# Unit 3 – Newton’s Laws of Motion!

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Unit 3 – Newton’s Laws of Motion!
Hand back test Lecture 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 = kilogram Weight = Newton So, what is a pound?
Mass or weight?

Calculating weight Weight 1 kg = 9.8 N
= mass X acceleration of gravity = mass x (9.8 m/s2) 1 kg = 9.8 N

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)

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

An engineer walks into a bar…
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?

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…

Lecture over Newton’s 3rd Law Friction Lab
October 28 Lecture over Newton’s 3rd Law Friction 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

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!

Friction notes Friction Lab
October 30 Friction notes Friction 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… If you double the force…
What happens to the acceleration? If you double the force… 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 Friction Friction 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.