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## High school physics - NGSS

### Unit 4: Lesson 2

Energy in systems

# LOL diagrams

David shows how LOL diagrams allow us to visually represent what we mean by conservation of energy as well as what we mean by an energy system. Created by David SantoPietro.

## Want to join the conversation?

• • • If a an object is sitting on the ground, it has zero Ug (assuming we've establish that ground level as 0). If a sink hole opens up under the object, the object will fall and have Kinetic energy due to gravity, but where did it get it from? Do we change the reference point after the fact to say that the new resting place of the object is now 0 and the original ground was actually X (whatever the height was in relation to the new 0) ? What if, a few minutes later, the ground opens up again and the object falls farther?
I guess what I'm asking is how do we deal with a changing frame of reference? Esp when it comes to defining Ug. • This used to confuse me too.
The important thing to recognize here is that "energy" is not an absolute amount. It's always relative between 2 different states of the object in question. So when the ball is on the table, you'd say it has 0 gravitational potential E relative to it being on the table surface. But it could have potential E relative to it being on the ground. You have to be comparing 2 states the object is in. (This is usually implied rather than stated explicitly)

Another example: When a moving ball "has kinetic E", it's usually meant in relation to if it were stationary on the floor. So the velocity you care about is the relative velocity between the ball and the floor. But if you zoomed out and considered the ball's kinetic E with respect to the sun (i.e. compared to if the ball were sitting stationary on the sun), the ball would have kinetic E of the Earth's movement as well. So the velocity you care about is the relative velocity between the ball and the sun.

You pick the reference point based on what objects are relevant to the question.
• What would happen if the force of the spring was so great that the mass flew into the air? Would it change the outcome or would it stay the same? • Say someone is bungee jumping, and the problem wants to know how much energy is transferred to the cord at the bottom of their jump. How would you solve this problem? • At , he says that there is no kinetic energy. If someone were to slightly push the mass down, however, would there be kinetic energy? • How do I know if something is in the energy system or not. Can't I randomly just move something in and out of it or does it depend on the question?
(1 vote) • Why is it shown as Ug and Ue We Our teacher showed it as Eg and Ee. Is there any difference or does It mean the same thing?
(1 vote) • What is the energy conversion for an object falling through the air at terminal velocity?
I know this sounds simple, but I'm confused about this because of "certain reasons". I was asked this question at school, for which I answered that gravitaional potential energy changes to kinetic. However, our teacher surprised us by saying GPE won't change to kinetic energy at all, "because the object is moving at a constant velocity". This can't be true, as all objects in motion are supposed to have kinetic energy (E = 1/2 mv^2).The _v_ in the formula shows that kinetic energy must be present if velocity is not zero.

I would really appreciate it if someone could answer soon as my exams are coming up...
(1 vote) • At terminal velocity the drag force from the air is equal to the gravitational force on the object so the net force is zero so the work done by gravity is also zero so kinetic energy doesn't change.

For example a standard baseball is about 145 g or 0.145 kg and has a terminal velocity of about 33 m/s so it has a kinetic energy of Ek = 0.145 * 33^2 = 0.145 * 1089 = 157.9 J.

So as the baseball accelerates from 0 to 33 m/s it will gain kinetic energy but when the velocity stops increasing the kinetic energy stops increasing as well.

The GPE is getting transferred to other things like sound, heat and turbulence in the air. 