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# HVAC Basic Science: Force & Energy

Updated: Jul 5

## In this unit, we will look at force and motion of an object and how they depends on its mass. We will also see what is moment arm (or lever arm) and how it is related to force. We will take a look at types of mechanical energy. Skip to quiz!

### Force

Force is what we use to make objects move. We do this by pushing and pulling the object.

For example, when riding a bike, we push on the pedals to go faster. To go slower, we pull on the brakes. The push and pull that makes the bike move is called force. We use Newton to measure force. We represent Newton with the capital letter N. One Newton is the force it will take us to move a 1 kg mass at a rate of 1 m/s². Meaning, the mass is moving 1 meter for every second that passes by.

Pressure is the force over an area. Using the previous bike example, when we push on the bike’s pedal, there is a force on the pedal. Pressure measures how much force is on the pedal.

We use pascals to measure pressure. We represent Pascals with Pa. Using the 1 kg block from the newton example, say it has 1 m². 1 Pascal is the pressure on the 1 m² block because of the 1 N force applied.

### Movement

Motion is when something moves from its position. A position is where we are at now. If we move from our position, then we are in motion.

Let us take the bike riding example from before. When we ride a bike, we move the bike from one place to another by using force. The movement of the bike from one place to another is called motion.

If the bike is heavy, the motion of the bike will be slower. If the bike lighter, the motion of the bike will be faster. How heavy and light the bike depends on the mass of the bike.

Mass is the amount of substance something has. If we fill an empty bottle with water, we make the bottle heavier. Here, water is the substance. The more substance something has, the heavier it is.

We have to make sure we do not confuse mass with weight. Weight is related to gravity. For this course, we will only focus on mass.

The heavier the mass of an object, the more force we need to put the object in motion. Let us look at how we can increase the force to lift heavier objects.

Levers help us to lift things that are hard to lift on our own. It does this by increasing the amount of force we put in it. This increased force is the leverage that helps us to lift things.

We can increase the amount of force the leverage puts out by increasing the moment arm. Let us define the moment arm.

The moment arm is the perpendicular distance from the force to the axis of rotation. Axis of rotation is the point the object rotates around. The higher the moment arm distance, the more force the leverage arm outputs.

Say I am on a seesaw by myself. By pushing the seesaw, I place force on the end I sat on. The distance between me and the midpoint of the seesaw is called the moment arm. We use a meter (m) to measure the moment arm since it is a distance.

There are two types of rotation of the moment arm:

• Positive rotation, and

• Negative rotation

The rotations depend on the direction of the force.

For example, if we sit on the right side of the seesaw, it would make the seesaw go down in a clockwise direction. If this is the case, we say that the moment arm is positive.

If we sit on the left side of the seesaw, it will make the seesaw go down in a counter-clockwise direction. If this is the case, we say that the moment arm is negative.

### Mechanical Energy

Energy allows us to do everyday tasks. There are many different types of energy. Mechanical energy is one type of energy.

Recall that when an object changes its position, it is in motion. When the object is in motion or is about to be in motion, the object has energy. This energy is called mechanical energy.

There are two types of mechanical energy:

• Kinetic Energy, and

• Potential Energy

Kinetic and potential energy depends on if an object is moving or is about to move.

An object has kinetic energy when it is in motion. Recall that motion is when an object moves from its position.

Energy stored in an object, not in motion, is called potential energy. As mentioned in the video, the potential energy of an object converts to kinetic energy when released.

An object that moves is said to be in motion. We need to apply force to set it in motion. The amount of force depends on how much mass the object has

An object that is in motion will have kinetic energy. An object that is about to be in motion will have potential energy.

## In this unit, we will define types of energy. We will also define power and learn how to convert between the units of power and energy. Skip to quiz!

### Types of Energy

Recall, energy is the ability to do work. Also, recall that kinetic and potential energy are the two types of mechanical energy. Let’s take a look at other types of energy :

• Light energy,

• Magnetic Energy, and

• Thermal energy

Light energy is a type of energy that we can see. Using light energy, we can see everything around us. The most common source of light energy is the sun.

We all have seen and used magnets. A magnet pulls metal objects made of iron, nickel, and cobalt towards itself. As mentioned in the video, the attraction between the metal and magnet is called magnetic energy.

Thermal energy is also called heat energy. It transfers heat from a hot to a cold place.

There are three types of heat in physics:

• Specific,

• Latent, and

• Sensible heat

Let us look at the three types of heat one by one.

Specific heat is the amount of heat needed to raise the temperature of 1 kg substance by 1℃. The specific heat of each substance is different. This is because different substances need different amounts of heat to raise their temperature.

Similar to specific heat, latent heat is also used on different substances. But, latent heat is used, not heat a substance. It is used to change a substance’s phase. Before we define latent heat more, let us first understand what phases are.

All the matter around us have three phases:

• Solid,

• Liquid, and

• Gas

Using many ways, we can make a matter go from one phase to another.

One way to change phases is by using latent heat. It is a type of thermal energy that changes matters’ phases without changing the temperature of the matter.

One way to change phases is by using latent heat. It is a type of thermal energy that changes matters’ phases without changing the temperature of the matter.

During phase change, the temperature of the matter does not change. This happens because all the thermal energy from the fire is used to separate the matter's molecules during phase change. None of the energy is used to increase the temperature of the matter.

Sensible heat is the heat that we can feel. For example, when we stand next to a campfire, we can feel the warmth of the fire. The warmth we feel is called sensible heat.

Sensible heat is the heat that increases the temperature of matter. But it does not change the phase of the matter. After the temperature has been increased, the matter's phase changes using latent heat.

There are many different units used for energy, such as:

• Joules (J), and

• British Thermal Unit (BTU).

Joules is the unit of energy in the metric system. BTU is the unit of energy in the imperial system.

Recall 1 Newton is the force it will take us to move a 1 kg mass at a rate of 1 m/s². Now, 1 Joule is the energy it will take to apply 1 newton of force over a distance of 1 meter.

1 BTU is the amount of heat we need to raise 1 pound of water to 1℉. As we saw in the many things around us gives off heat energy, including our computers.

### Power

Recall that we need the energy to do work. But how much work can we do? Power measures the amount of work we do in a given amount of time.

Similar to energy, power also has many units, such as:

• Horsepower (hp).

• Watts (W), and

Horsepower is the imperial unit for power. Watt is the metric unit for power.

Most of the time, we use horsepower to measure the power of an engine. 2 hp is equal to the power needed to lift 550 pounds over a distance of a foot for every second.

1 Watt of power is equal to 1 J/s. Meaning, it will take us 1 Watt of power to get 1 Joule of energy every second.

### Unit Conversion

As we have already discussed, a watt is one of the units of power. And, kilowatt (kW) is equal to 1000 watts. Recall, power is energy per time.

We write power using the following equation:

Power = Energy

Time

Power x Time = Energy

kW.h = Energy

This is why, kW-hr is a unit of energy.

Recall that BTU is a unit of energy. Also, recall that power is the energy per time. If we divide BTU by time, we will get a unit of power.

We can avoid doing all the calculations by using google to convert. It is the easiest way to convert between any units. Let's take a look at an example by converting BTU to kW-hr.

Here, we can write in the search bar when converting from BTU to kW - hr. Write the amount of BTU we to convert to KW - hr. We are converting 2 BTU to kW - hr, just like we did in the previous question.

When we hit search, two boxes will appear. Here, we are shown that the box on the left is BTU we are converting. The box on the right is the converted amount of kW - hr. As expected, our answer matches the question: 5.86 x 10⁻⁴ kW - hr.

This unit, we learned about three more types of energy: light, magnetic, and heat energy. We also learned about the units of heat and power. Lastly, we learned to convert the units using mathematics and Google.

## In this unit, we will define open and closed systems. We will also discuss how heat and energy move through the systems. Skip to quiz!

### Energy Transfer

Recall, energy is the ability to do work. We cannot destroy or create energy. We can only convert energy into different forms.

One type of energy can convert into another. Recall, potential energy converts into kinetic energy to make an object move.

An open system is exposed to the environment. Meaning things from the environment can go into the system. Also, things from the system can go into the environment.

On the other hand, a closed system is a cut-off from the environment. Nothing can go into the system from the environment. Also, nothing can go out of the system into the environment.

### Thermodynamics

Based on the definition of an open system, we are all considered an open system. We are constantly exchanging energy with our environment. Every time we exchange energy with the environment, the exchange must follow the laws of physics.

There are three laws for energy to transfer in physics. They are called the laws of thermodynamics.

• The first law of thermodynamics

• The second law of thermodynamics, and

• The zeroth law of thermodynamics

Conservation of energy is the first law of thermodynamics. Recall that energy cannot be created or destroyed. Energy can only convert into different forms. This is the definition of conservation of energy. Recall from the video that not all of the energy is fully converted. Some of the energy is released into the environment as heat or other energy. But what will happen to the energy when it is in a closed system?

The second law of thermodynamics explains what happens to energy in a closed system. It states that the entropy of a closed system always increases. But what is entropy?

Entropy measures how to spread out the energy is in a closed system. The energy spreads around more and more as time passes. This is known as entropy.

If a hot object is placed next to a cold object, the cold object will get hotter. Recall that heat flows from the hotter object to the cooler object. Over time, the energy from the hot object spreads into the systems making the cold object less cold.

Say we mix a hot substance with a cold substance. Because of entropy, over time, the heat from the hot substance will transfer to the cold substance. But does the heat transfer forever?

The zeroth law of thermodynamics states that the hot and the cold substance will mix until they reach thermal equilibrium.

Heat transfers from a hot object to a cold object until they are at thermal equilibrium. Meaning the two objects have the same temperature. Let us see how heat is transferred between different objects.

### Heat Transfer

There are three ways heat can travel:

• Conduction,

• Convection, and The way heat travels between something depends on the material.

In solid objects, heat is transferred using conduction. Let us understand how conduction works using the following examples.

Particles of a solid object transfer their energy to nearby particles. Solids such as metal are the best at transferring heat by conduction. Liquid and gas cannot transfer heat by conduction.

We transfer heat through a fluid using convection. Fluid can be any liquid and gas. Let us see how heat transfers between fluids by conviction.

The hot particle in fluid moves around to transfer heat. This is different from conduction because, in conduction, only the heat of the particles is transferred. But in convection, the particles themselves move to transfer heat.

In both conduction and convection, particles gain heat energy. The heat energy then gets transferred to other particles. But we can transfer heat without particles using radiation.

Heat is transferred by radiation waves without physically touching an object. Even if we don’t touch a hot object, radiation can still transfer the heat.

The most common example of radiation is the sun’s heat. We can feel the sun’s heat without touching it because the radiation transfers the heat using waves.

To summarize, energy can flow in two systems:

• Open system, and

• Closed system

All the energy that flows in the two systems must follow the three laws of thermodynamics.

A matter determines how heat flows through it. Solid matter transfers heat by conduction. Fluids transfer heat by convection. Heat transferred by radiation doesn’t need any matter in radiation, heat flow through waves.

Question #1: We make objects move by pushing and pulling.

1. True

2. False Force is created by pushing and pulling objects.

Question #2: What is the unit of measurement for pressure?

1. Pascals

2. Newtons

3. Inches

4. Meters We use Pascals to measure pressure.

Question #3: Motion is when something changes its position.

1. True

2. False Motion is when something moves from its position.

Question #4: There are no differences between mass and weight.

1. True

2. False Mass and weight are different. Mass is how much substance something has. Weight is related to gravity.

Question #5: Levers helps us _____ things.

1. Move

2. Lift

3. push

4. None of the above We use levers to help us lift things.

Question #6: Is the moment arm positive or negative when an object moves counter-clockwise?

1. Positive

2. Negative

3. Minus

4. None of the above Counter-clockwise rotation is negative.

Question #7: Potential energy is the energy stored in an object.

1. True

2. False Energy stored in an object, not in motion, is called potential energy.

Question #8: Heat is a form of energy that is transferred from _______ temperatures to ______ temperatures.

1. cold, colder

2. hot, cold

3. hot, hotter

4. cold, hot Heat transfers from hot temperatures to cold temperatures.

Question #9: Latent heat changes the phase of a matter without changing its temperature.

1. True

2. False Latent heat is thermal energy that changes matters’ phases without changing the temperature of the matter.

Question #10: How many joules are in 1 Newton meter?

1. 1 J

2. 10 J

3. 100 J

4. 20 J The size 5/16” - 15 means the bolt has a diameter of 5/16” and has 15 threads per inch.

Question #11: How many kWh of energy is in 2 BTU?

1. 5.86 x 10⁻⁴ kWh

2. 5.86 kWh

3. 5.86 x 10⁻³ kWh

4. 5.86 x 10⁴ kWh Multiply 2 by 2.93 x 10⁻⁴.

=2( 2.93 x 10⁻⁴)

= 5.86 x 10⁻⁴

Question #12: How many BTU/hr of power is in 2 kW?

1. 6824 BTU/hr

2. 6824 BTU - hr

3. 3412 BTU/hr

4. 6824 BTU Multiply 2 by 3412.

=2( 3412)

= 6824

Question #13: Which of the following is a closed system?

1. People

2. A cup with a lid on

3. A cup with a lid off

4. None of the above Answer: A cup with a lid on

A cup with a lid on cannot have anything going in or out. This is why it is a closed system.

Question #14: Scientist can create energy.

1. True

2. False The first law of thermodynamics states that energy cannot be created or destroyed. Energy can only convert into different forms.

Question #15: Entropy measures how spread out the energy is in a closed system.

1. True

2. False Entropy measures how spread out the energy is in a closed system.

Question #16: When does heat stops transferring from a hot to a cold object?

1. At thermal equilibrium

2. When the objects have the same temperature

3. When the hot object becomes cold

4. Never Heat stops transferring from a hot to a cold object when they are at thermal equilibrium. Meaning the objects have the same temperature.

Question #17: How is heat transferred in fluids?

1. Convection

2. Conduction

4. None of the above We transfer heat through a fluid using convection.