Transcript Newton’s Laws of Motion

Newton’s
Laws of
Motion
1st - Inertia.
2nd - F = ma
3rd - Action/Reaction
Take notes when see
Newton’s First Law
An object at rest tends to stay at rest
and an object in motion tends to stay
in motion unless acted upon by an
unbalanced force.
What does this mean?
Basically, an object will “keep doing what it
was doing” unless acted on by an
unbalanced force.
If the object was sitting still, it will remain
stationary. If it was moving at a constant
velocity, it will keep moving.
It takes force to change the motion of an
object.
Some Examples from Real Life
A soccer ball is sitting at rest. It
takes an unbalanced force of a kick
to change its motion.
Two teams are playing tug of war. They are both
exerting equal force on the rope in opposite
directions. This balanced force results in no
change of motion.
Newton’s First Law is also called
the Law of Inertia
Inertia: the tendency of an object to resist
changes in its state of motion
The First Law states that all objects have
inertia. The more mass an object has, the
more inertia it has (and the harder it is to
change its motion).
So, which has more inertia? A bowling ball or
a baseball?
Newton’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you)
resist changes in their motion. When the car
going 80 km/hour is stopped by the brick
wall, your body keeps moving at 80 m/hour.
If objects in motion tend to stay in motion,
why don’t moving objects keep moving
forever?
There are two forces that can slow motion:
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.
Newton’s Second Law
Force equals mass times acceleration.
F = ma
What does F = ma say?
F = ma basically means that the force of an object
comes from its mass and its acceleration.
Something very massive (high mass)
that’s changing speed very slowly (low
acceleration), like a glacier, can still
have great force.
Something very small (low mass) that’s
changing speed very quickly (high
acceleration), like a bullet, can still
have a great force. Something very
small changing speed very slowly will
have a very weak force.
2nd Law (F = m x a)
How much force is needed to accelerate a
1400 kg car 2 m/s2?
F
m a
• Write the formula
F=mxa
• Fill in given numbers and units
F = 1400 kg x 2 m/s2
• Solve for the unknown
2800 kg-m/s2 or 2800
N
What are the missing answers?
If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration.
Where you correct?
4
2
10
If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration.
Newton’s Third Law
For every action there is an equal and
opposite reaction.
What does this mean?
For every force acting on an object, there is an equal
force acting in the opposite direction.
Right now, gravity is pulling you down in your seat,
but Newton’s Third Law says your seat is pushing
up against you with equal force.
This is why you are not moving. There is a balanced
force acting on you– gravity pulling down, your seat
pushing up.
Action and Reaction forces act on
DIFFERENT OBJECTS!
Action: Your foot pushes on skateboard
Reaction: The skateboard pushes on
your foot.
Remember:
•There are always 2 forces
•They are always equal
•They act on different objects
Review
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third Law:
For every action there is an equal and
opposite reaction.
Other Motion Concepts
Gravity
Freefall
Weight
Momentum
What is Gravity?
GRAVITY - attraction force between all masses
Newton’s universal law of gravitation: Every
object in the universe exerts a gravitational
attraction to all other objects in the universe
Gravity depends on the mass of the objects
and the distance between the objects
The greater the mass, the greater the
force
The greater the distance, the less the
force
Acceleration due to gravity = 9.8 m/s/s or
9.8 m/s2
Free Fall
The rate of falling
increases by 9.8 m/s
every second.
A ball thrown horizontally
will fall at the same rate
as a ball dropped directly.
Air Resistance Effects Free Fall
• In air…
– A stone falls faster
than a feather
• Air resistance affects
stone less
• In a vacuum
– A stone and a
feather will fall at the
same speed.
Terminal Velocity – fastest speed
possible until air resistance slows you
down.
Free Fall
– A person in free fall
reaches a terminal
velocity of around
54 m/s
– With a parachute,
terminal velocity is
only 6.3 m/s
• Allows a safe landing
Weight vs. mass
• Weight and mass are not the same.
• Mass is a fundamental property of matter
measured in kilograms (kg).
• Weight is a force measured in newtons
(N).
• Weight depends on mass and gravity.
Weight depends on mass and gravity
A 10-kilogram rock has the same mass no matter where it
is in the universe. On Earth, the10 kg. rock weighs 98 N..
On the moon, the same rock only weighs 16 N.
Which of Newton’s Law does this look like?
Weight is a measure of the gravitational
force between two objects
The greater the mass the greater the
force (weight)
Measured in units called Newtons (N)
Weightlessness – free from the effects of
gravity
Momentum: The quantity of motion




A property of moving objects
Calculated by: P = mv
(p = momentum, m = mass, v = velocity)
Law of conservation of momentum: the total
amount of momentum of a group of objects
does not change unless outside forces act on
the objects