# Why and How are Mechanical and Kinetic Energy different?

1. Oct 23, 2013

### piareround

Hey guys,

I am in a teaching assistant in another 8th grade science class for my 2nd field experience in a title 1 school. The teacher is really struggling(*) to explain to the students the difference between Mechanical and Kinetic Energy.

The book their using defines Mechanical Energy as, "energy possessed by an object due to its motion or position. In other words, Mechanical Energy is the combination of gravitational potential and kinetic energy. Its the energy of moving things." On the other hand, the book defines Kinetic energy as, "energy of motion. The faster an object moves, The issue is the class really struggle with abstract math, so we cannot simply have them use the equation M = K+U more over the fact that both are energies involving directional motion confuses them.

Do you guys think you can help?

(*)For example, one person asked. "What about fly wheel powered-car hybrid, isn't it storing rotational kinetic energy? How can a fly wheel mechanical potential energy or stored energy from kinetic energy" He's talking about a car that instead of instead of loosing all its energy from braking stored some of it it stored in a giant fly wheel attached to the car. But Potential Energy is energy that you store not Kinetic Energy. I tried to explain to him that that is why we have Mechanical Energy, but I felt like my answer was weak.
(**)Another student asked, if we have "Gravitational and Kinetic, then why do we need mechanical energy?"
(***)In the past the teacher has noted she had one student who literally thought the states of energy were the 50 states. This is because she had never heard of the word of state.

Last edited: Oct 23, 2013
2. Oct 23, 2013

### nasu

Well, part of the problem is the use of "definitions" which are worded carelessly and of generalizations without support. For example, where does this sentence (bold) comes from?
What does it mean? Of course you can "store" kinetic energy. Every moving object "stores" kinetic energy, doesn't it?

The sentence "Its the energy of moving things" is confusing too. To say the least.
I suppose it should be "It (mechanical energy) is the energy of moving things". If they mean that only things that move have mechanical energy, then is not true.

For the (**) I don't see the problem. If we have oranges and apples, do we need fruit?

Last edited: Oct 23, 2013
3. Oct 23, 2013

Kinetic is the energy of objects in motion.

Mechanical is the Sum of Kinetic and Potential - but I would add in a mechanical system. ( A spring is a good example - as a spring oscillates it is exchanging Kinetic and Potential energy - but the Mechanical Energy remains constant.

But I believe the confusion comes from equating Potential Energy as Stored Energy. You can store energy in a kinetic system = flywheel.

The flywheel powered car is a good challenge. TECHNIALLY this is kinetic energy - but if you put the flywheel in the proverbial "black box" - and tell an engineer you have 1000 J of energy stored in there.... he does not need to care if it is stored in a compressed spring (potential), flywheel (kinetic), battery - connected to an electric motor, or gasoline and combustion engine - etc.... you have stored energy.

4. Oct 23, 2013

### piareround

Main Question Back:

Actually, I show the teacher your replies. Both she and I agree with you completely. What do you think would be a better definition that eight graders might understand?

Minor Comment:
Sorry I forgot to explain that the book literally defines "potential energy is stored energy, or energy of an object has due to its position or its shape." You and I both know that that from a certain frame of reference any object has kinetic energy because its moving and fundamentally an object because matter is always moving has internal kinetic energy that we often measure as thermal energy. However, 8th graders are not taught about reference frames until the end of the semester and they have not learned about thermal energy yet. However, I am limited by the definitions the 8th graders can understand. Basically this is the very similar UP-Goer Five problem. http://xkcd.com/1133/ Some of these student have never even heard of the words thermal, states, internal, potential, or even kinetic (Over 75% of the county lacks internet and 10% cann't afford have phones).

Minor Comment:
Oh good metaphor, I probably going to use that :)