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