Transcript Pre Lab PowerPoint - e

Who was Isaac Newton?
Sir Isaac Newton (1643-1727), an
English scientist and mathematician
famous for his discovery of the law of
gravity, also discovered the three
laws of motion. He published them in
his book Mathematic Principles of
Natural Philosophy in 1687. Today
these laws are known as Newton’s
Laws of Motion and describe the
motion of all objects we experience in
our everyday lives.
Newton’s Third Law States:
“For every action there is an
equal and opposite reaction.”
In Other Words….
• When one object exerts a force on a
second object, the second object
exerts an equal but opposite force on
the first.
What does this mean?
• This means that when you push on a wall, the
wall pushes back on you with a force equal in
strength to the force you exerted.
• The forces exerted by two objects on each other
are often called action-reaction force pairs.
What is Force?
• Force is any push or pull, and they
can be balanced or unbalanced.
Balanced results in no change in
speed or direction, while unbalanced
forces result in a change in speed
and/or direction.
Action Reaction Forces
• You constantly use
action-reaction force
pairs as you move
about. For instance,
when you jump, you
push down on the
ground. The ground
then pushes up on
you. It is this upward
force that pushes you
into the air.
Some Examples from Real Life
• When you walk forward, you push backward on
the ground.
• Your shoe pushes Earth backward, and Earth
pushes your shoe forward.
More Examples from Real Life
• When a bird flies, its wings push downward and
backward direction. With Newton’s third law, the
air pushes back on the bird in the opposite
directions—upward and forward. This force
keeps a bird in the air and propels it forward.
If forces are opposite and equal, then why do
we experience movement?
• Earth has much more mass
compared to you so it does
not move when you exert a
force on it but you move
when it exerts a force on
you. If you exert a force on
something that has less
mass than you do, like a
soccer ball, it will move and
you will not.
Forces Act in Pairs
•
Both objects that a force is exerted on
will accelerate.
•
The amount of acceleration depends on
the mass of the object.
•
Small mass  more acceleration
•
Large mass  less acceleration
Action and Reaction Forces act on
Different Objects
• How can a horse pull a cart if the cart is pulling
back on the horse with an equal but opposite
force? Aren’t these “balanced forces” resulting in
no acceleration? NO!!
• Forces are equal and opposite but act on
different objects.
Action and Reaction Forces act on
Different Objects
•
The hammer exerts a force on the nail to
the right.
•
The nail exerts an equal but opposite force
on the hammer to the left.
How Does a Rocket Use
Newton’s Third Law?
• A rocket contains fuel that, when ignited,
produces a hot gas. The gas molecules exert a
force on the inside walls of the rocket, and the
walls exert a force that pushes back on the gas.
The gas is pushed out of the bottom of the
rocket.
• This downward push is the action force and the
reaction force is the upward push on the rocket
engine by the gas molecules.
How Does a Rocket Use
Newton’s Third Law?
•
FG
FR
•
The rocket exerts a
downward force on
the exhaust gases.
(FR)
The gases exert an
equal but opposite
upward force on the
rocket. (FG)
Comprehension Check
• Describe Newton’s Third Law in your own words.
• Do action and reaction forces act on the same
object or on different objects? Explain your
answer.
• Kent Swim has rowed his boat within three feet
of the dock. Kent decides to jump onto the dock
and turn around and dock his boat. Explain to
Kent why this strategy is not a good strategy.
Try the next set of questions!
A truck driving down the road runs into a
bug flying the other way.
a) Which object has the greater force exerted on it?
• Both objects have the same small force exerted on them
Click to
see answer.
during the collision
according
to Newton’s third law.
b) Which object will change its motion the most?
• The bug squishes easily! The bug obviously changes its
motion significantly
more
thananswer.
the truck because of its
Click to
see
much lower inertia mass.
A boxer hits a piece of paper.
a) Which object has the greater force exerted on it?
• Both objects have the same small force exerted on them
Click to
see answer.
during the collision
according
to Newton’s third law.
b) Which object will change its motion the most?
• The paper obviously changes its motion significantly
Click
see ofanswer.
more than the
gloveto
because
its much lower mass.
A cannonball is fired from a cannon.
a) Which object has the greater force exerted on it?
• Both objects have the same force exerted on them
Click to
see answer.
during the collision
according
to Newton’s third law.
b) Which object will change its motion the most?
• The cannonball obviously changes its motion
significantlyClick
more than
actual
cannon because of its
to the
see
answer.
much lower mass.