Saturday, May 22, 2010

Apologia Physical Science, Module 10, Newton's Laws

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Friction Lesson - lots of information here.  Good for educators to read through and get a better understanding and a few ideas.  =)
Comparing Friction Mini-Lab - Page 3 of this pdf is pretty identical to one of the experiments in our book. (It is the 6th page of the document.)
Newton's Laws with Learning Cycles - great experiments!
Find these and more at Debbie's Educator's Resources.  (Thanks, Debbie!)

(1) p. 233-241a, Isaac Newton; Newton's First Law of Motion

Newton's 1st Law of Motion - The Law of Inertia
" An object in motion or at rest will tend to stay in motion or at  rest until it is acted upon by an outside force."

Inertia - start video at 1:20

Newton's Law of Inertia (seatbelt, headrest)

(2) p. 244-244, Friction

Newton's 1st Law of Motion - Outside forces can be friction and gravity as well as other obvious forces.  From this video: 0:00 - 1:50

(3a) p. 244b-250, Newton's Second Law of Motion

Newton's 2nd Law of Motion - the Law of Acceleration

"When an object is acted on by an outside force, the strength of that force is equal to the mass of the object times the resulting acceleration."  F=ma
Mass

Weight vs. Mass

Newtons' 2nd Law of Motion - F=ma
From this video: 1:50 - 2:48

Dr. Skateboard - Newton's 2nd Law, F=ma, and a=F/m

Overcoming Static Friction... to Kinetic Friction
From this video, 0:00 - 0:30

(3b) Doing the math:
Newton's Second Law of Motion: F = ma

The above video shows how to do the math to get the net force of Newtons.
In this example, there was no friction except for air, and it is so small that it isn't taken into account when finding the amount of force the javelin thrower exerted.
Since there was no friction, no other calculations were needed.

In doing the problems in your textbook, you learn about static friction and kinetic friction, and you learn to include those in your calculations.
Watch this video to understand what exactly is net force.

If you are pushing or pulling on an object, you are exerting force.
If you have friction caused by the object against a surface (like the floor), the applied force minus the frictional force will result in a net force.

If more than one person is pushing or pulling, you add those together for the total applied force.
If there is kinetic frictional force while the object is sliding, and maybe there is additional frictional force if the object is a human resisting you, or a pet dog resisting, you will add those to account for total frictional force.
Total Applied Force - Total Frictional Force = Net Force.

When you work the math as in the video above, you get the net force.
In your textbook, you usually need to do a second step to find either applied force or kinetic frictional force.

I have my kids in my class to, as they are reading the problem, circle any directions (east, south, downwards, etc.), and to circle what the problem is asking for (force a person is exerting or kinetic frictional force).
We then work the F = ma formula to find the net force.
Then we need to either find the applied force or the frictional force.
I have them write this on their paper:  _____     _____     _____
ap     -     ff     =     nf

They fill in net force with the number of Newtons they found in working out F = ma.
Next they fill in either the total applied force or the total frictional force that is given in the problem.
Then they are able to find out the answer that the problem is asking for.

Acceleration and Force is always in the same direction, and friction is always the opposite of motion.
This makes it easy to figure out which direction to include in their answer.

Example (and I am sure these are not sensible numbers, lol)
A man takes his iguana (mass = 51 kg) out for a walk.  The iguana is resisting the man with a force of 88 Newtons.  In addition, the static frictional force between the iguana and the ground is 76 Newtons, while the kinetic frictional force is 28 Newtons.  The man is determined to get the iguana moving so he pulls on the leash.  If the man drags the iguana with an acceleration of 2.8 meters/second² to the south, with what force is the man pulling?  __________________________________________________

First you work the formula, F = ma just like in the above video.  You will get 142.8 Newtons.
This is the net force.
If you were to able to move the iguana and still have this much net force even while he is resisting, adding in the kinetic frictional force of him sliding along the ground, you must be actually exerting much more than 142.8 in order to net 142.8 Newtons.

Fill in the blanks:  _____     _____     _____
ap     -     ff     =     nf

1.  Net force: 142.8 Newtons
2.  Frictional force: 88 + 28 = 116 Newtons (total frictional force)
The number 88 came from the resistance from the iguana, and the number 28 came from the kinetic frictional force.  Since the iguana is moving, we use the kinetic frictional force in our calculations instead of the static frictional force.
3.  The problem asks, "with what force is the man pulling?"  So we see that we need to add 142.8 Newtons and 116 Newtons to get the applied force of 258.8 Newtons.
4.  Since the man is pulling to the south, the answer is "258.8 Newtons south."
If you were looking for kinetic frictional force, you answer would say north because friction always opposes motion.

If you come across problems that say ignoring friction, or if something or someone is moving/sliding with no one pushing or pulling, etc. then you will only need to work the F = ma formula.
Why?
Because if you have a net force of 32, and either the friction equals 0 or the absent applied force equals 0, the second part of your problem will look something like this:  32  -  0  =  32

(4) p. 250b-254, Newton's Third Law of Motion

Newton's 3rd Law of Motion - the Law of Interaction
"For every action, there is an equal and opposite reaction."

Newton's 3rd Law of Motion - Science Theater 09

Baking Soda & Vinegar Rocket - Newton's 3rd Law

Newton's 3rd Law of Motion
From this video: 2:50 - end

Reviewing Newton's 3 Laws of Motion

Love this!