AP PHYSICS DAY 39 Its time to show the power of physics. 1) HW Turn In 2) We review some ENERGY, give you ideas of what’s going on. 3) We discuss the idea of Power, and I do an example, then you do an example. 4) We do some problems and situations to set us up for success. Oh yeah, my cookies.

HW TURN IN

HW D ISCUSSION

Student A Says: For each pair, the potential energy is decreasing, and is becoming kinetic energy. The ball is fastest at point 4, so from 3 to 4 is the largest change of KE Student B Says: Yeah, but the system energy is conserved in this case, so the total amount of energy is the same for each point, thus they must all have the same amount of change to keep it balanced. D Student C Says: The largest change in KE comes where the potential change is most, which is from 1 to 2. Therefore its A.

GRAPH IT

P OWER AS IT RELATES TO E NERGY Reminder of Mechanical Energy…. Mechanical is Kinetic and Potential combined (K + U) A difference between doing a lot of work at once and over a period of time… Lets talk about Mechanical Power. Its the Energy Per Second, or Work Per Second.

Many different kinds of power…

T HEREFORE Power is measured in J/s, called a WATT (w). Like horsepower, it’s the rate of doing work.

E XAMPLE O NE Emma lifts a 50N box from the floor straight up to a height of 40cm in 2 seconds. Julia lifts a 40N box from the floor to a height of 50cm in 1 second. Assume both did this at a constant velocity.

E XAMPLE O NE Compared to Emma, does Julia A) Do the same work but develops more power? B) Do the same work but develops less power? C) Do More work but develops less power? D) Do less work but develops more power?

E XAMPLE O NE Since it’s a constant velocity, The net work done is zero. We are looking at the work applied by each.

E XAMPLE O NE The – work done by gravity must be equal to the + work done by each. So lets find the work done by gravity That is the change in Ug, or F*d. (weight)*d for Julia: (40)*.5 = 20J For Emma: (50)*.4 = 20J It’s the same!

E XAMPLE O NE But Emma did that in 2 seconds, Julia did the same work in half the time. So she will have double the power. Choice A

E XAMPLE O NE NOW ITS ON YOU This pisses Emma off. So she now lifts the 50N box diagonally, 40cm to the right, and 40cm up in the same 2 seconds (again, at constant velocity). A) Compared to Emma’s work before, is this now more/less/same?

E XAMPLE O NE NOW ITS ON YOU This pisses Emma off. So she now lifts the 50N box diagonally, 40cm to the right, and 40cm up in the same 2 seconds (again, at constant velocity). A) Compared to Emma’s work before, is this now more/less/same?

E XAMPLE O NE NOW ITS ON YOU Well…If it’s a constant speed, that means that you can’t be applying an unbalanced force horizontally, or else it will not be at a constant velocity. Since there are no other horizontal forces, you CAN’T be applying a horizontal force. So you are still only working against gravity, and that means your work is the same as before.

E XAMPLE O NE NOW ITS ON YOU B) Is the power more/less/same compared? Same! Same work, same time, same power.

MINI LAB TIME Stairclimber. Lets read the goals and the ideas. This is my way to further teach you info, rather than assess you.