Unit 3 – Newton’s Laws of Motion!

Hand back testLecture on History of Motion

Introduction to Newton’s 1st & 2nd Law

Today you should learn…

What is mass? What is inertia? What did Aristotle think the natural

state of objects was? What is Newton’s 1st Law? If you were in outer space and you

threw a ball 10 m/s when would it stop?

A Brief History of Motion:

Aristotle’s Thoughts

It’s natural for objects to stop moving.

Aristotle continues to think…

A force is needed to move an object (which is against

nature) Ipso Facto: if the

force stops, the motion stops.

Aristotle, Aristotle, Aristotle…

Some natural motion exists (up and down), Violent motion are caused by an external

force.

Belief held for 2000 years. What do you

think?

Galileo’s Thoughts:

Every object resists a change to its present state of motion.

Inertia

An object’s tendency to keep moving… Thanks to Galileo!

Then came Newton…

Restates Galileo’s idea Things keep on doing what

they are already doing.

If at rest – resists moving

If in motion – resists changing that motion!

Newton’s First Law – Law of Inertia

Objects that are at rest stay at rest until a force acts on it.

An object in motion will stay in motion, until a force acts on it.

Back to Inertia Resistance to

change in velocity is caused by mass.

Big mass – takes a lot to change its velocity!

Different inertias The larger mass

… Larger resistance

to change in velocity

A ball is rolling on a counter…

And slowly comes to a stop. What would Aristotle have said

was the cause? What would Galileo and Newton

have said? And you? What do you say?

And force is …

A push Or a pull

In the SI unit of measurements A Newton (N)

“Net” Force

Takes into account ALL the forces acting on an object!

If you have a 6 N force pushing and a 4 N force pulling The net force is a 2 N force

pushing

Mass versus volume

Mass = amount of matter Volume = how much space

the object takes up.

Mass versus Weight

Mass = amount of matter. Weight = force of gravity on

an object.

SI Units

Mass = kilogramWeight = Newton

So, what is a pound? Mass or weight?

Calculating weight

Weight = mass X acceleration of gravity

= mass x (9.8 m/s2) 1 kg = 9.8 N

Check your understanding…

What is mass? What is inertia? What did Aristotle think the natural

state of objects was? What is Newton’s 1st Law? If you were in outer space and you

threw a ball 10 m/s when would it stop?

Newton’s Second Law:

A net force acting on an object will cause the object to accelerate in the direction of the net force.

Force = mass X acceleration

Free Body Diagrams: The size of the

arrow in a free-body diagram reflects the size of the force.

The direction of the arrow shows the direction the force is acting.

Forces

Weight Tension Friction Normal Springs (later)

October 26

Newton’s 2nd Law Lab II Finish with carts and graph.

Inertia What is Inertia? Resistance to

change in current state of motion (or lack of motion).

Inertia determined by…

Amount of mass.

What is Newton’s 1st Law?

An object in motion will remain in motion until an outside force acts on it. An object at rest will remain at rest until an outside object acts on it.

Forces

Weight: W = mg W = mass x 9.8 m/s2

Friction Tension Normal

Newton’s 2nd Law A force acting

on an object will cause the object to accelerate in the direction of the force.

F = ma

Force – Newtons Mass – kilograms Acceleration – m/s2

Force = mass x acceleration

Mass = Force / Acceleration

Acceleration = force / mass

A 95 kg person starts running and accelerates at a rate of 6.5 m/s2. How much force is he applying to get moving?

An engineer walks into a bar…

And a one, and a two…

I push a curling stone (mass = 35 kg) with a force of 300 N across a frictionless ice rink. What is the acceleration of the ball?

Why did the chicken fall out of the sky?

What is the mass of chicken that hits the floor while accelerating at 9.8 m/s2, if the force is measured to be 540 N?

A tree starts to walk out of a bar

“leafing so soon?” A van strikes a tree with a force of

10,500 N. It has a mass of 2100 kg. What was its acceleration?

Force is directly proportional to acceleration.

As Force gets bigger

Acceleration gets bigger

Assuming mass stays the same…

Mass is inversely proportional to acceleration

For today’s lab

Our force comes from weight W = mg We’re keeping mass

constant… So what does that tell us about

force and acceleration?

Acceleration and time

a = 2d/t2

If we “ignore” distance… Acceleration is proportional to 1/t2

Or if acceleration increases… 1/t2 increases

If we apply more force, we should see “1/t2” increase.

f = (1/t2)

Let’s see if we can prove it in lab…

October 28

Lecture over Newton’s 3rd LawFriction Lab

Newton’s 3rd Law Notes

•                       

3rd Law Whenever one

body exerts a force on a second body…

The second body exerts an equal and opposite force on the first.

Newton’s 3rd law

Or…

For every action

There is an equal and opposite reaction

Forces come in pairs

You lean against a wall…

You sit in your seat…

You stand up and walk forward in a canoe…

You jump up from the earth…

The earth pulls you back down…

More pairs

In space – you throw a ball

It goes forward

You go backwards

On earth – you pull a trailer with your car

It pulls back with the same force

So how do you move forwards?

Newton’s 3rd law

Whenever object A exerts a force on object B, object B exerts an equal and opposite force on object A.

Or… For every action

There is an equal and opposite reaction

Pairs of Forces One is called the

ACTION FORCE.

The other is called the REACTION FORCE

http://www.madison.k12.wi.us/jefferson/science/fmprojects.htm

You tell me…

Come up with an example of a pair of reaction forces related to riding a bike.

The “Great Misconception” If forces are equal

and opposite… HOW CAN

ANYTHING MOVE?

Because… A… B… ? The action force

acts on Body A, while the reaction force acts on Body B.

Remember that differing mass can cause objects to accelerate at differing rates.

If force is the same…

Small mass = high acceleration Large mass = low acceleration

Lab…

Lots of action/reaction pairs And some friction, too!

October 30

Friction notesFriction Lab

Forces?

Friction Tension Weight Spring “Normal”

Net Force? What are we talking about?

What forces are acting…

On a falling rock? On a book pushed across the floor? On a monkey swinging from a

rope?

F = ma

If you double the mass… What happens to the acceleration?

If you double the force… What happens to the acceleration?

What is your acceleration… If you move with a constant velocity?

What is the net force if that is true?

The Force of Friction

Determining factors:

Does surface area matter?

Other factors… Does the type of

surface matter? If so, which one?

More factors??? How about the amount of force between

the two surfaces (often caused by gravity).

Just what is going on with friction?

Surfaces are not totally smooth Pits and bumps

Atom to Atom interaction Electromagnetic forces

Either attract or repel.

Static? Moving? This atom to atom

attraction is called static friction.

Once object is moving, it is called sliding friction.

Once you get it moving… Electrostatic has

less effect Just bumping

along Moving friction is

less than the maximum static friction

Coefficient of Friction:Force friction = µ Force normal

µ is just a constant determined by the 2 surfaces involved.

force normal = weight in our case.

This lab…

Is a brick…

November 3 POTD

FrictionFriction Lab

The Force of Friction

Determining factors:

Does surface area matter?

Other factors… Does the type of

surface matter? If so, which one?

More factors??? How about the amount of force between

the two surfaces (often caused by gravity).

Just what is going on with friction?

Surfaces are not totally smooth Pits and bumps

Atom to Atom interaction Electromagnetic forces

Either attract or repel.

Static… sliding… This atom to atom

attraction is called static friction.

Harder to get an object moving Than to keep it

moving. If it is moving, it is

called sliding friction.

Once you get it moving… Electrostatic has

less effect Just bumping

along Moving friction is

less than the maximum static friction

Coefficient of Friction:

Force friction = µ Force normal

µ is just a constant determined by the 2 surfaces involved.

force normal = weight in our case.

F = ma = More Newton

November 5, 2006

Today

F = ma again! Short review/worksheet

Velocity is constant: F = ma

What are the forces? Tug of war Pushing a box over the ground Human Dominos

Velocity is changing: F = ma

Free fall: What is different? A piece of hail Or a bowling ball

What happens if you slide down a hill? What forces are different than free

fall?

Sledding in K. Falls

30 seconds to go 300 meters What was our acceleration? a = 2x/t2

What if you know the initial velocity and the final velocity? a = (vf – vi)/t

What if the slope wasn’t constant?

Parachuting

What is your acceleration if the air drag is equal to ½ your weight?

Worksheet

Work on your own first. When I let you know – work with

your lab partners to compare answers

Come and get your answers checked

Fix your answers and turn in