Mechanical energy surrounds us in the natural world and in the machines we build. Look around, and mechanical energy is found everywhere.
There are many types of mechanical energy, from a teenager riding a bike to high school to a wrecking ball demolishing a house.
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Read on and to discover more about mechanical energy.
What Is Mechanical Energy?
First, let’s consider what energy is. Energy is the ability to do work. There are many forms of energy, and mechanical energy is one of them.
Mechanical energy is defined as the ability of an object to do work. The amount of work an object can do depends on two things: its position and its motion.
We need to understand more about how an object’s position and motion affect its energy before looking further at mechanical energy.
What Is Potential Energy?
Potential energy is the energy of position, also called stored energy. Its energy exists because of the position of objects relative to each other. That’s to say, the energy of the object changes when the object’s position changes or when the object moves position.
For example, a book on the floor has relatively little potential energy — it has minimal capacity to move by itself or interact with other still or moving objects. Now, let’s lift that book and balance it on the edge of a shelf. It has a lot more potential energy at this point. Why? Because the book could fall and squash a passing bug, or hurt your toe, or make a loud bang when it hits the floor.
We have given the book gravitational potential energy. The book is held in a vertical position by the shelf. The Earth’s gravitational forces have given the book its stored energy of position, or potential energy. Suppose the mass of the object is greater. In that case, swap the book out for a 10-pound dumbbell — the object’s gravitational potential energy increases, too.
There is a second form of potential energy related to mechanical energy called elastic potential energy. This is potential energy that gets stored in an object that can be compressed or stretched.
Think of an archer stretching back the elastic string on a bow. The tensed string has elastic potential energy. Once released, the elastic string will propel an arrow forwards.
Both gravitational and elastic potential energies are energies of position and equally important factors when discussing mechanical energy.
What Is Kinetic Energy?
Kinetic energy refers to the energy of motion and movement. Think back to our example of the book, balanced on a shelf, full of potential energy.
Should the book fall to the ground, it has kinetic energy during its tumble — the book’s energy changes from that of potential energy to kinetic energy (motion). It’s the same with the arrow — its potential energy in a drawn bow becomes kinetic energy when the arrow fires and moves through the air.
Understanding Mechanical Energy, Potential Energy, and Kinetic Energy
As mentioned, mechanical energy is the ability of an object to do work. It’s also the sum of the potential energy of an object (gravitational or elastic) and its kinetic energy. Let’s find out how all three interact.
The law of conservation of energy states that energy cannot be created or destroyed. Energy can only convert from one energy form into another.
This law is vital for mechanical energy because it means we can understand the net force of a mechanical system. We can work out the mechanical energy of an object, because the total energy (potential + kinetic) doesn’t change.
What Are Examples of Mechanical Energy?
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Everyday life is full of mechanical energy. Here five examples:
- A wrecking ball that demolishes buildings has potential energy when the heavy ball is poised at the top of its swing. When the ball is released, starting a circular motion, it has kinetic energy. When the ball hits the building, it applies force to the building — mechanical energy — the ability of an object to do work. In this case, the work done is by the ball destroying the building.
- Wind turbines use mechanical energy to turn wind energy into electrical energy. The energy in the wind works on the blades by rotating them, which turns the turbines that create electricity. The wind has caused the work to be done using mechanical energy.
- A cyclist standing next to their bike has chemical potential energy thanks to the food in their stomach. A chemical process releases the chemical energy in the food to allow the cyclist to hop on their bike and apply force to the pedals. Pedaling is a form of mechanical energy.
- Similarly, a bowling ball has more potential energy once we pick it up at the bowling alley. Once we bowl it down the alley, that potential energy is turned into kinetic energy. The ball now has the ability to perform “work” on whatever it may encounter. In this case, it will (hopefully!) be the pins. It is mechanical energy that displaces the pins for what will be a strike.
- Nuclear energy comes from power plants that use heat from fission to turn electric generators to produce electricity. The turning of the generators is mechanical energy.
How Do You Calculate Mechanical Energy?
Mechanical energy is the sum of the potential energy of an object and an object’s kinetic energy. Both potential energy and kinetic energy are measured in joules.
We can find out an object’s gravitational potential via this formula: PE = mgh.
- PE is potential energy
- m is its mass in kilograms
- g is the gravitational acceleration of the earth (defined as 9.8 m/sec2)
- h is the height of the object above the earth’s surface in meters
If we hold a four-kilogram hammer (8 pounds, 13 ounces) some 1.5 meters (five feet) off the ground, its potential energy would be:
PE = 4 (kg) x 9.8 x 1.5 = 58.8 joules.
The formula for kinetic energy is KE = ½ m v2.
- KE is kinetic energy
- m is the object’s mass
- v is the object’s speed in meters per second
Let’s bash a nail into the wall with our four-kilogram hammer and assume we move at 20 meters per second (65 feet per second). That gives:
KE = 0.5 (½) x 4 (kg) x 20 (velocity) squared = 800 joules.
Mechanical energy is PE + KE, giving the hammer 858 joules of mechanical energy.
Factors Affecting an Object’s Mechanical Energy
Mechanical energy is subject to outside influences that affect its values. In many mechanical energy examples, energy is lost when the “work” is being completed.
Not all an object’s potential and kinetic energy always becomes mechanical energy. There is often a seepage of energy.
What Is the Conservation of Mechanical Energy?
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Picture a roller coaster moving along a straight track, using kinetic energy. The roller coaster has the exact amount of energy to reach the peak of an approaching climb, stopping at the top where it remains stationary. Its kinetic energy has converted into potential energy.
Inch by inch, the roller coaster summits the peak and descends the other side. The potential has been converted into an equivalent amount of kinetic energy.
The roller coaster car’s total mechanical energy hasn’t changed during the peaks and descents nor during the conversion from kinetic energy to potential energy and back to kinetic energy.
The energy of the system has remained the same. This is called the Principle of Conservation of Mechanical Energy.
Is Mechanical Energy Affected by Non-Conservative Forces?
The Principle of Conservation of Mechanical Energy assumes that the total mechanical energy of an object doesn’t change. (In our example, that would be the roller coaster.) No other force acts upon it. As such, no energy is lost as our roller coaster changes its kinetic energy for potential energy.
Of course, not every energy transfer is so simple. External forces and frictional forces play a part in the equation, some physical science behind the theoretical. Energy can get lost. How?
Frictional forces, also known as non-conservative forces, take energy away from the system. These forces include the roller coaster’s air resistance or thermal energy lost as the wheels heat the tracks. The system cannot recuperate this lost energy.
How Much Mechanical Energy Is Lost in a Collision?
Collisions are another way mechanical energy can be affected. There are two types of collisions.
An elastic collision is straightforward to understand in energy terms — no kinetic energy is lost in this scenario, and the system’s energy remains the same.
Imagine two trolleys traveling towards each other at the same velocity. Eventually, they bang into each other. The collision sets them off in a new direction but without any speed loss. This collision is defined as a perfectly elastic collision with no kinetic energy loss.
Such a perfectly elastic collision is probably unrealistic in real life. Collisions between atoms in gases are a better example of elastic collisions. A Newton’s Cradle is perhaps the closest practical example of an elastic collision, where negligible kinetic energy is lost as the balls swing back and forth.
An inelastic collision occurs when kinetic energy is lost in a collision. If those two trolleys collide and then continue their journey at a slower speed, then kinetic energy has been lost. Such a scenario is much more likely than an elastic collision.
If you bounce a ball and it doesn’t bounce up as high as the height from which it was dropped, that is an inelastic collision. A bowling ball smashing into pins is an inelastic collision because the bowling ball slows down after contact.
Who Discovered Mechanical Energy Could Be Lost?
James Prescott Joule, an English physicist, mathematician, and brewer, was a seemingly unlikely discoverer of the effects of energy loss in mechanical energy.
Joule had no formal physics education but a keen interest in mechanics. He observed and studied heat generated through various sources, including the stirring of water by a paddle and the expansion of a gas into a vacuum. Joule’s understanding that heat could be created by mechanical work changed scientific thinking in the 19th century.
His work became the backbone of the principle of conservation of energy and the First Law of Thermodynamics. This law states that heat is an energy that cannot be created or destroyed but can be transferred or converted into another energy type.
Understanding the Definition of Mechanical Energy
Mechanical energy is one of the most common types of energy. It describes the ability of an object to perform some form of work.
We see mechanical energy in action every day as we pick up stationary objects and make them perform tasks for us, from shooting hoops to playing the guitar.
Mechanical energy is the energy or power that objects have through motion or changing position.
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