ch 4-6 notes
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Ch 4-6 NotesMr. Russo
Beaumont High School
Objective: Ch 4.3-4.5 (Pg 40) We will be able to define inertia and
explain Newton’s first law of motion.
Ch 4.3 – 4.5 NotesForceForce – Any push or pull
FrictionFriction – Name given to force that
acts between materials that touch as they move past each other
InertiaInertia – Every material object resists
change in its state of motion (laziness of an object)
Figure 4.3
Newton’s 1st lawNewton’s 1st Law (Law of inertia) –
Every object continues in a state of rest or of motion in a straight line at constant speed, unless it is compelled to change that state by forces acting upon it. – Newton
An object at rest stays at rest, and an object in motion stays in motion without forces acting upon them
This reminds me of youStudents doing math 2 and 3 step math
problems
DemosHanging Mass
Penny in a cup
Paper and bottle
Why do objects slow down and stop?Because of outside forces, mostly
because of friction
What would happen if you threw an object from a space station?It would go forever
Inertia ExplainedThe more mass an object has, the more
inertia it has. Less mass, less inertia
Is it easier to pick up a toy car or a real car?
Inertia is laziness
What is mass?Mass – The amount of material present
in an objectMeasure of inertia of an objectMASS DOESN’T CHANGE
What do we measure mass in?Mass is measured in kilograms
What is the difference between mass, volume and weight?
Mass Volume WeightMeasure of how much
matter present (How much inertia)
How much space it is taking up.
Force of gravity on an
object.(Depends on
location)To calculate weight we use the formulaw = m*g
g = force of gravity
For ExampleMass of boulder = 10
kgWeight of boulder = 0
kgVolume of boulder =
10 L
On your white boardIf a woman has a mass of 50 kg,
calculate her weight in Newtons
w = mgw = (50kg)(9.8m/s2)w = 490 N
On your white boardCalculate in newtons the weight of a
2000 kg elephant
w = mgw = (2000kg)(9.8m/s2)w = 19,600 N
On your white boardCalculate in newtons the weight of a 2.5
kg melon. What is the weight in pounds? (4.45 N = 1lb)
w = mgw = (2.5kg)(9.8m/s2)w = 24.5 N24.5 N / 4.45 N = 5.5 lbs
On your white boardAn apple weighs about 1 N. What is its
mass in kilograms? What is its weight in pounds? (2.2 lbs = 1 kg)
w = mg1 N = (m)(9.8m/s2)m = 1N / 9.8 m/s2 = .1 kg.1 kg = .22 lbs
On your white boardSusie finds she weighs 300 N.
Calculate her mass.
w = mg300 N = (m)(9.8m/s2)m = 30.6 kg
Objective: Ch 4.6-4.9 (Pg 44)Given 2 or more forces we will be able
to calculate the net force exerted on an object
Notes 4.6 – 4.9, Force Net Force – The
combination of all forces acting on an object.Net force
changes an objects state of motion
What happens if you pull with equal and opposite forces?Nothing! If forces are equal and opposite the net
force is zero!
What is the minimum # of forces acting on an object at rest?2Force of gravity (Down)Normal Force (Up)
Normal ForceNormal Force – Upward force on an
objectalso called the support force
EquilibriumEquilibrium – When all forces on an
object cancel out. Net force is zeroObject will not move if at rest
Draw Figure 4.11
Tension ForceTension Force – When atoms are
stretched (As opposed to being compressed)
When the angle from vertical increases, what happens to the tension force?Tension always increases as the angle
away from vertical increases
Slogan:Net Force is zero, of course – TannerDo you move – JoseNet Force Zero, no excuses – MiaEquilibrium equals net force zero – MichaelAre you in motion – TimChuck Norris & Mr Russo, Net force ZERO!
– JJNet Force is zero, unless your Chuck Norris
- Chris
What happens if you flip a coin in an airplane while its moving?It behaves as if the plane were at rest.
Why?
Inertia
Objective
Ch 5.1-5.4 - What must happen for acceleration to occur?Forces cause acceleration
Hockey Puck at restNo Acceleration
Player hits puck Acceleration
Puck moving across iceNo acceleration
Acceleration is directly proportional to what?Acceleration is directly proportional to
the Net Force acting on it
More force = more acceleration
Less force = less acceleration
Newton’s 2nd LawNewton’s 2nd Law – The acceleration produced by
a net force 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. – Newton
More Force = More acceleration (Directly Related)
More Mass = Less Acceleration (Inversely Related)
What is the formula for acceleration?Acceleration = Net force / Mass
More commonly
F = ma
a = F/m
m = F/a
What are the units for the NewtonForce is measured in NEWTONS
Force is mass x acceleration
Units are kg*(m/s2)
FrictionFriction – between two objects
touching.
Always acts in direction opposite to state of motion
FluidsFluids – Gases
or liquids ( because they
flow)
Air ResistanceAir Resistance – Friction acting on
something moving through air
How can acceleration be zero when there is still a force applied?When there is a force applied, the force
of friction will balance it out and make net force zero
PressurePressure - force per unit area
P= F/A
Terminal Speed / Terminal VelocityTerminal Speed -
Object is falling and no longer is accelerating
Terminal Velocity – Same thing, direction is down
Ch 6.1-6.6 If you push against a wall, how come it doesn’t fall over?Because the wall is pushing back on
you.
InteractionInteraction – A mutual action between
objects where each object exerts equal and opposite forces
Newton’s 3rd LawNewton’s 3rd Law – Whenever one
object exerts a force on a 2nd object, the second object exerts an equal and opposite force on the 1st object.
In other words – For every action there is an equal and opposite reaction
Action / Reaction ForceAction /Reaction Forces – Co-parts of a
single interaction. One cannot exist without the other.
No such thing as a single force
ExampleAction: Earth’s gravity pulls down a
boulder
Reaction: The boulders gravity pulls up on the Earth
ExampleAction: Rocket pushes gas
Reaction: Gas pushes rocket
What pushes a car as you drive?Action: Tire pushes against the road
Reaction: Road pushes against tire
Figure 6.7
H0w come the cannon doesn’t move just as fast backwards as the cannon ball goes forward?
A smaller mass has greater accelerationA greater mass has less acceleration
ExampleHow come the Earth doesn’t move just
as fast up as the boulder goes down?
Larger masses have lessacceleration
Figure 6.8How does a rocket
accelerate?
Action Force: Rocket pushes air molecules down
Reaction Force: Air molecules pushing rocket up
Figure 6.13 – Horse and the CartHow come if forces are
equal and opposite, the cart still moves?
The friction between the horse and the ground is greater than the cart wheels and the ground
F - f
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