Archive for December, 2008

Christmas Physics

Some people go over board with decorations. It seems all the electricity being used would start a fire. However, this year Hallmark is responsible for the danger. The following is an article found on http://www.reuters.com/oddlyenough


Jumbo Snowglobes + Sunlight = Consumer Hazard

WASHINGTON (Reuters) – 

Some 7,000 jumbo-sized snow globes were reca

lled by Hallmark Cards Inc. because the holiday decorations can act as a magnifying glass when exposed to sunlight and ignite nearby combustible materials, the U.S. Consumer Product Safety Commission said on Tuesday.

The snowman-shaped snow globes were sold in October and November at Hallmark Gold Crown stores nationwide for about $100 each.

The consumer agency said Hallmark has received two reports of the snow globes igniting nearby materials but no injuries have been reported.

Consumers who bought the snow globes, which measure 11 by 12 by 17 inches (28 by 30 by 43cm), should immediately remove them from exposure to sunlight and return to a Hallmark Gold Crown store for a full refund. 

Details about the recall were posted at the government agency’s web site at: (the link didn’t work, go figure)

 

The Color of Light

I saw a great demonstration the other day in class about the color of light. It is similar to a lesson plan found at: http://science.hq.nasa.gov/kids/imagers/teachersite/UL1.htm


To begin, my classmates drew two venn diagrams. One had the colors of paint (red, yellow, and blue) and the other had the colors of light (red, green, and blue). We guessed which colors created which new colors (orange, green, purple and yellow, cyan, and magenta, respectively).
Next, they turned on three light bulbs (a red, green, and blue one). All other sources of light was covered and a large white projection screen was against one wall. One classmate put a meter stick in the way of the light and in front of the screen. It was amazing to see the meter stick separated the light into yellow, cyan, and magenta. Then the light bulbs were turned off one at a time. When the red light bulb was turned off, the background of the screen was cyan, while green and blue were separated by the meter stick. Similar results occurred when the green and blue light bulbs were turned off. 

It was just amazing!! Try it at home if you don’t believe it :o)

Vacation Science #2

I posted a slide show with a few of the pictures I’d consider for the vacation science wall…I’ll add more once I get the cds with pictures from Europe out!

Careful, don’t fall off the Earth!!

In class the other day we calculated the minimum acceleration we need to stay on the earth…what we found was fascinating!

First, we need to discover the diameter of the Earth as well as the time, in seconds, it takes the Earth to rotate once:
d=2*pi*radius=2*pi*6.3781 x 10^6
d=4.007 x 10^7 m
t=24hrs=1440min=8.64 x 10^4 sec
Now we have a change in displacement and time to calculate velocity:
v=(distance)/(time)=(4.007 x 10^7 m)/(8.64 x 10^4 sec)
v=463.77 m/s
We know the acceleration in circular motion is a=(v^2)/r, so plugging in the velocity and radius will give us the minimum acceleration of one point on the Earth without anything flying off!
v^2=2.1508 x 10^5 m^2/s^2
a=(v^2)/r=(2.1508 x 10^5)/(6.3781 x 10^6)
a=0.0337 m/s^2
So, we can see the minimum acceleration needed for us to stay on the Earth’s surface is much smaller than the 9.81 m/s^2 acceleration we experience every day. This is the exact reason why we are able to fall down.
Imagine if the centripetal acceleration were only .0337! We would merely hover over the surface of the earth — how crazy!?

Physics Jokes: Chickens

Why did the chicken cross the road?


Isaac Newton: Chickens at rest stay at rest. Chickens in motion cross roads.
Albert Einstein: It depends on your frame of reference, how do you know the road is not crossing the chicken?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There is this farmer who is having problems with his chickens. All of the sudden, they are all getting very sick and he doesn’t know what is wrong with them. After trying all conventional means, he calls a biologist, a chemist, and a physicist to see if they can figure out what is wrong. So the biologist looks at the chickens, examines them a bit, and says he has no clue what could be wrong with them. Then the chemist takes some tests and makes some measurements, but he can’t come to any conclusions either. So the physicist tries. He stands there and looks at the chickens for a long time without touching them or anything. Then all of the sudden he starts scribbling away in a notebook. Finally, after several gruesome calculations, he exclaims, ‘I’ve got it! But it only works for spherical chickens in a vacuum.’

Student Teaching Ideas

At this point, my two choices for middle school student teaching is Isaac Newton MS and Mott Hall II MS both in Manhattan.

Not sure about High School…I’d still like to observe at LaGuardia HS for Performing Arts!

Why Raindrops Don’t Kill Us…

I’d never really pondered the idea that raindrops might be a cause of death until the other day.

If we’re concerned about killer pennies from the top of the Empire State Building, how much more scary are little water bombs from thousands of feet in the sky?
No need to fear…physics is here!!!
Since raindrops are relatively small in volume, the reach a small terminal velocity quickly. An explanation follows:
When raindrops fall, they are met with air resistance. Air resistance is proportional to surface area, so small and big raindrops experience the same phenomenon. The air continues to resist the raindrop until air resistance reaches the same value as the gravitational force. This moment is when the raindrop reaches its “terminal velocity”. It can’t go any faster because it’s no longer accelerating. 
And since rain drops have little mass, they won’t have a very large velocity!!
Good thing there’s air resistance between the clouds and the ground!

Physics of Race Cars

In my Concepts in Physics course the other day at TC, we discusses race cars on a banked turn. We used free body diagrams (FBD) to illustrate where the centripetal force (the force that keeps the race car traveling in a circle).

Here is the basic FBD for a car on a banked turn. The dot in the middle of the rectangle (car) represents the car’s motion out of the page and toward the drawer or looker.
So, we’ve got the car on the incline. The force of gravity is always directly down — in the direction of a free hanging plumb line. The normal force (typically the force opposite the gravitational force) is always perpendicular to the surface, so not vertical in this case.
Now things start to get fun!!!!

We draw the components of the normal force in with dotted lines; pretty much, we’re making the normal force the hypotenuse of a triangle. You can clearly see the vertical leg of the triangle is equal and opposite to the gravitational force.
Left over is the little bit of horizontal normal force. Notice that it is pointing towards the center of the circle…it seeks the center. The definition for centripetal force is “center seeking”, so this must be the centripetal force on the car!!!!
The neat thing is, the faster the car travels around the bank, the higher it will go on the incline. The higher on the incline, the greater centripetal force. And the driver doesn’t have to do a thing! All these things happen naturally :o)
I thought this was a great demonstration to show students where this mysterious and confusing force comes from. It certainly isn’t magic!!!

Physics In The White House

I just read this article that President-Elect Obama has named Physicist Steven Chu as head of energy — yay Physics!! This would be a great current event for students in the classroom to see how physics effects their every day lives. :o)

Centripetal Acceleration Proof

I took notes on this proof the other day while observing Mr. Provo and Mr. DePalma team teach:
Start with a circle with the velocities pointing in the tangential direction to the circle. Draw the radius so students can see the velocity is perpendicular to the radius. Label the angles and radius so students can see they are the same (even if one angle is larger, it means the arc covered in that time is longer).
The next step in the proof is some vector addition. Placing two vectors together (not using the initial vector since it has no vertical component), students can find the direction and magnitude of the acceleration. According to the image below, the acceleration will be toward the center of the circle.

 

This triangle (when drawn well) will be a similar triangle to those in the circle. This means we can create a ratio.
The arc(ab) can be written as the velocity multiplied by the change in time – the circle image. Therefore, the first part of the ratio can be written as v*(delta)t/r. The triangle diagram shows the change in velocity divided by the magnitude of the velocity is analogous to the first part.
v*(delta)t/r ~ (delta)v/v
Now we want to get the deltas on the same side.
(v^2)/r ~ (delta)v/(delta)t
And we know that (delta)v/(delta)t is also equal to acceleration. So…
(v^2)/r ~ a
But, this is only an approximation, so we’ve got to figure out how to make it exact. Well, as the angle between the vectors approaches zero (again referring to the circle diagram), the arc(ab) approaches a straight line. And as the change in time approaches zero (as measurements become more instantaneous), the velocity becomes perpendicular to the acceleration. This means the approximation we made can be exact – both the velocity and radius are perpendicular to the acceleration.
(v^2)/r = a !!!
This isn’t the most clearly written proof, but I’ll work on it.
Things to remember:
– As velocity increases, so does acceleration.
– As the radius increases, the acceleration decreases.