Last two equations today – work is calculating the effort required to do something, and power is looking at the relationship of work and time. The faster you do something the more power it takes, but the actual work done is the same.
Make sure you know this is a SCIENCE use of the words work and power… don’t think the words mean the same thing in the big bad scarey world.
Homework was questions one and two from page 80 and 81.
So today there were another couple of equations, both of them for calculating the amount of energy something has. The units are Joules (J) Kinetic energy or E(k) is working out how much energy a moving object has. Potential energy is how much energy is stored in a high object – like the energy in the water of a dam, or something lifted off the earth.Potential energy is E(p)=m.g.h So the higher you lift something the more energy it has. The larger the object you lift the more energy it has. g is for gravity, which remains a constant on this earth and you will be told g=10m.s-2. Try it with some simple numbers eg m=4g, g=10m.s-2, and h=12m, and then double the mass to 8g. Try doubling the height to 24m… When they went to the moon gravity was less. But imagine (or calculate) the effect of going to Jupiter where gravity =40m.s-2 The homework was to finish the question given in class and do Question Four of page 81 in your green book…The class question was: A metal weight (2700g) in the gymnasium was left on a shelf 2m above the floor. It falls. It takes 0.8secs to fall (me bad, thanks for noticing Gavin)How much energy did it have on the shelf?How fast was it going when it hit the ground?What was its acceleration?With what force did it hit the ground?
If you grab your green textbook and turn to page 77, you’ll see our new friend the formula for acceleration, and below that the distance-time graph. Note that over the page is a comment that the area under a graph will give you the distance travelled – we will have to cover that in class. You may have covered it in maths (but I’ll pretend you haven’t and teach it anyway).
Try question one on page 78. Note that you have to convert the units for a). There will almost certainly be at least one question at the end of the year on converting units – its one of those doddle questions they put in for easy ‘A’s. Always double check these ones if you are arithmetically challenged like me. It’s far too easy to put the decimal point in the wrong place!
For d) when they say use the graph, what they really mean is use the numbers on the graph – you do this with maths, getting the numbers from the picture. They want to know you know how to read a graph, and do not tie yourself in knots trying to figure a new way to figure out v=d/t.
Calculating deceleration is the same as calculating acceleration but you’ll get a NEGATIVE number.
Let me know how you get on….