Transcript Newton*s Laws of Motion


A scientific law is something that will
happen in the physical world each and
every time that conditions allow for it.
◦ Gravity
◦ Conservation of momentum
◦ Newton’s laws
An object in motion or at
rest will stay in constant
motion or at rest forever and
ever unless an outside force
acts on it.
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The moon will keep
revolving around the
earth forever, unless
acted on by an
unbalanced force.
Why then, do we observe
every day objects in motion
slowing down and
becoming motionless
seemingly without an
outside force?
• Moon in orbit around
earth
 Once
airborne,
unless acted
on by an
unbalanced
force, it
would never
stop!
 Unless
acted
upon by an
unbalanced
force, this
golf ball
would sit on
the tee
forever.
What is this unbalanced force that acts on an object in motion?

There are four main types of friction:
◦ Sliding friction: friction between a sliding
object and the surface it is sliding on.
◦ Rolling friction: friction between a rolling
object and the surface it is rolling on.
◦ Static friction: friction that keeps a
stationary object in place
◦ Fluid friction (air or liquid): air or water
resistance
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Sliding
◦ +
◦ -
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Rolling
◦ +
◦ -
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Static
◦ +
◦ -
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Fluid
◦ Air
 +
 -
◦ Water
 +
 -
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Slide a book across a table
and watch it slide to a rest
position. The book comes to
a rest because of the
presence of a force.
What force?
Sliding Friction
In the absence of a force
of friction, the book would
continue in motion with
the same speed and
direction - forever! (Or at
least to the end of the
table top.)
Don’t let this be you. Wear seat belts.
The Law of Inertia states that an object in motion stays in
motion until acted on by an outside force. Let’s make that
outside force the seatbelt and not the windshield or
pavement!
 Newton’s
1st Law in Action.
 An person will remain in
motion until acted upon by
an outside force!
 Can you identify the outside
forces?
An object will accelerate in
the direction of the greatest
force acting on it.
 The total force of an object
is equal to its mass times
its acceleration.


F=mxa
◦ Mass in kilograms
◦ Acceleration in m/s2
◦ Force is kg x m/s2 or Newtons

One Newton is equal to the force required to
accelerate one kilogram of mass at one m/s2 .

How much force is needed to accelerate a
1400 Kilogram car 2 m/s2 ?
Write the formula
F=mxa
Fill in given numbers and units
F = 1400 kg x 2 m/s2
Solve for the unknown

2800 kg X m/s2 or
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2800 N
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1. How much force is needed to accelerate a 66-kg skier 1 m/s2 ?

2. What is the force on a 1000-kg elevator that is falling freely at 9.8 m/s 2 ?
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3. What acceleration will result when a 12 N net force applied to a 3-kg
object?
4. A net force of 16 N causes a poodle to accelerate at a rate of 5 m/s2.
Determine the mass of the poodle.

1. How much force is needed to accelerate a 66 kg skier 1 m/s2 ?
66 N = 66 kg x 1 m/s2
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2. What is the force on a 1000 kg elevator that is falling freely at 9.8
m/s2 ?
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9800 N = 1000 kg x 9.8 m/s2
3. What acceleration will result when a 12 N net force applied to a 3 kg
object?
12 N = 3 kg X 4 m/s2
4. A net force of 16 N causes a poodle to accelerate at a rate of 5 m/s2.
Determine the poodle’s mass.
16 N = 3.2 kg x 5 m/s2
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The fastest speed/velocity a falling object can
go.
This occurs when downward gravity is
balanced by upward air resistance.
If enough time exists, all falling objects
eventually reach a constant speed.
This is the only time on earth that a moving
object has balanced forces acting on it.
Questions:
Gravity = 9.8m/s2
Terminal Velocity of
Feather
What is the only
difference between the
two objects?
Which object will
accelerate for a
longer period of
time?
Air Resistance
Terminal Velocity of Ball
Which object will have
the greatest terminal
velocity? Why?
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Occurs when objects move in a
circular/orbital path.
The force that keeps these objects moving in
that path.
These objects always have an inertia that is
perpendicular to the orbit.
Example: The Moon and Earth.
◦ What would happen if we had a switch to flip and
turn off Earth’s gravity?

How much force does a 2000 kg meteor hit the
earth with?

What is the acceleration of a 55 kg swimmer
jumping off a cliff at Raystown?
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On Earth, when dropped, why does a bowling ball
hit the ground before a feather?
On the moon, when dropped, which one will hit
the lunar surface first and why?
http://www.youtube.com/watch?v=5C5_dOEyAfk
 For
every action, there is an
equal force and opposite
direction reaction.
 Draw a diagram for each of
the following slides.
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The action force is the
arrow pushing
backward on the
string.
The reaction force the
string pushing
forward on the arrow.
The mass of the bow
+ Indian and the
acceleration of the
string is transferred to
the mass of the arrow
giving it acceleration.
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Projectile Motion
The action force is the
mass of the bullet
accelerating forward.
The reaction force is the
mass of the gun
accelerating backward.
Therefore, guns kick!
The bigger and faster
the bullet, the more
kick.
The lighter the gun, the
more it kicks.
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Pitching a
baseball/softball.
The action force is
the pitcher pushing
backward on the
mound.
The reaction is the
mound pushing the
pitcher toward
home plate!

The baseball forces
the bat to the left
(an action); the bat
forces the ball to
the right (the
reaction).
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Consider the propulsion
of a fish through the
water. A fish uses its fins
to push water backwards.
In turn, the water reacts
by pushing the fish
forwards, propelling the
fish through the water.
The size of the force on
the water equals the size
of the force on the fish;
the direction of the force
on the water (backwards)
is opposite the direction
of the force on the fish
(forwards).
Flying gracefully
through the air, birds
depend on Newton’s
third law of motion. As
the birds push down on
the air with their wings,
the air pushes their
wings up and gives
them lift.
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Consider the motion
of a car on the way to
school. A car is
equipped with wheels
which spin
backwards. As the
wheels spin
backwards, they grip
the road and push
the road backwards.
The reaction of a rocket is
an application of the third
law of motion. Various
fuels are burned in the
engine, producing hot
gases.
The hot gases push against
the inside tube of the rocket
and escape out the bottom
of the tube. As the gases
move downward, the rocket
moves in the opposite
direction.