Transcript Newton`s Laws of Motion

Newton’s
Laws of
Motion
While most people know
what Newton's laws say,
many people do not know
what they mean (or simply do
not believe what they mean).
Newton’s Laws of Motion

1st Law – Law of Inertia: An object at rest
will stay at rest, and an object in motion
will stay in motion at constant velocity,
unless acted upon by an unbalanced force.
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2nd Law – Law of Acceleration, or F=ma
3rd Law – Law of Interaction: For every
action there is an equal and opposite
reaction.
1st Law of Motion
(Law of Inertia)
An object at rest will stay at
rest, and an object in motion
will stay in motion at
constant velocity, unless acted
upon by an unbalanced force.
1st Law

Inertia is the
tendency of an
object to resist
changes in its
velocity:
whether in
motion or
motionless.
These pumpkins will not move unless acted on
by an unbalanced force.
1st Law

Once airborne,
unless acted on
by an
unbalanced force
(gravity and air
– fluid friction),
it would never
stop!
1st Law

Unless acted
upon by an
unbalanced
force, this golf
ball would sit on
the tee forever.
Check for Understanding
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If you hold a coin above your head while in a bus that is
not moving, the coin will land at your feet when you drop
it. Where will it land if the bus is moving in a straight line
at constant speed?
It will fall in the same spot as if the bus wasn't moving.
This is because once the bus is moving at a constant
velocity, the coin is also moving at the velocity, and so are
you. So in reality, when you dropped the coin, it moved
forward in addition to falling down. However since you are
moving the same speed as the coin, it appears to fall
straight down. Motion is relative.
Check for Understanding

In the cabin of a jetliner that cruises at 600
km/hr, a pillow drops from an overhead
compartment into your lap below. Since the
jetliner is going so fast, why doesn’t the pillow
slam into the rear of the plane when it falls
out? What is the speed of the pillow relative to
the jetliner? To the ground?
Net Force
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The sum of all forces on an object. The net
force changes an object’s state of motion.
Ex: pushing a book across a table.
 Gravity, air friction, & your pushing
muscles = the net force
Newton (N) is the scientific unit of
force/weight
Net Force
Equilibrium for Objects at Rest
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(Demo spring scale & weight)
Tension – the state of being stretched (the
spring scale experiences a “stretching” force)
Tension force acts upwards & weight acts
downwards. 2 forces equal & opposite = 0, so
it’s resting
Equilibrium Rule
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When the net force on an object is 0, its state
of motion doesn’t change, & the object is in
mechanical equilibrium
Σ = vector (direction)
SUM OF
F = forces
Forces acting upwards (+)
balance w/forces acting downwards (-)
(+) and (-) = 0
Review (Speed & Equilib)
Rosa’s sports car takes 60.0 seconds to cover 1200 meters.
What’s the average speed in meters per second?
2)
Marisela walks at an average speed of 0.5 m/s from the Bay
Farm bridge to the Hornet (aircraft carrier). The trip takes her
1300 seconds. How far did she walk in meters?
3)
A snail travels at an average speed of 0.005 m/s across a 3.0
meter walkway. How long does the snail take to cross the
walkway?
4)
Mario weighs 400N & stands in the middle of a board that
weighs 100N that is hanging from 2 scales (one 1 on each side).
Find the upward force of each scale.
5)
Mario moves to the left, and the reading on the left scale (the
one closest to him) is 350. What is the reading on the scale on
the right?
(DRAW DIAGRAM)
1)
Review - Equilibrium
1)
2)
Mario weighs 400N & stands in the middle of
a board that weighs 100N that is hanging from
2 scales (1 on each side). Find the upward
force of each scale.
Mario moves to the left, and the reading on the
left scale (the one closest to him) is 350. What
is the reading on the scale on the right?
DRAW A PICTURE!
???
Why then, do we observe every day
objects in motion slowing down and
becoming motionless seemingly
without an outside force?
It’s a force we sometimes cannot see
– FRICTION.
Objects on earth, unlike the
frictionless space the moon
travels through, are under the
influence of friction.
What is this unbalanced force that acts on an object in
motion?
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A force that arises when an object rubs against
something else
There are four main types of friction:
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Sliding friction: ice skating
Rolling friction: bowling
Fluid friction (air or liquid): air or water resistance
Static friction: initial friction when moving an object
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 that force being the
force of friction which brings the book
to a rest position.

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.)
Newtons’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.
Support Force
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The force that supports an object against gravity
Ex: book lying on table
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Support force is the upward force from the table
Support force is equal to weight of book
Support force + gravity = 0
Ex: bathroom scale (2 forces)
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Downward pull of gravity (your weight)
upward support of floor
Scale shows support force
Equilibrium for Moving Objects
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A moving object is in equilibrium if it moves
steadily, without changing its state of motion
A rolling ball is in equilibrium if it moves at
constant velocity
ΣF=0
2nd Law
The acceleration produced by a
net force acting on an object is
directly proportional to the
magnitude of the net force, is in the
same direction as the net force, and
is inversely proportional to the mass
of the object.
2nd Law
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When mass is in kilograms and acceleration is
in m/s/s (m/s2), the unit of force is in newtons
(N).
One Newton is equal to the force required to
accelerate one kilogram of mass at one
meter/second/second.
2nd Law (F = m x a)
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How much force is needed to accelerate a 1400
kilogram car 2 meters per second/per second?
Write the formula
F=mxa
Fill in given numbers and units
F = 1400 kg x 2 meters per second/second
Solve for the unknown
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2800 kg-meters/second/second or 2800
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N
If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration.
Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd Law we know
that they don’t hit the
ground with the same
force.
F = ma
F = ma
98 N = 10 kg x 9.8 m/s/s
9.8 N = 1 kg x 9.8 m/s/s
Check Your Understanding
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1. What acceleration will result when a 24 N net force
applied to a 4 kg object? A 8 kg object?
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2. A net force of 36 N causes a mass to accelerate at a rate
of 4 m/s2. Determine the mass.
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3. What is the force on a 1000 kg elevator that is falling freely at 9.8
m/sec/sec?
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4. A frictional force of 10 N acts against a forward force of 25 N. If the
forward force is applied to a 12-kg object, what is the acceleration of the
object?
Check Your Understanding
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1. What acceleration will result when a 24 N net force applied to a 4 kg object?
8 kg?
24 N = 4 kg x 6 m/s2
24 N = 8 kg x 3 m/s2
2. A net force of 36 N causes a mass to accelerate at a rate of 4 m/s2. Determine
the mass.
36 N = 9 kg x 4 m/s2
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3. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s2?
9800 kg-m/s2 or 9800 N
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4. A frictional force of 10 N acts against a forward force of 25 N. If the forward
force is applied to a 12-kg object, what is the acceleration of the object?
1.25 m/s2
Mass vs Weight
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MASS IS NOT WEIGHT!
Mass – the amount of matter in an object.
Weight – the force due to gravity that acts on
an object’s mass
The 2 are directly proportional: object with
large mass has large weight, object with little
mass has little weight.
3rd Law

For every action, there is an
equal and opposite reaction.
3rd Law
According to Newton,
whenever objects A and
B interact with each
other, they exert forces
upon each other. When
you sit in your chair,
your body exerts a
downward force on the
chair and the chair
exerts an upward force
on your body.
3rd Law
There are two forces
resulting from this
interaction - a force on
the chair and a force on
your body. These two
forces are called action
and reaction forces.
Newton’s 3rd Law in Nature
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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).
3rd Law
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 flying motion of birds. A bird flies by
use of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing the bird
upwards.
The size of the force on the air equals the size of the
force on the bird; the direction of the force on the air
(downwards) is opposite the direction of the force on
the bird (upwards).
Action-reaction force pairs make it possible for birds
to fly.
Other examples of Newton’s
Third Law

The baseball forces the
bat to the left (an
action); the bat forces
the ball to the right (the
reaction).
3rd Law

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.
3rd Law
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.
Check for Understanding
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A speeding bus makes contact with a bug that
splatters onto the windshield.
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Compare the force of the bug on the windshield to
the force of the windshield on the bug.
Compare the acceleration of the bug and the
acceleration of the bus.
Check for Understanding

How does a rocket in outer space move when
there is no air to “push” against?