result of
the gases
pushing
downward from
the rocket.
Third Law of Motion
Key Points
The third law can be used to explain how a wing
produces lift and a jet engine produces thrust.
On a wing, the air is pushed
downward because of the
wing’s unique shape (an
airfoil). In reaction, the air
pushes the wing upward.
A jet engine produces hot exhaust
gases which flow out the back of
the engine. In reaction, a
thrusting force is produced in the
opposite direction.
Image Resources
Microsoft, Inc. (2009). Clip art. Retrieved June 16, 2009, from
http://office.microsoft.com/en-us/clipart/default.aspx
National Aeronautics and Space Administration (NASA). (n.d.).
Newton’s third law of motion. Retrieved June 19, 2009, from
http://www.grc.nasa.gov/WWW/K-12/airplane/newton3.html