Transcript (Chapter 4) Newton’s Laws of Motion Fill in the blanks on your guided notes during the PowerPoint.

(Chapter 4)
Newton’s Laws of Motion
Fill in the blanks on your guided notes
during the PowerPoint
Newton’s Laws of Motion
• To quote Bill Nye: “Consider the Following”:
1. Lifting a backpack:
• When you lift a backpack, you exert a force on
it that causes it to move (pull it up)
• The backpack initially was at rest (not in
motion)
• The force you exerted caused the backpack’s
velocity to change.
Newton’s Laws of Motion
2. Pushing on a table:
• Push down on a table and the table does not
move.
• The force you applied did not cause the
velocity of the table to change.
Newton’s Laws of Motion
• Sir Isaac Newton (1642-1727) came up with
three laws to explain how motion and forces
are related:
First Law of Motion
First Law of Motion aka The Law of Inertia:
• An object at rest will remain at rest and an
object in motion will remain in motion with a
constant speed & direction (velocity) unless
acted upon by an external unbalanced force.
(Gravity and Friction are unbalanced forces)
First Law of Motion
• If an object is not pushed or pulled upon, it’s
speed & direction (velocity) will naturally
remain constant….which means:
– Once moving at a steady speed in a straight
line…it will continue to move at a steady
speed in a straight line.
– Once standing still…it will stay still.
• (Note: a motionless object is maintaining a
constant velocity of 0 m/s)
First Law of Motion
Example:
• A skateboard is at rest and doesn’t move
unless you give it a push and increases its
velocity while you push it.
• After the skateboard leaves your hand, it
slows down and stops because friction acts on
it as it rolls.
First Law of Motion
• Inertia: the tendency of an object to resist a
change in motion.
– Objects want to remain in uniform motion in a
straight line so they resist changes in speed or
direction
First Law of Motion
(Inertia Continued)
– Depends on mass: Increase Mass, Increase
Inertia
• Heavier objects are harder to move because
they have more inertia or more resistance to
change in motion.
• Pushing a heavier person on a swing is
harder than pushing a lighter person.
First Law of Motion
Example:
A bowling ball and a volleyball are rolling
towards you. You would have to exert a
greater force on the bowling ball to make it
stop. The bowling ball has more inertia than
the volleyball because it has more mass.
First Law of Motion
Example:
You are pushing a cart with a box on top and
come to a sudden stop, causing the box to
slide off the cart. The inertia of the boxes
causes them to keep moving even after the
cart stops.
First Law of Motion
Example:
A car traveling at 85 mph crashes head-on
with something solid, crumples, slows and
finally stops within 0.1s. Anyone in the car not
wearing a seatbelt continues to move forward
at the same speed the car was traveling (85
mph), slamming into the windshield,
dashboard etc.
Note: 85mph is about the speed falling from a
three-story building.
Bonus Lesson: Always wear your seatbelt!
Second Law of Motion
Second Law of Motion:
The acceleration of an object is dependent
upon the force acting upon the object and
the mass of the object.
Second Law of Motion
Newton’s Second Law can be written
as the equation:
F = ma
or
Force = mass x acceleration
F = force (N)
m= mass (kg)
a = acceleration (m/s/s or m/s2)
1N = 1kg.m/s2
Second Law of Motion
This equation demonstrates the
following two important
relationships between force, mass
and acceleration:
Second Law of Motion
1. The greater the force on an object, the greater
the acceleration of an object.
(Increase Force, Increase Acceleration)
Example:
In baseball, if I hit a ball as hard as I can, the ball
accelerates more than if I was trying to bunt the
ball.
Second Law of Motion
2. The greater the mass of an object, the smaller
the acceleration.
(Increase Mass, Decrease Acceleration)
Example:
In baseball, if I hit a 50 gram ball and a 25 gram
ball with the same force, the 50-gram ball would
go slower than the 25 gram ball because it has a
greater mass.
Second Law of Motion
Acceleration is directly proportional to the
force:
*you push twice as hard and it accelerates
twice as much.
(Increase Force, Increase Acceleration)
Second Law of Motion
Acceleration is inversely proportional to the
mass:
*If it gets twice the mass, it accelerates half
as much.
(Increase Mass, Decrease Acceleration)
Third Law of Motion
Third Law of Motion:
*For every action there is an equal
and opposite reaction*
Third Law of Motion
• When one object exerts a force on a second object, the
second object exerts the same amount of force back on the
first object but in the opposite direction.
• If a student were to jump off of a desk onto the floor, their
feet would sting. This is because the student’s feet would
exert a force on the ground but the ground exerts a force
back on the student’s feet, causing their feet to sting.
• You jump on a trampoline and exert a downward force
while the trampoline exerts an equal force upward, sending
you high in the air.
Momentum
Momentum: a property of a moving object that
equals its mass x velocity.
Formula: P = mv
m = mass (kg)
V= velocity (m/s)
P=Momentum (Kg.m/s)
Law of Conservation of Momentum
Law of Conservation of Momentum: total
momentum is conserved unless an outside
force acts on the objects.
Friction is an outside force.
Law of Conservation of Momentum
• The momentum of an object doesn’t change
unless its mass, velocity or both change.
• If a group of objects exerts forces only on each
other, their total momentum doesn’t change.
• Momentum can be transferred from one
object to another, but none is lost.
Gravity
Gravity: the attractive force between any two
objects that depends on the mass of the
objects and the distance between them.
Gravity
• The earth is close enough and has a large
enough mass that you can feel its gravitational
attraction.
• The sun has much more mass than the earth,
but it is too far away to notice a gravitational
attraction to you.
Gravity
The Law of Universal Gravitation: The
gravitational force between two masses
decreases rapidly as the distance between the
masses increases.
i.e Increase Distance, Decrease Gravitational
Attraction
Gravity
Gravitational Force Equation:
F=mg
Gravitational Force (N) =
mass (kg) x gravitational acceleration (m/s2)
g=9.8 m/s2
Gravity
Weight: the gravitational force exerted on an
object
W=mg
Weight (N) =
mass (kg) x gravitational acceleration (m/s2)
Gravity
Weight and Mass are NOT the same thing!
Weight is a force and mass is a measure of the
amount of matter in an object. You weigh less
on the moon because gravity is 1.6 m/s2.
Your mass is the same.