Sunday, February 21, 2010

Energy Reflection

Part A:
In the Energy Unit I have learned that energy is never created but just reused. Because of that, in one system there will always be the same amount of energy. For example, when someone bungee jumps the system would be the earth, the bungee jumper, and the bungee cord. Before the bungee jumper jumps, quantitatively he has 4 units of gravitational potential energy (Eg) and after he jumps, and is hanging by the cord, he could have 1 unit of Eg, 2 units of elastic energies (Eel), and 1 unit of internal energy (Eint). The most difficult thing about understanding the conservation of energy would definitely be being able to plug in equations with other equations and knowing what to substitute. I am studying all of the equations because being able to quantitatively solve the equations, it is important to know  what substitutes for what. I am working really hard to learn all of them before the test. The one that I usually get mixed up is when to use co-sine in the work equations. I just need to remember that when the force (F) and the distance (x) are parallel to eachother you use the equation W=Fx but when they are not parallel but rather at an angle of eachother you use the equation W=Fxcosθ. However I have found reading the problems in this unit a lot more straightforward than in the previous units. Overall, I think that the most important thing to walk away with is that there will always be the same amount of energy in a system. It is a good way to check yourself because no matter what the initial energy will equal the final energy but just in different forms. The energy can transfer to one form or another by moving, being at a height, or preforms work. If it is moving it has kinetic energy (Ek), if it has height it has potential gravitational energy (Eg) and if it a force is being applied to a distance it is doing work, another type of energy. 
Part B:
Over this unit I realized that everything we do uses energy. No matter what you do you will always use energy which I personally find to be amazing. Physics really is everywhere, who would've thought it? One common example among teenagers is car crashes. While a car is moving it has kinetic energy but lets say they hit a wall. All of the energy becomes internal.

Monday, February 1, 2010

Seatbelt FBD and equations

 Here is the FBD for the person on the seat of a car.

sum of the Fy= Fn(bottom)- Fg = 0

sum of the Fx= Fseatbelt - Fn(seatbelt) = ma
Fn(seatbelt) is the seat belt on the person and Fseatbelt is the force of the seat belt. 

Fn(seatbelt) would not equal Fseatbelt because the person does actually go forward a little but after a very short distance the person stops.

Safety first

Here is a short comic that gives an example of why you should wear a seat belt. In the comic the car was going 40 m/hr down Midway when all of the sudden a car pulled out in front of them. Unfortunately, they crashed without even having time to put on the brakes.  The man was wearing his seat belt so as their car hit the other car his seat belt kept him from continuing to move forward, staying relatively unharmed. The woman however was not as lucky, she was not wearing her seat belt. Newton's First Law says, an object in motion tends to stay in motion unless acted upon by an unbalanced force. Since the woman was moving as a separate object she continued to move despite the fact that the car had stopped.The man wore his seat belt so when the car stopped the seat belt locked creating the unbalanced force that is required to stop an object that is in motion. In the end the woman sadly did not survive the crash. Only if she had put on her seat belt...

Always buckle up

Ever since you were little you would always hear your mom say "Buckle up!". It has become like second nature to me, as soon as I get into the car I put my seat belt on even if we are just going around the corner. But how does wearing a gray belt across your lap and chest help save you when a car is coming straight at you? Well, the purpose of a seat belt is that it will stop you when the car stops because unless acted upon by an unbalanced force an object in motion will stay in motion and just because the car stopped doesn't mean that you did. If you weren't wearing a seat belt and the car suddenly stops, due to inertia your body would continue to move forward. This could be potentially fatal. You could end up going through the windshield continuing onto the pavement. Wearing a seat belt reduces the impact force of the crash on you. By no means am I saying it won't hurt, because it will hurt, but it won't hurt as bad. In fact, according to the website on the physics behind seatbelts, if you are wearing a seat belt the impact force is about one fifth of the amount it would have been without a seat belt. So maybe your mom was right and you really should always wear your seat belt!