Download Newton`s Three Laws of Motion

Newton's Laws of
Motion
Isaac Newton
(17th century scientist)
Sir Isaac Newton
• Lived from 1642-1727 in England.
• He was a dedicated physicist and
mathematician, and is considered to be
one of the most brilliant scientists of all
time.
• He is most famous for his three laws of
motion and his universal law of
gravitation, but did much more.
Mass & Weight
• Mass is the amount of “stuff” or matter that
an object contains.
• Weight and Mass are different.
• Weight, or how much an object weighs,
depends on the amount of gravity acting
upon it.
– Ex. You weigh less on the moon then on Earth.
Weight on other worlds
Force
• A force is what we call a push or a pull,
or any action that has the ability to
change motion of an object.
• The metric unit used to describe force is
called the Newton (N).
One Newton is equal to:
1 Kg x 1 m/s/s
Thus, one Newton of force
causes a one kilogram object to
accelerate at a rate of one meter
per second squared.
Your weight in Newtons!!!
Balanced and Unbalanced Forces
• The motion of all objects depends on the
total of all forces acting on the object.
• We call the total of all forces the net
force.
• Reminder – add forces acting in same
direction, subtract, when in opposite
direction
Balanced and Unbalanced Forces
Newton’s First Law of Motion
• An object at rest will remain at rest, and an
object in motion will remain in motion
unless acted upon by an unbalanced force.
• Unbalanced forces cause acceleration.
– When you roll a ball, why doesn’t it roll on
forever? FRICTION
– This concept is also
referred to as the
Law of Inertia
The Law of Inertia
Inertia
• Inertia is defined as the property of an
object to resist changing its state of motion.
• The amount of inertia that an object has
depends on the amount of mass that it has.
• The more massive the object is the harder it
is to move or stop.
Which is easier to push? Which is easier to stop?
Solving Problems
A car drives along the highway at
constant velocity. Find the car’s
weight and the friction force if the
engine produces a force of 2,000
newtons between the tires and the
road and the normal force on the car
is 12,000 N.
Concept Check:
Ben Tooclose is being chased
through the woods by a massive bull
moose If Ben 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.
Concept Check:
Mr. Wegley spends most
Sunday afternoons at rest on
the sofa consuming large
quantities of food. What
effect (if any) does this
practice have upon his
inertia? Explain.
Newton’s Second Law of Motion
• Newton’s second law relates the
applied force on an object, the mass of
an object and the acceleration.
• It states: F = M x A
• Another form of this equation says:
• A= ___
F
force causes acceleration
M
mass resists acceleration
Newton’s second law
•
There are three main ideas related to Newton’s
Second Law:
1. Acceleration is the result of unbalanced forces.
2. A larger force makes a proportionally larger
acceleration.
3. Acceleration is inversely proportional to mass.
1
3
2
4
Concept Check:
Why does a tractor trailer have a
bigger engine than a car?
Why would a truck driver want to
leave more space between the car
in front of it than a normal car?
Newton’s Third Law of Motion
• For every action there is an
equal and opposite reaction.
• If you push on something it
pushes back with the exact
same force.
• Forces come in
pairs
Newton’s Third Law of Motion
• Sometimes the reaction force is
noticeable and causes motion
– (ex – recoil from a gun, pushing off
while on ice)
• Sometimes it is more
subtle, but forces
ALWAYS act in pairs.
– (ex - sitting in a chair,
walking)
Concept Check:
Explain the net force in
each graph
Videos
• Football and Hockey
• Newtons Three Laws
6.3 Collisions
•
Newton’s third law tells us that any time two
objects hit each other, they exert equal and
opposite forces on each other.
•
The effect of the force is not always the same.
6.3 Momentum
• Momentum is the mass of an object times
its velocity.
• The units for momentum are
kilogram-meter per second (kg·m/s).
6.3 Momentum
• The law of
conservation of
momentum states
that as long as the
interacting objects
are not influenced by
outside forces (like
friction) the total
amount of
momentum is
constant or does not
change.
Pbefore =Pafter
m1v1 = m2v2
6.3 Momentum
We use positive and
negative numbers to show
opposite directions.
• The result of a
skateboarder throwing a
1-kg ball at a speed of 20 m/sec is that he and
the skateboard with a
total mass of 40 kg
move backward at a
speed of +0.5 m/sec (if
you ignore friction).
6.3 Collisions
• When a large truck
hits a small car, the
forces are equal.
• The small car
experiences a much
greater change in
velocity much more
rapidly than the big
truck.
Which vehicle ends up
with more damage?
Solving Problems
• If an astronaut in space
were to release a 2kilogram wrench at a
speed of 10 m/s, the
astronaut would move
backward at what
speed?
• The astronaut’s mass is
100 kilograms.
Solving Problems
1. Looking for:
–
… the velocity of the astronaut (backward)
2. Given
–
…velocity1 = 10 m/s; mass1= 2 kg;
–
...mass2 = 100 kg;
3. Relationships:
–
m1v1 = m2v2
4. Solution
–
Draw a free body diagram.