Download Newton`s First Law

Newton’s First Law
(law of inertia)
An object at rest tends to stay at
rest and an object in motion
tends to stay in motion unless
acted upon by an unbalanced
force.
What does this mean?
Basically, an object will “keep doing what it
was doing” unless acted on by an
____________ force.
If the object was sitting still, it will remain
____________. If it was moving at a
constant velocity, it will keep moving.
It takes ________to change the motion of an
object.
Balanced Force
Equal forces in opposite
directions produce no motion
Unbalanced Forces
Unequal opposing forces
produce an unbalanced force
causing motion
Newton’s First Law
(law of inertia)
• INERTIA is a property of an
object that describes how
____________________________ the
motion of the object
• more _____ means more _______
Which person in this ring will be
harder to move? The sumo wrestler
or the little boy?
Inertia
Don’t let this be you!
What common safety device is used to
counter your inertia in a crash?
Things don’t keep moving forever
because there’s almost always an
unbalanced force acting upon them.
A book sliding across a
table slows down and
stops because of the force
of friction.
If you throw a ball
upwards it will eventually
slow down and fall
because of the force of
gravity.
• Can be helpful or harmful
• There are four main types of friction:
– Sliding friction: ice skating
– Rolling friction: bowling
– Fluid friction (air or liquid): air or water
resistance
– Static friction: initial friction when moving
an object
Newton’s Second Law
Force equals mass times
acceleration.
F = ma
What does F = ma say?
F=ma basically means that the force of an
object comes from its mass and its
acceleration.
Something with a high mass
that’s changing speed very
slowly (low acceleration), like a
glacier, can still have great
force.
Something very small (low mass)
that’s changing speed very
quickly (high acceleration), like
a bullet, can still have a great
force. Something very small
changing speed very slowly will
have a very low force.
nd
2
Law (F = m x a)
How much force is needed to accelerate a
1400 kg car 2 m/s2?
• Write the formula
F=mxa
• Fill in given numbers and units
F = 1400 kg x 2 m/s2
• Solve for the unknown
2800 kg-m/s2 or
F
m a
2800 N
Using the 2nd Law of Motion
You want to
know
You know
Equation to
Use
a
(acceleration)
mass (m) and
force (F)
a = F/m
F
(Force)
acceleration (a)
and mass (m)
F = ma
m
(mass)
Acceleration
(a) and Force
(F)
m = F/a
If mass remains constant, doubling the
acceleration, doubles the force. If force remains
constant, doubling the mass, halves the
acceleration.
Check Your Understanding
1. What acceleration will result when a 12 N net
force applied to a 3 kg object? A 6 kg object?
2. A net force of 16 N causes a mass to accelerate
at a rate of 5 m/s2. Determine the mass.
3. How much force is needed to accelerate a 66 kg
skier 1 m/sec/sec?
4. What is the force on a 1000 kg elevator that is
falling freely at 9.8 m/sec/sec?
Check Your Understanding
1. What acceleration will result when a 12 N net
force applied to a 3 kg object?
12 N = 3 kg x 4 m/s/s
2. A net force of 16 N causes a mass to
accelerate at a rate of 5 m/s2. Determine the
mass.
16 N = 3.2 kg x 5 m/s/s
3. How much force is needed to accelerate a 66
kg skier 1 m/sec/sec?
66 kg-m/sec/sec or 66 N
4. What is the force on a 1000 kg elevator that is
falling freely at 9.8 m/sec/sec?
9800 kg-m/sec/sec or 9800 N
Newton’s Second
Law
•One rock weighs 5
newtons.
•The other rock weighs
0.5 newtons.
•How much more force
will be required to
accelerate the first rock
at the same rate as the
second rock?
Ten times as much
What is Gravity?
The greater the mass, the
greater the force
The greater the distance, the
less the force
Acceleration due to gravity =
9.8 m/s/s or 9.8 m/s2
Newton’s Second
Law
• WEIGHT is a measure of
the force of ________ on the
mass of an object. It is
measured in __________
• Mass is the amount of
_______ in an object.
Weight depends on mass and gravity
A 10-kilogram rock has the same mass no
matter where it is in the universe. On Earth,
the10 kg. rock weighs 98 N. On the moon, the
same rock only weighs 16 N.
Which of Newton’s Law does this
look like?
Newton’s Second Law
ACCELERATION of GRAVITY(Earth) = 9.8 m/s2
Moon’s gravity is 1/6 of the Earth’s
If you weigh 420 Newtons on earth,
what will you weigh on the Moon?
420/6 = 70 Newtons
If your mass is 41.5Kg on Earth
what is your mass on the Moon?
Free Fall
The rate of
falling
increases by
9.8 m/s every
second.
Newton’s Third
Law
For every action there is
an equal and opposite
reaction.
rd
Newton’s 3 Law
• For every force there is an equal and
opposite force.
Book to Table to
earth
book
Thought Question:
How is the force that the Earth exerts
on you related to the force you exert on
it?
A. Earth exerts a larger force on
you.
B. I exert a larger force on Earth.
C. Earth and I exert equal and
opposite forces on each other.
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 1000 kg 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!
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
(b) the force applied to the bat by
the ball
Net Force
• The sum of all of the forces acting
on an object.
Balanced forces & net forces
• An unbalanced force (non zero net force)
acting on an object changes its speed
and/or direction of motion.
• If the forces are balanced (in equilibrium),
then the net force = 0 and the object will
not accelerate and the velocity will
remain constant.
Balanced forces versus a net force
Thought Question:
Is there a net force? Y/N
1. A car coming to a stop.
2. A bus speeding up.
3. An elevator moving up at constant
speed.
4. A bicycle going around a curve.
5. A moon orbiting Jupiter.
rd
Newton’s 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.
Terminal Velocity
The velocity at which a
falling body moves
through a medium, as
air, when the force of
resistance of the
medium is equal in
magnitude and
opposite in direction to
the force of gravity.
Terminal Velocity
Why does the elephant
hit the ground before
the feather?
The feather quickly
reaches terminal
velocity and stops
accelerating.
TRUE or FALSE:
• The elephant encounters a smaller force of air
resistance than the feather and therefore falls
faster.
• The elephant has a greater acceleration of
gravity than the feather and therefore falls
faster.
• Both elephant and feather have the same force of
gravity, yet the acceleration of gravity is greatest
for the elephant.
• Both elephant and feather have the same force of
gravity, yet the feather experiences a greater air
resistance.
• Each object experiences the same amount of air resistance, yet
the elephant experiences the greatest force of gravity.
• Each object experiences the same amount of air resistance, yet
the feather experiences the greatest force of gravity.
• The feather weighs more than the elephant, and therefore will
not accelerate as rapidly as the elephant.
• Both elephant and feather weigh the same amount, yet the
greater mass of the feather leads to a smaller acceleration.
• The elephant experiences less air resistance and than the
feather and thus reaches a larger terminal velocity.
• The feather experiences more air resistance than the elephant
and thus reaches a smaller terminal velocity.
• The elephant and the feather encounter the same amount of air
resistance, yet the elephant has a greater terminal velocity.
Notes for Newton’s Laws of Motion
Newton’s First Law (law of inertia)
An object at _____________ tends to stay at _____________ and an object in _____________ tends to stay in
_____________ unless acted upon by an unbalanced force.
What does this mean?
Basically, an object will “keep doing what it was doing” unless acted on by an ____________ force.
If the object was sitting still, it will remain ____________. If it was moving at a constant velocity, it will keep
moving.
It takes ________to change the motion of an object.
Balanced Force - ____________ forces in opposite directions produce no ____________.
Unbalanced Forces - Unequal _________________ forces produce an ______________ force causing motion.
Newton’s First Law (law of inertia)
•
INERTIA is a property of an object that describes how ____________________________ the motion of
the object
•
more __________ means more __________
Which person in this ring will be harder to move? (circle) The sumo wrestler or the little boy?
What common safety device is used to counter your inertia in a crash? ______________________________
If objects in motion tend to stay in motion, why don’t moving objects keep moving forever?
•
Things don’t keep moving forever because there’s almost always an ____________________________
____________________________ acting upon them.
•
A book sliding across a table slows down and stops because of the force of _______________________.
•
If you throw a ball upwards it will eventually slow down and fall because of the force of ____________.
Friction
•
Can be helpful or ____________________________
•
There are four main types of friction:
• Sliding friction: ____________________________
• Rolling friction: ____________________________
• Fluid friction (air or liquid): ____________________________
• Static friction: ________________________________________________________________________