Transcript Newton*s 1st Law of Motion

Newton’s
Laws of
Motion
Newton’s
st
1 Law of Motion
(law of inertia)
Newton’s 1st Law of Motion
An object in motion
tends to stay in
motion and an object
at rest tends to stay at
rest, unless the object
is acted upon by an
outside force.
The Law of Inertia-due
to an object’s mass
We feel the effects of Newton's First Law
every day, but usually don't notice
them because other forces interfere. In
space, the First Law is much more
obvious. Objects will follow their
natural path until they are stopped by
an outside force. On Earth, the
atmosphere will eventually slow down
all moving objects, but in a vacuum
(basically an empty space with no air or
atmosphere), like space, it will be more
obvious that objects obey Newton's
Laws.
Some Examples
• The person is standing on
the floor.
• The only forces acting on
the person are the force
due to gravity pulling
down & the normal force
pushing up. The net
force is zero and the
person remains still.
Another Example
One of the most common places people
feel the First Law is in a fast moving
vehicle, such as a car or a bus, that comes
to a stop. An outside force stops the
vehicle, but the passengers, who have been
moving at a high speed, are not stopped
and continue to move at the same speed.
Balanced vs. Unbalanced Forces
Some Problems
An astronaut in
outer space
away from
gravitational or
frictional forces
throws a rock.
The rock will…..
And now the answer
…continue to move in a
straight line at a constant
speed.
The rock’s tendency to do
this is called INERTIA.
A rock is being
whirled at the end
of a string in a
clockwise
direction. If the
string breaks, the
path of the rock is
It will follow an “inertial path”
so it will follow path “C”.
Once the rock leaves the
string, there are no
unbalanced forces to affect
its motion.
A Weighty Problem
I spend most Sunday
afternoons at rest on
the sofa, watching
football and
consuming large
quantities of food.
What effect (if any)
does this practice
have upon my
inertia? Explain.
My inertia will most
definitely increase.
My mass will increase
because of this
practice and if mass
increases, then inertia
increases.
How much force?
0N
An 2-kg object is
moving horizontally
with a speed of 4 m/s. An object in motion will
How much net force
maintain its state of
is required to keep
motion. The presence
the object moving
of an unbalanced force
with the same speed
will change the
and in the same
velocity of an object.
direction?
Is a force required?
If you were in a
weightless
environment in
space, would it
require a force to
set an object in
motion?
Yes, because even in outer
space, an object has mass.
If an object has mass then
the object is going to
resist changes in its
motion. A force must be
applied to set the object
in motion. Newton’s
Laws rule—Everywhere!
A Final Problem
Why isn't the
girl hurt when
the nail is
driven into the
block of wood?
And the answer is….
• Due to the large mass of the
books, the force of the hammer is
sufficiently resisted (inertia).
• This is demonstrated by the fact
that the blow of the hammer is
not felt by the girl.
Newton’s
Second
Law of
Motion
NEWTON'S 2nd LAW OF MOTION
F
a
m
F
M
a
m
F
F
a
m
M
a
m
m
F
m
1
a
m
F
a
 
aF
or
M
a


F  ma
Newtons’ Second Law
• F=ma
• The acceleration of an
object is directly
proportional to the net
force acting on the
object…
• …and inversely
proportional to the mass of
the object.
Example Questions
• How much acceleration does a 747 jumbo
jet of mass 30,000kg experience in takeoff
when the thrust of all of the engines is
120,000N?
• F= ma  a = F/m
• a = 120000 N/30000 kg
•a= 4
2
m/s
Example Questions
F
F
• The same net force is applied to two
blocks.
• If the blue one has a smaller mass than
the yellow one, which one will have the
LARGER acceleration?
If the NET FORCE is parallel to the velocity,
then the speed of the object increases.
If the NET FORCE is anti-parallel (or
opposite) to the velocity, then the speed of the
object decreases.
If the net force is
perpendicular to the velocity,
the direction of the velocity
changes.
• Force and acceleration are vector
quantities.
• If v is parallel to F, speed
increases.
• If v is antiparallel to F, speed
decreases.
• If v perpendicular to F, direction
of v changes.
When Acceleration Is Zero...
• …we say the object is in Mechanical
Equilibrium.
• …the net force is zero.
• For Static Equilibrium the velocity is
zero.
• For Dynamic Equilibrium the velocity is
constant.
When Acceleration Is Zero - Equilibrium
Scales pushing up
Static Equilibrium
Velocity is zero
Examples:
Computer setting on a table
Normal up
Weight down
Weighing yourself on a set of scales
Hanging from a tree
Car parked on an incline
Friction
Tree
pulling up
Weight down
Normal
Weight down
Weight down
Dynamic Equilibrium
Velocity is nonzero and constant
Examples:
Driving at constant velocity
Normal up
Air resistance
Force from road
Friction
Weight down
Terminal velocity in parachuting
Weight down
Newton’s 3rd Law
• For every action there is an equal and opposite
reaction.
Book to
earth
Table to
book
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall? You slide in the opposite direction
(away from the wall), because you pushed on
the wall but the wall pushed back on you with
equal and opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on a
rock, the rock exerts an equal force back on
your toe. The harder you hit your toe against
it, the more force the rock exerts back on your
toe (and the more your toe hurts).
Newton’s Third Law
• A bug with a mass of
5 grams flies into the
windshield of a
moving 1000kg bus.
• Which will have the
most force?
• The bug on the bus
• The bus on the bug
Newton’s Third Law
• The force would be
the same.
• Force (bug)= m x A
• Force (bus)= M x a
Think I look bad?
You should see
the other guy!
Action and Reaction on Different Masses
Consider you and the earth
Action: earth pulls on you
Reaction: you pull on earth
Reaction: road pushes on tire
Action: tire pushes on road
Reaction: gases push on rocket
Action: rocket pushes on gases
Consider hitting a baseball with a bat. If we
call the force applied to the ball by the bat the
action force, identify the reaction force.
(a) the force applied to the bat by the hands
(b) the force applied to the bat by the ball
(c) the force the ball carries with it in flight
(d) the centrifugal force in the swing
Newton’s
rd
3
Law
• Suppose you are taking a space
walk near the space shuttle, and
your safety line breaks. How
would you get back to the shuttle?
Newton’s
rd
3
Law
• The thing to do would be to take one of the tools
from your tool belt and throw it is hard as you can
directly away from the shuttle. Then, with the help
of Newton's second and third laws, you will
accelerate back towards the shuttle. As you throw
the tool, you push against it, causing it to
accelerate. At the same time, by Newton's third
law, the tool is pushing back against you in the
opposite direction, which causes you to accelerate
back towards the shuttle, as desired.
What Laws are represented?
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.
1stlaw: Homer is large and
has much mass, therefore he
has much inertia. Friction
and gravity oppose his
motion.
2nd law: Homer’s mass x
9.8 m/s/s equals his
weight, which is a force.
3rd law: Homer pushes
against the ground and it
pushes back.