This unit started out with something we get SO much of at the Asheville School. WORK. No, not homework or classwork. But this nifty other kind of work.
WORK= f x d
*work is measured in JOULES
but you have to be careful because the force has to be parallel to the distance or NO WORK GETS DONE
it also means that if you are walking up a 4 meter staircase carrying a 2N weight you're doing 8J of work. If the stairs on the staircase were to become wider your work STILL WOULDN'T CHANGE (becuase your distance remains the same).
another cool thing about work is that it will always stay the same. or WORK in = WORK out this property is known as conservation of energy. It states that energy is never lost it is only converted.
So say some guy comes to you and says, "Here's this really cool pulley, if you put 200J in you'll get 250J out!" Well you shouldn't trust him because that's just not possible. your work in must equal your work out.
because of this property we can do something nifty like this
WORK= f x d or WORK = f x d
which means you can increase the force and decrease the distance of decrease the force by increasing the distance. This will pop back up when we talk about MACHINES.
Next we talked about POWER
POWER= WORK/TIME
*Power is measured in JOULES per SECOND or J/s
So remember how you were carrying a 2N weight up the staircase and were doing 8J of work? Well let's say you just did that in 2 seconds. Because POWER is equal to work divided y time your power would be 4J/s.
But wait. Isn't power that thing we buy from the power company to use our microwave?
NOPE!
We buy ENERGY from the power company.
*Energy is measured in JOULES (and sometimes Watts).
Two specific types of energy we studied in this unit were POTENTIAL and KINETIC energy.
POTENTIAL ENERGY- is the energy an object can have, or the energy an object has at rest.
The formula for potential energy is
PE= mgh
KINETIC ENERGY- is the energy of motion. Kinetic energy is how much energy an ojbect has as it moves.
The formula for kinetic energy is
KE= 1/2 mv^2
one super cool thing about the change in Kinetic energy is that it is equal to work!
So say you have a lead ball attached to a string and you hold it so it is right under your nose. When you let that ball go is it going to come back and give you a black eye?
NOPE!
When the ball is released it has 100% Potential Energy but as it moves down it's swing that potential energy is converted into kinetic energy.
One of our big problems from the unit (and by big I mean this thing was EVERYWHERE) was as follows:
A 10kg car accelerated from 10m/s to 20m/s in 2 seconds. In that time it traveled 10m.
a. What was the change in Kinetic energy the car experienced?
Well to answer this you have to plug both velocities into the formula for KE
1/2(10)(10)^2= 500J
1/2(10)(20)^2=2000J
2000J-500J=
1500J
b. How much work was done?
We know work= to change in KE so
1500J
c. What was the force that caused the car to accelerate?
work= f x d
1500J= f x 10
1500/10=
150N
d. What was the power during the acceleration?
power= work/time
1500/2
750J/s
Last, but certainly not least we talked about MACHINES
Machines are objects that make our lives a little bit easier. In this unit we talked about three different machines.
Pulleys, Inclined Planes, and car Jacks.
all three of these make work easier by increasing the distance traveled therefore decreasing the force!
We used pulleys in our lab where we attempted to move Mrs. Lawrence's care using only a rope and some
clips used for rock climbing. By threading the rope through the clips we created a pulley system that increased the distance therefore decreasing the force.Inclined planes make things easier much the same way. We use inclined planes in problems like this:
Uhaul truck beds (deck height) are approximately 1 meter high. The ramp is approximately 3 meters long. You are trying to load a 30N box into the truck.
a. How much work will it take to get the box from the ground into the truck?
work = f x d
work = (1)(30)
30J
b. How much Potential energy will the box have if it was just lifted up to the deck?
PE=mgh
PE=(30)(1)(1)
PE= 30J
c. How much Potential Energy will the ox have if it goes up the ramp to get into the truck?
PE=mgh
PE= (30)(1)(3)
PE= 90J
d. What force do you use to pull the box up the ramp?
work= f x d
30J= f x 3
f= 10J
But machines aren't 100% efficient, meaning all the energy put in does not go directly into say loading the box. Some energy is released as heat! This doesn't violate the Law of Conservation of Energy though NO WAY! because the amount of energy you put in still equals the energy you put out!
****************************************
I really enjoyed this unit, it has been my favorite unit all year. I think it's fun and fascinating to see what makes our lives possible in terms of machines and even something as simple as lifting a box!
There were some concepts I found were hard to wrap my head around, specifically how energy is never lost but can be expelled through heat. It took me a few days to fully comprehend how those two statements did not contradict one another. By looking over my notes and reading the section in the book that talked about that specific property, and even doing a few extra homework problems to make sure I understood I was able to overcome that minor bit of confusion.
I was really happy with the effort I put in this unit, I missed one blog post which I'm bummed about and my goal for next unit is to miss NO blog posts by maybe even getting some done early (NO WAY!). My problem solving skills really came through in this unit with my quick grasp of Work and Power and Kinetic and Potential energy. I think, however, this was a product of sustained effort it all area's, I worked hard with my podcast group and talked in class, I asked questions and turned in my homework. Overall I'm feeling super confident with my quiz grades and the effort I've put into this reflection that I will be able to get an A on this Physics test!
No comments:
Post a Comment