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Chapter 11: Lesson 3 - Work-Energy Theorem, Law of Conservation of Energy, and Collisions
Chapter 11: Lesson 3 - Work-Energy Theorem, Law of Conservation of Energy, and Collisions
Work-Energy Theorem
Work-Energy Theorem
If you remember from chapter 10, the work energy theorem states that the net work done on an object is equal to its change in kinetic energy.
The work-energy theorem indicates that if the net work is positive, the kinetic energy increases.
Look at the picture of the spike and the ram. When held at a height, the ram possess mechanical energy in the form of potential energy. As it falls, the ram possess mechanical energy in the form of kinetic energy. As it strikes the spike, the ram applies a force to move the spike, therefore the ram does work on the spike.
The work energy equation has three parts. Remember, you only need two parts to make an equation, so you can pick which of the two out of the three parts that you will use to make up an equation. This is useful if you know the net work done on an object and need to know the change in kinetic energy.
Law of Conservation of Energy
Law of Conservation of Energy
The law of conservation of energy states that within a closed, isolated system, energy cannot be created or destroyed, it can only be transferred from one form to another. The total amount of energy never changes.
The equation below shows that the sum of the initial kinetic and potential energy are equal to the sum of the final kinetic and potential energy.
Mechanical Energy
Mechanical Energy
Mechanical energy is the sum of the potential and kinetic energy that an object has.
Toying Around with Energy
Toying Around with Energy
Is there a relationship between winding a wind-up toy and the distance that the toy moves?
Conservation of Energy Examples:
Conservation of Energy Examples:
Just like the previous video, this shows 10 J of energy being added into the wind up toy. 8J of energy goes into the movement of the toy and 2 J of energy is released as heat due to friction. The amount of energy that is added into the toy is the same amount of energy that is released from the toy.
These four pictures show the transfer of potential energy into kinetic energy when bouncing on a trampoline.
This picture also shows the transfer of potential energy to kinetic energy on a rollercoaster.
This is a picture of a girl jumping out of a building. In each position, the total amount of energy is 10,000 J. That never changes. What does changes is the amount of kinetic and potential energy at each position. At the top, she has the most amount of potential energy and that PE changes to kinetic energy as she reaches the ground.
Energy and Collisions
Energy and Collisions
We talked about elastic and inelastic collisions in chapter 9.
In an elastic collision, the two objects bounce off one another. The potential energy is completely converted into kinetic energy after the collision.
In an inelastic collision, the two objects stick together. Some of the kinetic energy is changed into other forms of energy.
Check Your Understanding:
Check Your Understanding:
Click on the down arrow when you have your answer to check to see if you are correct.
When bouncing a ball, why doesn't the ball bounce back to the original height in which it was dropped?
The ball has a certain amount of potential energy when it is held at the drop height. When the ball is dropped, the potential energy changes to kinetic energy. When the ball hits the ground, some of the energy is transferred into heat and sound. When the ball bounces back up, it has less energy than it originally had, so it can't go up as high.
2. Look at the picture above. Which position in the rollercoaster picture has the most amount of potential energy?
Point A has the most amount of potential energy. It is the highest position from the ground, the reference level.
3. Look at the picture above. Which position in the rollercoaster picture has the most amount of kinetic energy?
Point B has the most amount of kinetic energy. This is the point in the rollercoaster where the cart is traveling the fastest.
Roller Coaster Physics
Roller Coaster Physics
Here is a great video on the physics behind roller coasters. Fast forward the video to the 1:10 mark to start the physics part.
We will be making our own rollercoasters in class, so this is a great video to find ideas for your rollercoaster!
You have finished Lesson 3!
You have finished Lesson 3!
Be sure to head over to google classroom and fill out the exit pass.
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