The reason why we put the forces into two different categories will be obvious when we start solving physics problems, but for now, you can think of it like this: when total amount of work is done on an object is done by only internal forces, energy will change forms (like going from kinetic to potential energy), and the total amount of mechanical energy is conserved, and the forces are also conserved. When the total amount of work done is done by an external force, the forces are not conserved and the object with either gain or lose energy. [am4show have='p9;p58;' guest_error='Guest error message' user_error='User error message' ]
balldroppeke As Phillip holds the ball at the top of the building, the ball has 100 Joules of potential energy (the number is just an example). When he drops it, the potential energy of the ball drops since the height of the ball gets less and less. At the same time, however, kinetic energy increases because the speed of the ball increases. All the way down, the sum of the two energies equals 100. No energy gets lost, it only gets transferred. Energy is conserved. Now here’s a question you may be asking yourself, “If energy is neither created or destroyed in a closed system then why doesn’t a pendulum swing forever?” That’s a very good question. Energy is neither created or destroyed, but it can be transferred into non- useful energy. In the case of a pendulum, every swing loses a little bit of energy,which is why each swing goes slightly less high (achieves slightly less PE) than the swing before it. Where does that energy go? To heat. The second law of thermodynamics states basically that eventually all energy ends up as heat. If you could measure it, you’d find that the string, and the weight have a slightly higher temperature then they did when they started due to friction. The energy of your pendulum is lost to heat! If you could prevent the loss of energy to useless energy, you could create a perpetual motion machine. A machine that works forever! There have been a lot of folks who have spent a lot of time trying to make a perpetual motion machine. So far, they have all failed. A perpetual motion machine is one that is said to be 100% energy efficient. In other words all the energy that goes into it goes to useful energy. Your pendulum could be said to be about 90% efficient.Very little energy is converted into useless energy. In most systems, energy is converted to useless heat and sound energy.

Click here to go to next lesson on Energy Exchange between Kinetic and Potential Energy.

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2 Responses to “Mechanical Energy Relationship”

  1. The temperature change of the air, ball and string are sooo very slight that for everyday usefulness, we tend to neglect it. It’s important to know about, otherwise it wouldn’t make sense why the pendulum slows and eventually stops.

  2. Karen Daley says:

    Karis asks: If a pendulum is swinging back and forth and the friction increases the temperature of the string and ball, why does the air temperature around it get lower?