UNIT 5
Over the course of this unit we have studied machines, work and power.
Work
Lets say I'm walking up some stairs and I start to wonder, wow there must be some kind of physics going on that allows me to walk up these stairs. Well there is, it's called work.
Work is the amount of force that you apply over a certain distance, or in equation form:
work = force times distance.
As I start climbing up those stairs I apply a certain amount of force over a certain amount of time that allows me to get up those stairs.
Lets say that I weigh 600N and I want to get to the top of a flight of stairs 4 meters tall. How much work will I do?
since work = f times d all we need to do is start plugging in numbers, so:
600N times 4 equals 2400 Joules.
It took us 2400 J's to get to the top of the stairs. Wait, what in the world are Joules? A Joule is equal to the amount done by using the force of one Newton over the distance of one meter. But all you really need to know is that it is used as the unit of measurement for work.
Now lets say you go out to a nice restaurant to eat and you ( being a smart physics student) notices that the waiter is applying a certain amount of force on his food tray, while he is walking a certain distance to deliver the food. And you say that some work is being, thats actually not true. Because in order for there to be any work done force and distance must be parallel. the force that the waiter used to pick up the tray needed some work, but the act of walking did not.
Power
Now theres this other thing called power, which is how quickly work is done.
The equation for power is Power = work over time
Which is pretty simple, if you push a box for 10 seconds while doing a total work on it of 100 J than you would have a total of 10 watts of power. Notice that the measurement for power is watts, which is the same as a lightbulb. a lightbulb is a great example of power, Say you have 60 watt light bulb, that means that the lightbulb uses 600 J of work every 10 seconds.
Work and Kinetic Energy
Notice that we have two velocities and a distance. But how are we going to find the distance it took to stop? We could use the work formula because it has a distance, although we don't have a force. But don't worry physics has an answer, it's called the energy of movement or Kinetic Energy and lucky for us it is equal to work.
Like most things in physics KE has a formula, which is: KE = 1/2mv^2
Now how do we know we're going to survive, well we know that 20m/s is two times greater than 10m/s so we must have skid to a halt in 6m. Unfortunately, no.
According to the formula for KE (1/2mv^2) the velocity is squared, making us go four times as far. 4 times 3 equals 12. Lets hope we survive the crash.
Just to prove it to you i'll even work out the equation for you:
KE = 1/2m (2v)^2
= 4(1/2mv^2)
Work = KE
4(work) = F time 4(distance)
Distance equals 12 meters.
Machines
Heres another basic physics principle for you, it's called a machine.
Lets say you are moving houses and have rented a Uhaul truck, But the box is to darn heavy for you to carry. Lucky for us Uhaul provides a free ramp with the truck and all you need to do is push the box up the ramp and into the truck.
How in the world does that make things easier? Aren't you still pushing the box up the same height? Yes the box is being push up the exact same height, but theres something a little different. Remember the formula for work, W = F times D, well when you push that box up the ramp or are able to lift it up you exert some work. It was to hard for you to carry up because you couldn't apply enough force, but when you push it up a ramp the distance was increased enough so that a smaller amount of force could be used to push it onto the truck.
But lets say you got a stronger friend of yours and she was able to lift it up without a ramp, she exerted more force over a shorter distance in order to bring that box up.
This is where we get our formula for Machines, Work in = Work out.
You used a longer ramp and less force while you'r friend used more force and less distance, but you both still lifted a box into the same truck bed. So both of your forces are equal. You simply used a longer distance in order to make things easier.
In formula form it would look like this:
Work in = work out
F times D = F times D
Therefore, machines like the one above are used to decrease the force needed by increasing the distance needed.
Potential and Kinetic Energy:
Lets say there is a large rock sitting precariously on the edge of a cliff that weighs 1000J, this means that the rock has a total PE of 1000J, if for some reason a strong gust of wind came along and pushed that rock off the cliff it would immediately start converting PE into KE to the point where right before the rock hit the ground KE would equal 1000J. Because change in KE = change in PE the more KE we have the less PE we have.
As soon as that rock settled itself on the ground the PE would be 1000J again, right? Well not according to the formula for PE, Which is PE = mgh. The h in the formula represents height and if the rock is settled on the ground then it has no height and so PE = zero.
Conservation Of Energy:
Have you ever been on a roller coaster? Well I haven't, but I do have a small insight on how it works thanks to physics. Lets say you're sitting on the top of a roller coaster with approximately 1000J of PE. As you roll down and reach the bottom of the area in between the other hill your PE equals zero and your KE equals 1000J! But as soon as you crest that hill, lets say you now have 300J of PE but still have 700J of KE left over so that you easily make it over the hill. Once again you fall down into the dip between hill, PE equals 0 and KE equals 1000J. Then cresting a larger hill than the last you have 600 J of PE and 400J of KE. Notice that all of these values equal up to 1000, which was the starting amount of Joules. Energy is conserved on a roller coaster and the cart can continue moving (excluding outside forces such as air and friction) as long as the very first hill is longer than all the other hills.
In case you didn't understand how the conservation of energy works, heres my units podcast:
Important connections:
Because it's imperative that we know how things connect together i've compiled some of the formulas of this section and shown you how they relate
Work = Force times Distance
Power = Work over time
PE= mgh
KE= 1/2mv^2
change in KE = change in PE
change in KE = work
change in PE = work
change in KE = KE initial - KE final
Efficiency = Work out/ work in
Work in = Work out
Review:
For some reason this unit has been a little difficult for me. I thinks it's mostly because i haven't been completing my homework (I did complete all my homework) but I just didn't check over it again to make sure I got them right. I really think this impaired on my last two quizzes, which when I looked back over they were fairly simple. I think next unit i'm really going to pay attention to what I missed on homework assignments, instead of day dreaming in class.
I liked this unit because it explained a lot about how objects move and why they move. It was really interesting to learn that work, force and energy are not all the same thing, but entirely diffe
rent concepts on there own, although they are related to each other
