Fundamentals of Engineering Module #5 – More Energy! Stored, Mechanical, More

what is mechanical energy

Some random atom model.

Before we start, can I just give you all a cheesy thanks?  Like most of you, I enjoy learning to a degree.  Seeing as how I started this back in 1992, this type of work not the site lol, it has been a constant learning process.

By me trying to structure a website that is helpful and informative, I have to obviously go back and learn some stuff that I haven’t looked at in a while.  And by doing so, I remember why I love engineering and energy so much.  Everything has to follow the rules and laws – Newton, Ohm.

Today, let’s go over mechanical energy.  What it is, how potential and kinetic energies factor in, stored energy maybe, work.  Makaukau (get ready in Hawaiian.)

Mechanical Energy

mechanical potential energy

Falling water generates tons of electricity.
(pic from water-technology.net)

We are concerned with 2 forms of stored mechanical energy for now.  There may be more who knows ha ha.

Mechanical potential energy exists because of the relative positions of two or more objects.  Such as:

  • a person right on the edge of a cliff that will fall freely if pushed has mechanical potential energy.
  • Water just itching to be released at the top of a dam has mechanical potential energy.
  • A cart being held at the top of a ramp has mechanical potential energy.

Mechanical kinetic energy exists because of the relative velocities of two or more objects.  You do remember velocities right?

If you shove that person (perhaps I should change that?,) open the flood gates for the dam, or let go of the cart, a something is gonna herpen.  Something will move.  The person will plunge, the water will flow, and the cart will roll down the ramp.



In all 3 examples, the mechanical potential energy will be changed to mechanical kinetic energy.  Another way to think of it is that the energy of position will be changed to the energy of motion.

About the examples, did you notice that an external source of energy is used to get things rolling like Limpbizkit?  Energy from some outside source is required to shove the unsuspecting person, open the gate of the dam, or release the cart.

Pretty much all machines and processes require this type of kick start from an energy source outside the system.   Energy can’t be pulled from thin air right?  We talked about this in an earlier post sorta.

For example, a tremendous amount of chemical energy is stored in the gasoline for your car.  This energy will not turn your engine over until you have expended some energy to cause the gas to combust.

Hold up a sec

It’s much easier to learn the basic principles of energy by forgetting about all the energy systems that might be involved in or affected by each energy process, for now.  Out in the real engineering world, building or otherwise, you have to consider it.  From now on, let’s consider only one energy process or system at a time, disregarding both the energy “kick starts” that may be received from external systems and the energy transfers that may take place.

Back at it

Notice that both mechanical potential energy and mechanical kinetic energy are stored forms of energy. It’s easy to see why we regard mechanical potential energy as being stored, but not so easy to see with mechanical kinetic energy.

Confusion arises because mechanical kinetic energy is often referred to as energy of motion, which leads to the false conclusion that energy in transition is somehow involved.  This is not the case, however.

Work is the only form of mechanical energy that can be properly considered as energy in transition.

mechanical kinetic energy

Cool pic.
(from garylellis.org)

If you have trouble with the idea that mechanical kinetic energy is stored, rather than in transition, think of it like this:

  • A bullet fired from a rifle has mechanical kinetic energy because it is in motion.
  • The faster the bullet is moving, the more kinetic energy it has.
  • We all know that the bullet has the capacity to produce an effect, so we may safely say that it has energy.
  • Although the bullet is not in transition, the energy of the bullet is not transferred to any other object until the bullet strikes some object that resists its motion.
  • When the bullet impacts a resisting object, then, and only then, can we say that energy in transition exists, in the form of heat and work.

In this example, we are ignoring the fact that some work is done against the resistance of the air and that some heat results from the friction of the air.  But this does not change the basic idea that kinetic energy is stored energy rather than energy in transition.  Think of the air as a resisting object, which causes some of the stored kinetic energy of the bullet to be converted into energy in transition (heat and work, maybe sound (not the report but the actual bullet hissing as it cuts the air)) while the bullet is flying.

However, the major part of the stored kinetic energy does not become energy in transition until the bullet hits something more firm than the air it was knifing through.


How we measure this stuff
stored energy

Potential and kinetic energies in action.

Mechanical potential energy is measured in foot-pounds (ft-lb).  Consider, for example, our unsuspecting victim at the top of the cliff.  If they weigh 100 pounds and the distance from the person to the base of the cliff is 100 feet, 10,000 ft-lb of mechanical potential energy exists because of the relative positions of the rock and the earth.

An equation for mechanical potential energy:

PE=WxD , where:

  • PE = total potential energy of the object (in ft-lb)
  • W = total weight of the object (in pounds)
  • D = distance between the earth and the object (in feet)

Mechanical kinetic energy is also measured in ft-lb.  The amount of kinetic energy present is directly related to the velocity of the moving object and to the weight of said object.

Energy Conversions

Mechanical potential energy can be changed into mechanical kinetic energy.

And this is why engineering is awesome – these laws, how to use them to get work done efficiently…

If you nudge that 100 lb person over the edge of the 100’ cliff, they what?  Fall.  As they fall, they lose potential energy and gain kinetic energy.   Too easy right?  Remember the post on pumps and how we talked about pumps having to force water up 45 stories?  And how by using the potential energy at the top of the building and transferring it to kinetic when the water comes back down…

At any given moment, the total mechanical energy (potential plus kinetic) stored in the system is the same, 10,000 ft-lb.  The proportions of potential energy and kinetic energy are changing all the time as they are falling.  Just before they hit the earth, and their body becomes one with their shadow, all the stored mechanical energy is kinetic energy.  As they hit, the kinetic energy is changed into energy in transition, work and heat.

Mechanical kinetic energy can likewise be changed into mechanical potential energy.

potential kinetic energies

Simple baseball example
(from thescienceroom.pbworks.com)

Let’s say you throw a baseball straight up into the air.  The ball has kinetic energy while it is in motion, but the kinetic energy decreases and the potential energy increases as the ball travels upward.  When the ball has reached its zenith, apex, top point, just before it starts to fall back to earth, it has only potential energy.

Then, as it falls back toward the earth, the potential energy is changed into kinetic energy again.  Friggin’ awesome.  And what is bringing it back to earth?  Another law – gravity.

Work son

Mechanical energy in transition is called work.

When an object is moved through a distance against a resisting force, work has been done.  The formula for calculating work is:

W=F×D, where:

  • W = work
  • F = force
  • D = distance

So to find out how much work is done you need to know how much force is exerted and the distance through which the force acts.  The unit of force is the pound.  When work is done against gravity, the force required to move an object is equal to the weight of the object.

Why?  Weight is a measure of the force of gravity or a measure of the force of attraction between an object and the earth.

How much work will you do if you carry that 100 lb person from the bottom of the cliff back to the top?

You will do 10,000 ft-lb of work—the weight of the object (100 pounds) times the distance ( 100 feet) that you move it against the force of gravity.

Other examples

We also do work against forces other than the force of gravity.

When you push an object across the floor, you are doing work against friction.  The force you work against is not only the weight of the object (gravity drawing it to earth,) but also the force required to overcome the friction and slide the object over the surface.  This is why carpet is harder than tile, way more friction.

If you are unable to generate enough force to overcome gravity, friction, etc., the object ain’t movin’.

Notice that mechanical potential energy, mechanical kinetic energy, and work are all measured in the same unit, ft-lb.

One ft-lb of work is done when a force of 1 pound acts through a distance of 1 foot.  One ft-lb of mechanical potential energy or mechanical kinetic energy is the amount of energy that is required to accomplish 1 ft-lb of work.

horsepower

One horsepower.

The amount of work done has nothing at all to do with how long it takes to do it.  When you lift a weight of 1 pound over a distance of 1 foot, you have done 1 ft-lb of work, whether you do it in half a second or half an hour.

The rate at which work is done is called power. The common unit of measurement for power is the horsepower (hp).

By definition, 1 hp is equal to 33,000 ft-lb of work per minute or 550 ft-lb of work per second.  So a machine that is capable of doing 550 ft-lb of work per second is said to be a 1-hp machine.

Conclusion

Whew.  Thank god for the internet, reference materials, and the nuclear Navy.   In this post we covered mechanical energy.  This brought us to talking about forms of stored energy and energy in transition.  It also went over the equations for both mechanical potential and kinetic energies as well as work.  I used some killer examples where we shoved a person off a cliff to measure their energies ha ha.  And lastly, we defined a horsepower.

The quiz for this module is up.  I realize some of you will probably never see this edit lol.  If you think you have learned enough about stored mechanical energies to pass my quick quiz, click here to take it.

Question, comment, disagreement?  Let us know.  The next post is up, module #6, and it covers thermal energy.  Now we’re getting somewhere.