# Mechanical Energy

1. Apr 16, 2005

### knownot

Hey guys!
I am thoroughly confused.Hope u can help me.
Can we say that living things have mechanical energy?
Is muscular energy also mechanical energy?
Can mechanical energy be equal to potential energy?If so, why is the definition "energy of a moving object is mechanical energy"?
If a person picks up the stone ,does the person and the stone have mechanical energy?

2. Apr 16, 2005

### marlon

mechanical energy is just the sum of kinetic energy and potential energy.

Kinetic energy is associated with the actual movement (depending on your mass and how fast you go) and Potential energy is the stored energy in an object due to it's position, structure, composition...When you fall down, potential energy is converted into kientic energy and during the entire motion, their sum : ie the mechanical energy will remain constant

marlon

3. Apr 18, 2005

### knownot

thank you marlon.
But I am still confused regarding the last part of my question.

4. Apr 18, 2005

### whozum

Can mechanical energy be equal to potential energy?If so, why is the definition "energy of a moving object is mechanical energy"?
If a person picks up the stone ,does the person and the stone have mechanical energy?

That definition is incorrect, energy of a moving object is kinetic.
Think of it like this:

If you are in a position where you can POTENTIALLY move (just because you are there), then your mechanical energy is potential energy. It is PURELY potential as long as you are not moving.

KE = 1/2 mv^2, not moving means v = 0, and so KE = 0.

If you are in a position where you are MOVING and have absolutely NO potential energy (infinitely far from any sort of gravitational or magnetic/electric field - not possilbe) then your mechanical energy is purely kinetic.

Use my hints to answer the question you put forth. While you are holding a ball infront of you, is it moving? Can it move? These two questions should give you the answer.

You can also tell me the energy while you are lifting the ball from the ground, is it moving? Can it move?

5. Apr 18, 2005

### kusal

I agree with whozum.An object doesn't necessarilly have to move for it to have mechanical energy. Even an apple hanging from a branch has mechanical energy that is potential energy.
And the rock that was picked up contains potential energy which is a form of mechanical enrgy.We cant consider that the man has mechanical energy if we consider that earth is the base becase he is neither moving nor is ellevated.

6. Apr 19, 2005

### dextercioby

As for muscular energy,that's chemical energy used by our conscient brain (usually) to do work...

Daniel.

7. Apr 19, 2005

### HallsofIvy

Staff Emeritus
Are you sure you've copied ""energy of a moving object is mechanical energy" correctly?

At http://www.glenbrook.k12.il.us/gbssci/phys/Class/energy/u5l1d.html [Broken]
I found
"Mechanical energy is the energy which is possessed by an object due to its motion or its stored energy of position."

Notice the last phrase (my emphasis).

Last edited by a moderator: May 2, 2017
8. Apr 19, 2005

### dextercioby

I think that fallacious definition just wiped out one of the most important conservation laws in nonrelativistic mechanics...:roleyes:

Daniel.

9. Apr 19, 2005

### knownot

Thank you all for the replies.
But do you think that Living things have mechanical energy since they have energy stored in them and hence the potential to do work.
If so then all living things have mechanical energy even when at rest

10. Apr 19, 2005

### knownot

whozum,

when I lift a stone I use mechanical energy to lift it and since the stone moves , it also has mechanical energy.Am I right?

11. Apr 19, 2005

### misskitty

Everything has potential energy even when its at rest. Your statement makes sense. From reading Daniel and Ivy's posts, I would conclude that living things have mechanical energy; since we have potential energy and use kinetic energy every time we move.

12. Apr 19, 2005

### misskitty

No the stone has no mechanical energy. It is moving because you exerted your energy to move it. It did not move on its own. If you threw it then it would have mechanical energy. If you dropped it then it would have kinetic energy. You simply picking it up does not give it energy. It becomes a part of you so to speak in that you are holding it and making it move. It's not moving on its own. So its part of your system.You simply moving it doesn't give it energy. That would be pretty sketchy if some random rock sitting on the ground suddenly started bouncing on its own.

13. Apr 19, 2005

### Staff: Mentor

When you lift the stone you add to its gravitational potential energy. If the stone moves, it also has kinetic energy. Either way, it has mechanical energy (which is just the sum of potential and kinetic energy).

14. Apr 19, 2005

### misskitty

Doc actually probably has the simplist answer of all of us. Why do we always have to make simple things so complicated?

15. Apr 22, 2005

### knownot

Thank you all.

So basically all living things have mechanical energy since they have chemical potential energy stored in them!

16. Apr 22, 2005

### Staff: Mentor

All things, living or not, have mechanical energy due to their having either (or both):
(1) potential energy due to their macroscopic position: this could be gravitational PE (due to height above the ground) or elastic PE (due to the stretching of a spring), or
(2) kinetic energy, due their having a speed.​
I would not include chemical potential energy as a form of mechanical energy. (The line between mechanical and non-mechanical energy is somewhat fuzzy and arbitrary, but useful in expressing certain conservation laws.)

17. Apr 24, 2005

### knownot

Oh ! Doc Al!

Now I am confused.Why is chemical potential energy not a form of mechanical energy?

When I am standing am I not having mechanical energy since I have the potential energy(in the form of chemical pot. energy) in me which I can use to jump(then I have KE),play , dance walk etc.?

18. Apr 24, 2005

### whozum

Mechanical Energy is the sumo f any kind of energy you have.
Kinetic energy is energy you have due to moving.
Potential energy is energy due to position or ability.

Chemical potential energy is just that, potential energy, in chemical form. Your body CAN cause the chemicals to react, producing some sort of motion (I guess).

19. Apr 26, 2005

### Staff: Mentor

It's just a definition, admittedly imperfect. The label "mechanical" is meant to distinguish certain kinds of energy (see my last post) from other types (like chemical, nuclear, thermal, etc.).

It is certainly true that you can transform one type of energy (chemical energy) to another type (mechanical energy, for instance).

I'll have to disagree with you on that one. One's total energy is the sum of all the various types of energy you might have; mechanical energy is just a part of that.