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Newton's Laws
of Motion
Newton built upon the work of
others...
-1686, Sir Issac Newton publishes his book
Principia in which he describes 3 laws relating
forces to motion of objects
-did not discover all 3 laws himself, but
combined previous discoveries by other
scientists and explained them in a way that
people could understand
-as a result, the 3 laws are commonly known as
Newton's Laws of Motion
Newton's First Law
An object at rest remains at rest and an
object in motion remains in motion at a
constant speed and in a straight line
unless acted on by an unbalanced force.
-this law describes an object with a net force of zero
acting on it
EXPLAIN.
-often referred to as the Law of Inertia (an object's
resistance to any change in motion)
WHAT IS THE LINK BETWEEN MASS AND INTERTIA?
-this law is often difficult to observe because of friction
WHY?
Newton's Second Law
The accelertion of an object depends on the
mass of the object and the amount of force
applied.
-describes the motion of an object when an unbalanced
force is acting on it
EXPLAIN
-acceleration decreases as mass increases &
acceleration increases as mass decreases
DOES THIS MAKE SENSE?
-acceleration increases as force increases and
acceleration decreases as force decreases
HOW, THEN, ARE MASS AND FORCE RELATED TO
ACCELERATION?
Newton's Second Law expressed
mathematically
-Newton's second law can be expressed mathematically
as a formula that shows the relationship between mass,
force, and acceleration:
acceleration =
Force
mass
OR
a=
F
m
-it is often rearranged to look like this:
Force = mass x acceleration OR F = m x a
-either form can be used to solve problems as long as
you know 2 of the variables
EXAMPLES:
1. What is the acceleration of a 7 kg mass with a force of
68.6 N moving it toward the Earth? (1 N = 1 kg x 1m/s/s)
2. What force is needed to accelerate a 1,250 kg car at
a rate of 40 m/s/s?
3. What is the mass of an object if a force of 34 N
produces an acceleration of 4 m/s/s?
Newton's Third Law
Whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first.
HOW IS THIS LAW COMMONLY PHRASED?
-describes how all forces act in pairs
-the interaction of force pairs affects the motion of
objects
-action force is the initial force exerted on an object
-reaction force is the force exerted back from the object
IS IT POSSIBLE FOR FORCE PAIRS TO ACT ON THE
SAME OBJECT? WHY?
-effect of reaction force (especially on falling objects)
can be difficult to see
USE NEWTON'S 2ND LAW TO EXPLAIN WHY THIS IS SO.
Examples of force pairs
The bat exerts a force on the ball, sending it
toward the outfield. The ball exerts an
equal force on the bat in the opposite
direction, but the bat doesn't fly backwards
because the batter is exerting another
force on it.
The air inside the balloon exerts an
outward force, while the air outside the
balloon exerts and equal force inward.
The bowling ball exerts a force on the pin,
pushing it to the left. At the same time, the pin
exerts an equal force on the ball, pushing it to
the right.
CAN YOU THINK OF SOME OTHER FORCE PAIRS?
WHY IS IT THAT THE PIN FALLS DOWN, BUT THE BOWLING BALL
CONTINUES TO ROLL? THE BALL HAS MOMENTUM.
Momentum
-momentum is a property of a moving object that
depends on the object's mass and velocity
-an object with a large mass or high velocity, or both has
greater momentum than an object with a small mass or
low velocity or both
-the more momentum an object has, the harder it is to
stop it, or make it change direction
-momentum can be calculated using the following
formula:
momentum(p) = mass(m) x velocity(v)
EX:
1. What is the momentum of a 15,000 kg semi traveling
at a constant velocity of 30 m/s?
2. What is the momentum of a 1,300 kg car traveling at
that same velocity?
WHAT DO YOU THINK
WILL HAPPEN IF
THESE 2 VEHICLES
WERE TO COLLIDE?
(YOUR ANSWER
SHOULD BE
SCIENTIFIC!)
Momentum is Conserved
-when a moving object hits another object, momentum
is transferred to the other object
-it may be all or only some of the momentum that is
transferred
-if it is only some, the rest of the momentum stays with
the first object
-this is why the bowling ball continues to roll after
hitting the pin and is called The law of conservation of
momentum
-any time two or more objects interact, they may
exchange momentum, but the total amount of
momentum stays the same
NAME SOME MORE EXAMPLES OF THIS LAW.
HOW DOES NEWTON'S THIRD LAW HELP TO EXPLAIN
CONSERVATION OF MOMENTUM.