Honors Physics - Chapter 11: Lesson 1

Chapter 11: Lesson 1

Potential Energy

Chapter 11: Lesson 1 - Potential Energy

Potential Energy

Potential energy is energy that is stored. It has the POTENTIAL to do work. We abbreviate potential energy with a PE. The unit for potential energy is the same for all forms of energy, Joules (J.) This also called gravitational potential energy.

The equation for potential is PE=mgh, where m=mass, g=gravity, and h=height. The height is the height above (or below) the reference level.

Reference Level

The reference level is where the potential energy is chosen to be zero. We can choose anything to be a reference level. Look at the picture of the book sitting on the table. If we chose the top of the table to be the reference level, then the book would have no potential energy. If the top of the table was the reference level and we dropped the book off the table to the floor, then the book would have a negative potential energy because it would be below the reference level. If we moved our reference level to the floor, the book, which is now sitting on the floor, would have no potential energy. Potential energy can be positive or negative, depending on where the reference level is located. Unless it is noted, we can assume that the reference level will be the floor for any problem that you encounter in this class.

Potential Energy Depends on the Height Above the Reference Level

The more height above the reference level an object has, the more potential energy the object has.

It doesn't matter how you get to a certain height, lift a rock 2 m, roll it up a ramp, or walk it up a set of steps, as long as the rock is some height above the reference level and has mass, it will have potential energy.

Check Your Understanding:

Use the picture below to answer the following 5 questions. Click on the down arrow when you have your answer to check to see if you are correct.

This picture shows red spheres at various points on three grey objects that are all the same height. Find the potential energy at each of the five spots.

What is the potential energy at point A?

30 J

The tops of all three grey objects are the same height. Since the potential energy at the top of the first object is 30 J, the top of the second object is also 30 J.

2. What is the potential energy at point B?

30 J

The tops of all three grey objects are the same height. Since the potential energy at the top of the first object is 30 J, the top of the second and the third object is also 30 J.

3. What is the potential energy at point C?

20 J

The top of the grey object is 30 J and there are three steps. Each step decreases by 10 J.

4. What is the potential energy at point D?

10 J

The top of the grey object is 30 J and there are three steps. Each step decreases by 10 J. The first step down from the top is 20 J, so the second step down from the top is 10 J.

5. What is the potential energy at point E?

0 J

Point E is at the bottom, the reference level, so the potential energy is 0 J.

Elastic Potential Energy

Energy can also be stored in the bending or stretching of an object. When energy is being stored because something is compressed, stretched, or bent, then we call that elastic potential energy.

Elastic Potential Energy and Pole Vaulting

Pole vaulters run horizontally then plant a pole into the ground to propel them over a bar. As the runner plants the pole in the ground, the pole flexes. The flex in the pole is almost exactly like the compression of a slinky. The more the pole bends, the greater the stored elastic potential energy. This stored elastic potential energy comes from the kinetic energy of the vaulter. As the horizontal motion stops, the pole then releases this stored elastic energy as it pushes the vaulter upward. So, in short the pole takes horizontal kinetic energy and stores it before using it to increase the gravitational potential energy of the vaulter.

Watch the video to see a slow motion video of Dana Wehbi a class of 2019 graduate in action!

You have finished Lesson 1!

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