Tuesday, November 29, 2011
Monday, November 28, 2011
An electromagnetic wave is made up of alternating electric fields, a charge exposed to it will experience forces regularly changing in direction. For water molecules, which are dipoles, the net effect would force the molecules into rotation.. These agitated water molecules then posess heat energy to transfer into the food, thus cooking it. The key note here is that the microwave acts as a protective box, directing the waves directly at your food. Microwaves operate at a frequency of up to 300 GHz.
Cell phones cannot direct all that energy straight into a tiny popcorn kernel since it is not in a closed box like a microwave. Additionally, they do not emit enough energy (they operate at a frequency of roughly 2 gHZ) to heat up the water in the kernels to make them pop, and if they did, they would probably boil the water in your hand while you were on the phone and eventually make your hand explode!
Sunday, November 27, 2011
Wednesday, November 23, 2011
Tuesday, November 22, 2011
High temperature and pressure converted to work
Thermodynamic Limit (Efficiency): 37%
Most engines: average efficiency of 18-20%
NOT Frictionless, Ideal Gases, Perfect Insulator
Monday, November 21, 2011
If we were to look at this problem in a vacuum the air resistance could be ignored and it would be very clear that the force due to gravity would be the same on both bullets. The force that the gun applies to the fired bullet is only acting horizontally and thus determines how far in the x-direction the bullet will move, not how long it will take. Since the height the bullets are dropped from is the same and the force causing their downward motion is the same both bullets would hit the ground at the same time.
However, as the MythBusters found out this is not exactly true. Because of air resistance acting on the bullets there is a slight difference in the time it takes for them to reach the ground, though very, very small. As discussed in class air resistance (or the drag force) is expressed as
But in order to determine this force on the fired bullet as it relates to the time that it takes for the bullet to fall we must break this force into components. Because of this breakdown into components it can be determined that the air resistance on the fired bullet actually causes it to take longer to hit the ground than the dropped bullet, also experiencing air resistance.
The MythBusters demonstration made it clear that, though the bullets won’t hit at exactly the same time the difference in the time is so small it has to be measured with a high-speed camera and using the images can be calculated to be about 39.6 milliseconds, a time that is smaller than the human eye can detect.
http://www.youtube.com/watch?v=D9wQVIEdKh8 (start at around 2:10)
This is a fun animation which shows all the forces acting on a skydiver, check it out in motion at this site!
Here is a link to an extreme skydiver
If you wanted to calculate this terminal velocity all you would need to do is set the drag force equal to the force of gravity and solve for v.
Because air density increases with decreasing altitude, this causes an increased buoyant force near the surface of the earth. The buoyant force causes the terminal velocity to decrease by 1% every 525ft.
The world record for skydiving speed of 614 mph was obtained at high altitude where the less dense air reduces drag and buoyant effects.