Oct. 16 – Newton's Law of Universal Gravitation

First of we discussed the results of the lab I handed back.  Here are some tips for improvement:

Lab Improvements

Only need to show sample calculations.  You don't have to write out the same calculation over and over again for each trial.  Just show the calculations for the first trial.  Do not put calculations in a table.  In a table you should only have numbers with the units at the top.

Units everywhere.  On every number, in the tile of tables, in the middle steps of calculations.  Everywhere.

% difference = precision

% error = accuracy

Don't confuse them.  Ask me if you don't know the difference.

Sources of Error

- Calculations are NOT a source of error.

- Measurement errors.

- Reaction time.

- Friction and other assumptions.

Improvements for Lab

Any improvements should affect your procedures or the equipment. "Measure more precisely," is not an improvement because I already expect you to measure as precisely as you possibly can.

For the next lab...

• independent variable: values that you are changing.
• dependent variable: the value that responds to the change, what you are measuring.
• controlled variable: what you tried to keep constant.

Then the next topic...

Newton's Law of Universal Gravitation

Here are some examples of what gravitational field might look like if we could see it:

The lines show how objects might move when they fly past the Earth.

If the little dot in the middle is the Earth, these vectors show the gravitational force at different locations around the Earth.

Another way to illustrate what a field might do.

Finally, here's what people use to represent the gravitational field around a black hole:

The gravitational field affects all areas of space.  The only way you can avoid feeling the effects is if you're in free, such as in this ride:

The brief moment when you are falling is what it would feel like there were no gravitational field.  You can also get the same effect if you were in an airplane that is diving.  We call the effect weightlessness.  Here are some videos of what that would look like:

Here's what it would look like to pop water balloons in a weightless environment:

Instead of an airplane, you could also go to the international space station to experience weightlessness.

Oct. 15 – Lab and Test 1 Results

Congratulations on completing the Lab today!  I hope you all got a lot out of the experience.  Let me know if you have questions on how to write it up.

For now I handed back the first Unit Test.  Here are the test solutions:

• Test Solutions Period 2
• Test Solutions Period 2 - Thinking
• Test Solutions Period 5
• Test Solutions Period 5 - Thinking

Oct. 14 – Lab Prep

Welcome back from the long weekend!

Today we did some examples and then went over the lab for tomorrow.

Look at this image...

Can you explain how this sprinkler works using Newton's Laws?

According to Newton's 3rd Law, every action has an equal and opposite reaction.  The sprinkler pushes the water out sideways and the water pushes the sprinkler in the opposite direction, therefore the sprinkler spins around.

Read this Manga to if you want some more explanation about Newton's 3rd Law

• Newton's 3rd Law from the Manga Guide to Physics

Here are other examples we worked on:

Then we discussed the lab, have a look at the handouts.

Handouts

• Examples of N2 and N3
• Newton's 2nd Law Lab

Homework

• Read and prepare for the lab.
• Complete any skipped homework from previous days.

Oct. 10 – Newton's 3rd Law

Here's quick review and demo of Newton's Second Law:

Next we talked about Newton's 3rd Law.  I did a demo with a rolling chair.  Here are the notes:

Newton's 3rd law may be easy to write down, but it's can be tricky to understand.  Here are more videos that demonstrate the law:

How do water jetpacks work?  Heres'a diagram:

The jetpack forces water down, the reaction force is the water pushing the person up.  If the upwards force is greater than gravity, the person will accelerate upwards.

Here are more examples of Newton's 3rd Law:

Every time you walk, you are using Newton's 3rd Law!  You push the ground backwards and the ground pushes you forwards!

Homework

• Follow the unit plan and do the homework for day 4.

Here are some solutions for day 3 homework:

Oct. 9 – Newton's 2nd Law

Today we covered Newton's 2nd Law.  Here are the notes:

Handouts

• Some Newton's 2nd Law Problems
• Net Forces Worksheet

Homework

• Finish the handout and do the first half of the day 3 homework from the unit plan: 
• P. 126 #1, 2
• P. 134 #2, 3
• P. 135 #1, 2

Oct. 8 – Newton's First Law and Inertia

I couldn't be in class yesterday due to a field trip for another class.  Here are the handouts I left with the supply teacher:

• Free Body Diagrams 2
• Free Body Diagrams Worksheet

Here are the solutions to the worksheet:

More on Newton's 1st Law (Inertia)

Inertia

• The tendency for an object to keep moving (or stay motionless).
• Inertia is directly related to mass.  More mass means more inertia.

There were two space probes launched in 1977 called Voyager 1 and Voyager 2.  They have no rockets and no source of propulsion on board.  They are now over 18 billion km away.  Here's a tracker from NASA http://voyager.jpl.nasa.gov/where/

•        What will happen to them?

Since there is no forces acting on them, they will keep going due to Newton's First Law.

• An object in motion tends to stay in motion unless acted upon by an external force.

In fact, they will keep going forever unless they encounter something to exert a force on them.  In the space between our sun and other starts, there's really not much that will stop them!

Here's an image taken by Voyager:

The tiny speck in the beam on the right is the Earth!

Here's what Carl Sagan had to say about this image:

From this distant vantage point, the Earth might not seem of any particular interest. But for us, it's different. Consider again that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam.

—Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space, 1997 reprint, pp. xv–xvi

On Earth, we also see Newton's First Law in action all the time:

A force from the tires stops the bike, but this force doesn't act on the rider, so he keeps going!

The same is true for cars.  If there's not enough friction to stop a car, say on an icy road for instance, cars will have a difficult time stopping:

Using inertia, can you explain how this works?

Possible solution:

When the card is flicked, a force acts on the card.  No addition force is applied to the coin (small bit of friction), therefore, due to inertia, the coin remains motionless.  Once the card is removed, there is no longer a normal force on the coin, the net force is gravity, causing the coin to fall into the cup.

Homework

• You can now do the homework from Day 1 and Day 2 of the unit plan.

Oct. 6 – Unit 2: Introduction and FBD

Congratulations on completing the unit test!  I am busy marking them now and will get you the marks as soon as possible.

In the meantime, we are starting unit 2...

Unit 2: Forces

Here's what movies like Star Wars thinks a "force" is,

What are forces in real life?  Watch this,

Here are the notes I gave in class:

 What is a force?

 - A push or a pull on an object.

 - Cause objects to “move” (acceleration).

Four Fundamental Forces

1. Gravity

        - weakest

        - can only attract (pull)

        - acts on anything with mass

2. Electromagnetism (EM)

        - electricity and magnetism

        - stronger than gravity

        - can attract or repel (push or pull)

        - holds atoms and molecules together

        - most forces you feel on a everyday basis are electromagnetic!

3. Weak Force

        - stronger than EM

        - only affects radioactive nuclei

4. Strong Force

        - strongest of forces

        - holds together protons to create nuclei inside atoms.

Forces are vectors!

        - have magnitude and direction

        - we can represent them with arrows

        - we organize forces with Free Body Diagrams (FBD)

        - draw all the forces acting on an object

Examples: 

An object falling due to gravity.

An object falling due to gravity with air friction included.

Forces add like any vectors would.

The total force on an object is called NET Force (the sum of all force vectors), ∑F.

In the above example: ∑F = F(gravity) + F(air)

NEWTON’S FIRST LAW

(Law of Inertia)

"An object in motion (or at rest) tends to stay in motion (or at rest) unless acted upon by an external force"

More precisely, we can say:

When the net force on an object is zero, the object will move at constant (or zero) velocity.

If ∑F = 0, then a = 0.

or

When an object is moving at constant (or zero) velocity, the net force is zero.

If a = 0, then ∑F = 0.

Example:

An object sitting on the ground.

Using Newton's First Law, the object is not accelerating, therefore, the net force must be zero.  We see that there must be a force opposing gravity, otherwise the object will fall right through the Earth!

This force is 90° from the ground, therefore it’s called the Normal Force.  (Normal = 90°)

The Normal Force comes from contact with a surface (contact force).

Other common forces…

 - Tension: pulling force from a string or rope.

 - Friction: contact force that opposes the direction of motion.

Handouts

• Unit Outline
• Free Body Diagrams