Energy

Energy

What is Energy? 

The simplest definition of energy is "the ability to do work". Energy is how things change and move. It's everywhere around us and takes all sorts of forms. It takes energy to cook food, to drive to school, and to jump in the air. 

Different forms of Energy 

Energy can take a number of different forms. Here are some examples:

• Chemical - Chemical energy comes from atoms and molecules and how they interact.
• Electrical - Electrical energy is generated by the movement of electrons.
• Gravitational - Large objects such as the Earth and the Sun create gravity and gravitational energy.
• Heat - Heat energy is also called thermal energy. It comes from molecules of different temperatures interacting.
• Light - Light is called radiant energy. The Earth gets a lot of its energy from the light of the Sun.
• Motion - Anything that is moving has energy. This is also called kinetic energy.
• Nuclear - Huge amounts of nuclear energy can be generated by splitting atoms.
• Potential - Potential energy is energy that is stored. One example of this is a spring that is pressed all the way down. Another example is a book sitting high on a shelf.

Units of Measure for Energy 

In physics, the standard unit of measure for energy is the joule which is abbreviated as J. There are other units of measure for energy that are used throughout the world including kilowatt-hours, calories, newton-meters, therms, and foot-pounds. 

Law of Conservation of Energy 

This law states that energy is never created or destroyed, it is only changed from one state to another. One example is the chemical energy in food that we turn into kinetic energy when we move. 

Renewable and Nonrenewable 

As humans we use a lot of energy to drive our cars, heat and cool our houses, watch TV, and more. This energy comes from a variety of places and in a number of forms. Conservationists classify the energy we use into two types: renewable and nonrenewable. Nonrenewable energy uses up resources that we cannot recreate. Some examples of this are gas to run our car and coal burned in power plants. Once they are used, they are gone forever. A renewable energy source is one that can be replenished. Examples of this include hydropower from turbines in a dam, wind power from windmills, and solar power from the sun. 

The more renewable power we use the better for our planet and for future generations as they won't run out of resources someday. 

Fun Facts about Energy

• In 2008 about 7% of the energy used in the United States was from renewable sources.
• A modern windmill or turbine can generate enough electricity to power around 300 homes.
• People have used waterpower to grind grain for over 2,000 years.
• Geothermal power uses energy from geysers, hot springs, and volcanoes.
• The entire world could be powered for a year from the energy from the sun that falls on the Earth's surface in one hour. We just need to figure out how to harness it!3

Laws of Motion

Laws of Motion

A force is anything that can change the state of motion of an object, like a push or a pull. You use force when you push a letter on the computer keyboard or when you kick a ball. Forces are everywhere. Gravity acts as a constant force on your body, keeping you secure on planet Earth so you don't float away. 

To describe a force we use the direction and strength. For example when you kick a ball you are exerting force in a specific direction. That is the direction the ball will travel. Also, the harder you kick the ball the stronger the force you place on it and the farther it will go. 

Laws of Motion 

A scientist named Isaac Newton came up with three Laws of Motion to describe how things move scientifically. He also described how gravity works, which is an important force that affects everything. 

First Law of Motion 

The first law says that any object in motion will continue to move in the same direction and speed unless forces act on it. 

That means if you kick a ball it will fly forever unless some sort of forces act on it! As strange as this may sound, it's true. When you kick a ball, forces start to act on it immediately. These include resistance or friction from the air and gravity. Gravity pulls the ball down to the ground and the air resistance slows it down. 

Second Law of Motion 

The second law states that the greater the mass of an object, the more force it will take to accelerate the object. There is even an equation that says Force = mass x acceleration or F=ma. 

This also means that the harder you kick a ball the farther it will go. This seems kind of obvious to us, but having an equation to figure out the math and science is very helpful to scientists. 

Third Law of Motion 

The third law states that for every action, there is an equal and opposite reaction. This means that there are always two forces that are the same. In the example where you kicked the ball there is the force of your foot on the ball, but there is also the same amount of force that the ball puts on your foot. This force is in the exact opposite direction. 

Fun facts about Forces and Motion

• It is said that Isaac Newton got the idea for gravity when an apple fell off a tree and hit him on the head.
• Forces are measured in Newtons. This is after Isaac Newton, not fig newtons, even if they are tasty.
• Gases and liquids push out in equal forces in all directions. This is called Pascal's Law because it was discovered by the scientist Blaise Pascal.
• When you go upside down in a roller coaster loop-the-loop, a special kind of force called "centripetal force" keeps you in your seat and from falling out.

Learn English Conversation Unit 21 At the Restaurant

Learn English Conversation Unit 21 At the Restaurant

Friction

Friction

What is friction? 

Friction is the resistance of motion when one object rubs against another. Anytime two objects rub against each other, they cause friction. Friction works against the motion and acts in the opposite direction. 

Friction and Energy 

When one object is sliding on another it starts to slow down due to friction. This means it loses energy. However, the energy doesn't disappear. It changes from moving energy (also call kinetic energy) to heat energy. This is why we rub our hands together when it's cold. By rubbing them together we generate friction and, therefore, heat. 

The force F of friction pushes back on the block.

Preventing Friction 

In some cases we want to prevent friction so it's easier to move. A good example of this is a ball or wheel. They roll to help reduce friction. Another way to reduce friction is with a lubricant like grease or oil. Machines and engines use grease and oil to reduce friction and wear so they can last longer. 

Another way to reduce friction is to change the types of materials in contact with one another. For example, ice contacting with steel would produce less friction than rubber would on concrete. This is why ice skates slide so easily on the ice, but you don't slip when wearing rubber shoes on the sidewalk. These different materials are said to have different "coefficients of friction". 

Using Friction 

Friction is also a great help to us. After all, we would all just be sliding around everywhere if there wasn't friction to keep us steady. Friction is used in car brakes, when we walk or climb a hill, making a fire, skiing down a hill, and more. 

Experiment with Friction 

Different types of surfaces create different amounts of friction. Some materials are much smoother than others. Take three flat objects with different types of surfaces. Set them on one end of a tray and slowly lift it. The item with the least friction will start to slide first. 

There are two main factors that will influence the total amount of friction: 1) the roughness of the surfaces (or the "coefficient of friction") and 2) the force between the two objects. In this example, the weight of the object combined with the angle of the tray will change the force between the two objects. Play around with different objects and see how these two factors change the friction. 

Types of friction

• Dry Friction - This is what we've been mostly talking about here. Dry friction occurs when two solid objects touch each other. If they are not moving, it is called static friction. If they are moving, it is called kinetic or sliding friction.
• Fluid Friction - Fluid friction involves a fluid or air. The air resistance on an airplane or water resistance on a boat is fluid friction.
• Rolling Friction - Rolling friction occurs when a round surface rolls over a surface, like a ball or wheel.

Fun facts about Friction

• Although wheels are great for rolling and reducing friction, they couldn't work without friction.
• It would be really tough just to stand up without friction.
• Friction can generate static electricity.
• The harder two surfaces are pressed together, the more force it takes to overcome the friction and get them to slide.
• Fluid friction is used a lot in water parks so we can slide smoothly and fast down giant slides.

Черные дыры - от рождения до смерти

Gravity

                   Gravity

What is gravity? 

Gravity is the mysterious force that makes everything fall down towards the Earth. But what is it? 

It turns out that all objects have gravity. It's just that some objects, like the Earth and the Sun, have a lot more gravity than others. 

How much gravity an object has depends on how big it is. To be specific, how much mass it has. It also depends on how close you are to the object. The closer you are, the stronger the gravity. 

Why is gravity important? 

Gravity is very important to our everyday lives. Without Earth's gravity we would fly right off it. We'd all have to be strapped down. If you kicked a ball, it would fly off forever. While it might be fun to try for a few minutes, we certainly couldn't live without gravity. 

Gravity also is important on a larger scale. It is the Sun's gravity that keeps the Earth in orbit around the Sun. Life on Earth needs the Sun's light and warmth to survive. Gravity helps the Earth to stay just the right distance from the Sun, so it's not too hot or too cold. 

Who discovered gravity? 

The first person who dropped something heavy on their toe knew something was going on, but gravity was first mathematically described by the scientist Isaac Newton. His theory is called Newton's law of universal gravitation. Later, Albert Einstein would make some improvements on this theory in his theory of relativity. 

What is weight? 

Weight is the force of gravity on an object. Our weight on Earth is how much force the Earth's gravity has on us and how hard it is pulling us toward the surface. 

Do objects fall at the same speed? 

Yes, this is called the equivalence principle. Objects of different masses will fall to the Earth at the same speed. If you take two balls of different masses to the top of a building and drop them, they will hit the ground at the same time. There is actually a specific acceleration that all objects fall at called a standard gravity, or "g". It equals 9.807 meters per second squared (m/s²). 

Fun facts about gravity

• Ocean tides are caused by the gravity of the moon.
• Mars is smaller and has less mass than Earth. As a result it has less gravity. If you weigh 100 pounds on Earth, you would weigh 38 pounds on Mars.
• The standard gravity from Earth is 1 g force. When riding a roller coaster you may feel a lot more g forces at times. Maybe as much as 4 or 5 g's. Fighter pilots or astronauts may feel even more.
• At some point when falling, the friction from the air will equal the force of gravity and the object will be at a constant speed. This is called the terminal velocity. For a sky diver this speed is around 122 miles per hour!

Acceleration

Acceleration

When we discussed velocity and speed, we assumed a constant velocity. However, this is rarely the case in the real world. In the real world the velocity of an object in motion is often changing. 

What is acceleration? 

Acceleration is the measurement of change in an object's velocity. When you press down on the gas pedal in a car, the car surges forward going faster and faster. This change in velocity is acceleration. 

The equation for calculating acceleration is: 

Acceleration = (change in velocity)/(change in time) 
or 
a = Δv ÷ Δt

How to Measure Acceleration 

The standard unit of measurement for acceleration is meters per second squared or m/s². You can calculate this from the above formula where velocity is meters per second and time is in seconds. 

Acceleration is a Vector 

In physics acceleration not only has a magnitude (which is the m/s² number we discussed above), but also has a direction. This makes acceleration a vector. 

Force and Acceleration 

Newton's second law of motion states that the force on an object equals the mass times the acceleration. This is written in the following equation: 

Force = mass * acceleration 
or 
F = ma

We can use this formula to also figure out the acceleration if we know the mass and force on an object. This formula is: 

acceleration = force/mass 
or 
a = F/m

Constant Acceleration 

When an object is changing velocity by a constant amount over time, this is called constant acceleration. An object with constant positive acceleration will be going faster and faster. Its velocity will be increasing constantly. 

Interval 1st second 2nd second 3rd second

Acceleration 5 m/s2 5 m/s2 5 m/s2

Velocity 10 m/s 15 m/s 20 m/s

An example of constant acceleration of 5 m/s².

Free Fall: A Type of Acceleration 

One example of constant acceleration is an object in free fall. During free fall, gravity applies a constant force on the object causing a constant increase in velocity. If you were to measure the distance an object fell, each second it would fall further because it is constantly picking up speed. 

Note: In the real world there would be the additional force of air friction on the object. At some point the object would reach "terminal velocity". This means that it would no longer accelerate and the speed of the fall would stay the same. The terminal velocity of a skydiver falling face down is around 122 miles per hour. 

Average Acceleration 

The average acceleration is the total change in velocity divided by the total time. This can be found using the equation a = Δv ÷ Δt. 

For example, if the velocity of an object changes from 20 m/s to 50 m/s over the course of 5 seconds the average acceleration would be: 

a = (50 m/s - 20 m/s) ÷ 5s 
a = 30 m/s ÷ 5s 
a = 6 m/s² 

Deceleration or Negative Acceleration 

When the velocity of an object decreases (slows down) this is called deceleration. It may also be represented by a negative acceleration. This means the direction or vector of the acceleration is pointing in the opposite direction of the movement of the object. 

For example, if the velocity of an object changes from 40 m/s to 10 m/s over a time interval of 2 seconds the average acceleration would be: 

a = (10 m/s - 40 m/s) ÷ 2s 
a = -30 ms ÷ 2s 
a = -15 m/s² 
This could also be called a deceleration of 15 m/s².