Energy Basics

The basic concepts of work, force, energy and power are fundamental physics concepts utilized in many engineering calculations and design. Every engineered device that moves, lifts or pushes requires energy. An engineer must know how to calculate the power and energy needed to do the necessary work or provide the required heat.

Most of the world uses metric units to quantify engineering terms, while the US still performs some of its engineering work using the old British system. Metric units are all based on fundamental physics quantities. That makes the metric Joule, Newton and watt much easier to use and calculate than the British BTU (British thermal unit), horse power, pound force, and slug. In this unit, metric units are used, but a few British units are included as reference point because they are more familiar for many energy measurements.

After this lesson, students should be able to:

• Define and contrast energy, work and power.
• Given mass, distance, and time, calculate work, force and power using appropriate units.
• Given the conversion formulas, convert between horsepower and kilowatts.
• Use measurement tools to apply the concepts of work, power and energy to real-life examples.

Energy is the ability to do work and is used in order to perform work.

Power is the rate at which work is done.

There are mathematical representations of work and power.

There are various units of energy, work and power.

The concepts and definitions of work, energy and power can be illustrated with a few examples:

A force is a push, pull, or twist

• Force = mass X acceleration
• Drop a ball . What is pulling it towards the ground? (Gravity. Acceleration of gravity [9.81 m/s²].)
• Who first formulated scientific understanding of gravity? (Sir Isaac Newton. We name the unit of measure of force after him.)
• 1 Newton (N) = kg *m/s²
• In the metric system, if you can remember the equation (F=m•a) then you can remember the corresponding units (or vice versa).

Energy is the ability to do work.

• Work is a force acting over a distance to move an object.
• Work = force X distance
• Metric units are measured in Joules (J=N•m)
• Reinforce the concept of work with the Student Push/Pull Demo
• Ask for two volunteers.
• Have one push against the wall and one push on a chair so that the chair moves at least 0.5 meters.

Discuss as a class:

• Did either one or both of these students do any work? The one pushing on the chair did work; the other did not. "Work" requires that an object be moved.
• Did either of these expend energy? Yes – energy contributes to doing work, but not all energy successfully does work. (Heat was generated by the student who did not do work.)

Introduce Newton's second law to have students apply their knowledge of forces, work, and energy:

• Energy transfer is described with Newton's first law. An object in motion will remain in motion unless acted on by another force.
• What happens to the energy of an object when its motion changes?

Show Newton's second law with the following example / demonstration:

Scenario 1:

• Consider a hockey puck is pushed on a table surface. Once in motion will the hockey puck remain in motion? (Students: No, the puck will stop.) Demonstrate by pushing a hockey puck, or other object, across a table.
• Well, I thought that Newton's law tells us that an object will remain in motion unless acted upon by another force. Is there another force acting on the object? (Let students respond.)
• Yes, friction is the force acting on the hockey puck, causing its motion to stop. Energy was applied to the puck via the initial push, and the friction force opposed the puck's motion causing it to stop. What happened to the energy that the puck had? (Let students respond.)
• The energy was transferred to the table (or lost) in the form of heat.

Scenario 2:

CopyrightCopyright Santeri Viinamäki, 2017. Wikimedia. https://commons.wikimedia.org/wiki/File:Ice_hockey_stick_and_puck_20170120.jpg

• Imagine the hockey puck is pushed on an air hockey table, or on ice. Will the puck remain in motion? (Let students respond. Students: Yes!) If possible, demonstrate with a mini air hockey table, or freeze water in an aluminum pie pan.
• Teacher: Why does it remain in motion? What happened to the friction? (Let students respond.)
• The air coming up from the table reduces the amount of friction between the table and the puck, so once a force is applied to the puck it will maintain its energy.
• What happens if another force acts on the object, like a hit to the puck from an air hockey player? (Let students respond.)
• The motion of the puck changes, because energy was added to the puck from the hit.

Power is how fast work is done (or the rate at which work is done).

• Power = work/time OR Power = energy used/time
• Power is now measured in units of watts after James Watt. Mr. Watt built the first steam engine. When he was selling it, he advertised to farmers and miners that it could give more power than a horse. He said that it had 1.5 horsepower. Although the unit of horsepower is still used today, it does not accurately describe how many horses it replaces because not every horse is the same.
• A watt is defined based on the equation W=J/s

Here are a few examples:

• Jane is moving into her new house and is moving furniture around. If Jane pushes a large dresser with a force of 200 N moving it 0.5 m in 2 seconds, what is her Work and Power? (Answers: W= (200 N)(0.5 m)=100 J, P=(100 J)/(2 s) = 50 W)
• A crane lifts a car that weighs 2000 kg a distance of 5 m in 2 min (convert minutes to seconds with the class: 120s). What is the Force (g=9.8 m/s²), Work and Power? (Answers: F=19600 N, W= 98000 J, P= 817 W or .082 kW)

Students will have been introduced to the global energy problem and how their choices impact the energy situation.The purpose of this lesson is to introduce students to the basic concepts of energy and to apply that knowledge in the associated activity, Human Power, to determine how much a power a human being can provide. To begin stirring thoughts on energy, consider the following questions:

• What do you think about when you hear the term "energy?"
• What do students think about when they hear the term "energy?"
• Where/how do you use energy in your lives? Name a few things that we do that use energy. Can use examples from game.
• What happens when we do not have access to energy (i.g. electric power), such as during summer blackouts or ice storms)?

Engineers and scientists have learned how to "capture" energy to run machines, provide power and heat our homes. The term "capture" is really not appropriate. Thermodynamic principles tell us that energy can neither be created or destroyed, only converted into a different form. Sometime that form helps us do work, while other times it creates heat. Most of the time, energy transformations create both heat and work.

First Day

• Give out energy basics homework.
• Reinforce energy, work and power.
• Ask the students: What did we learn today? Can anyone define work, power, energy?

Post-Introduction Assessment: Assign students the attached Homework Handout, in which they complete word problems that require calculations of force, work, energy and power.

Lesson Summary Assessment: A quiz at the end of the lesson requires students to apply knowledge to specific calculations of force, work and power, and apply some of these scientific fundamentals to the unit project.