Transcript Newton`s Law

What are Newton’s Laws?
The Nature of Forces
What are Balanced Forces?
• Have you ever had an arm wrestling
competition with someone?
• If you compete against someone who is
just about as strong as you
• There will probably be a time when both of
you are pushing as hard as you can, but
your arms stay in the same place.
• This is an example of balanced forces.
Unbalanced Forces
• Imagine that you are competing against
Mr. Zim - a world famous arm wrestler.
• Chances are that his force will be much
greater than yours.
• Your arms will move in the direction he is
pushing with a force that is equal to his
force minus your force. It will look like this.
Force can add to each other
• Imagine that your family's car breaks down
on the road and you have to push it into a
parking lot.
• If you and your brother both push on the
car, the resulting force on the car will be
the sum of your forces
• The figure below shows how this would
work.
Who was Isaac Newton?
• Isaac Newton (a 17th century scientist) put
forth three laws which explain why objects
move (or don't move).
• These three laws have become known as
Newton's three laws of motion.
Isaac Newton’s 1st Law of Motion
• An object at rest tends to stay at rest
• And an object in motion tends to stay in
motion with the same speed and in the
same direction unless acted upon by an
another force.
Pass the Water
• If students participate in a relay race carrying a
plastic container of water
• the water will have a tendency to spill from the
container at specific locations
• the container is at rest and you attempt to move it
• the container is in motion and you attempt to stop it
• the container is moving in one direction and you
attempt to change its direction.
Should you wear seatbelts?
Check Your Understanding
• 1. Imagine a place in the cosmos far from
all gravitational and frictional influences.
Suppose an astronaut in that place throws
a rock. The rock will:
• a) gradually stop.
• b) continue in motion in the same direction
at constant speed.
Check Your Understanding
• If you were in a weightless environment in
space, would it require a force to set an
object in motion?
Check Your Understanding
• Mr. Zim is being chased through the
woods by a bull moose which he was
attempting to photograph. The enormous
mass of the bull moose is extremely
intimidating. Yet, if Mr. Zim makes a zigzag
pattern through the woods, he will be able
to use the large mass of the moose to his
own advantage. Explain this in terms of
inertia and Newton's first law of motion.
Check Your Understanding
• Two bricks are resting on the edge of a lab table.
She acquires an intense desire to know which of
the two bricks is more massive. Since Shirley is
vertically challenged, she is unable to reach high
enough and lift the bricks; she can, however,
reach high enough to give each brick a push.
• Discuss how the process of pushing the bricks
will allow Shirley to determine which of the two
bricks is more massive.
What is Inertia
• Inertia is the tendency of an object to
resist changes in its state of motion.
• Inertia is the tendency of an object to
resist changes in its velocity.
What is Inertia
• An object at rest has zero velocity and (in the
absence of an unbalanced force) will remain at a
zero velocity; it will not change its state of motion
• An object in motion with a velocity of 2 m/s, East
(in the absence of an unbalanced force) will
remain in motion with a velocity of 2 m/s, East; it
will not change its state of motion (i.e., its
velocity).
Newton's 2nd Law of Motion
• F=ma
• Force need = mass x acceleration
Newton's Third Law of Motion
• According to Newton's third law, for every
action force there is an equal (in size) and
opposite (in direction) reaction force.
Forces
Newton’s 3rd Law of Motion
Blow up a balloon
Hold the opening downward and
release the balloon.
Repeat this several times, and observe
what happens.
Now describe what happened using
Newton's third law of motion.
Try these Problems
• 1.Two tugboats are moving a barge.
Tugboat A exerts a force of 3000 newtons
on the barge. Tugboat B exerts a force of
5000 newtons in the same direction. What
is the combined force on the barge?
• 2.Draw arrows showing the individual and
combined forces of the tugboats in #1.
Try these Problems
• Now suppose that Tugboat A exerts a
force of 2000 newtons on the barge and
Tugboat B exerts a force of 4000 newtons
in the opposite direction. What is the
combined force on the barge?
• Draw arrows showing the individual and
combined forces of the tugboats in #3.
Try these Problems
• 5.Could there ever be a case when
Tugboat A and Tugboat B are both
exerting a force on the barge but the barge
doesn't move? Draw arrows showing the
individual and combined forces in such a
situation.
Which law is this?