Chapter 12: Work & Energy Section 2 – What is energy ?

Energy comes in many forms that we are familiar with. –Light, heat, and sound are a few examples of energy that we encounter every day. –Definition: energy – a measure of the ability to do work. Energy is measured in Joules. What is Energy?

Energy is very closely related to work. In fact… –Energy must be transferred to do work! Energy & Work

Energy comes in many forms… –Definition: mechanical energy – energy that can be used to do physical work. Examples: –Sound Waves –Objects in Motion –Definition: chemical energy – energy stored in the bonds of atoms. Examples: –Burning Gasoline –Batteries –Food Energy

–Definition: electrical energy – energy resulting from the flow of electrons. Examples: –Electricity –Lightning –Definition: radiant energy – energy travelling as waves. Examples: –Sunlight –Ultraviolet Waves Energy

–When you pull a rubber band back, you are doing work on the rubber band. –By doing that work, you are transferring some of your energy into the rubber band. You used energy to do the work that stretched the rubber band! Now, the rubber band has the energy you used!

Mechanical Energy comes in 2 Great- Tasting Flavors! KINETIC Potential

When you stretched the rubber band, the energy you transferred to it was held as “potential energy”…. Definition: potential energy – the stored energy that results from an object’s position or condition. Potential Energy

–When an object is stretched or compressed, it has “elastic” potential energy. Potential Energy

–When an object is above the ground, it has “gravitational” potential energy. –We are only worried about GPE. Potential Energy

–GPE depends on mass and height of an object. –We can find the potential energy of an object with the GPE equation. –The GPE equation: PE = mgh –m = mass (kg) –g = gravitational acceleration (9.8 m/s 2 ) –h = height (m) Gravitational Potential Energy

–Knowing the PE of an object is VERY important to engineers. –Buildings, bridges, roller coasters, and other things rely on PE calculations! Gravitational Potential Energy

In the example with the rubber band…after you released the rubber band… –It had kinetic energy as it snapped back into place. –Definition: kinetic energy – the energy an object has because of its motion. –Objects with kinetic energy can do work. What would happen to a bottle cap if the rubber band hit it? Kinetic Energy

OMG!!! Kinetic Energy!

–The kinetic energy equation: KE = ½ mv 2 –m = mass (kg) –v = velocity (m/s) Because velocity is squared, KE depends more on velocity! –In other words, a higher velocity gives your much more KE than a higher mass. Kinetic Energy

–Automobile makers use this equation when testing cars. –They test the cars at different speeds to see how they hold up in a crash. Kinetic Energy

Chapter 12: Work & Energy Section 4 – The Conservation of Energy.

When you hit a baseball, what happens to the energy that you transferred to the bat? Did the energy disappear, or did it just change into other forms…? What happens to energy??

When you hit the baseball, the kinetic energy of the swinging bat is transferred to the baseball. The baseball flies away! What happens to energy??

Energy is also used to produce the cracking sound.. And some energy is used to heat up the bat and the baseball! What happens to energy??

What happened with the baseball is an example of a very important law… The Law of Conservation of Energy. What happens to energy??

The Law of Conservation of Energy states: ENERGY CAN NEVER BE CREATED OR DESTROYED. EVER!

So, energy cannot be created… –And it cannot be destroyed. –Energy can change forms. –And it changes form constantly! –Energy can change into sound, light & heat. –Kinetic Energy can change into potential energy… –And PE can be come KE. Energy can change forms!

Suppose you hit a pop-up. As the baseball flies upward, it has KE. At its maximum height, it no longer has KE. –It has PE! –As the baseball falls, the PE becomes KE once again. What happens to energy??

Energy can change forms!

What point has the most PE? What about the most KE?

Chapter 13: Heat and Temperature Section 1 – Temperature.

Generally, temperature is a measure of how hot or cold something is –Scientifically… –Definition: temperature – a measure of the average kinetic energy of particles. Faster moving particles have a higher temperature than slower moving particles. What is temperature?

If we could look at the molecules of hot air, we would see… The molecules would be moving very fast!

What is temperature? If we could look at the molecules of cold air, we would see… The molecules would be moving very slowly!

Temperature change indicates energy transfer. Imagine that you are holding a piece of ice in your hand. What happens to the ice? The ice melts, of course, but what happens in scientific terms…? Temperature & Energy Transfer

Your hand is much warmer than the ice –That means that it has more kinetic energy. As you hold the ice in your hand, heat energy is transferred from your hand to the ice. Temperature & Energy Transfer

As a result, the molecules in the ice move faster… –And their kinetic energy increases, which causes the ice to melt. –Definition: heat – the energy transfer between two objects that have different temperatures. Temperature & Energy Transfer