Potential and Kinetic Energy

Paraphrase:

People have struggled to understand gravity for many centuries, but Aristotle, Galileo, and Newton are three important people who thought about it differently and made interesting observations.

Show students the definition of gravity to the right. Encourage them to ask questions about gravity and what the basic definition means.

Gravity n.

A force that acts between all masses, pulling them together.

Is the definition helpful?

• Sort of.
• Not really.
• Seriously?

Read and discuss the passage to the right with the class.

When discussing, have students offer a variety of ideas.

Allow students to drop books and single sheets of paper.

All through history people have asked the same questions, like “What's for dinner?” and “Why do things fall?” Aristotle, who lived in Athens almost 2,500 years ago, argued that the center of the Earth was the center of the universe. So naturally everything tended to fall in that direction! Wrong on TWO counts. He also thought that heavier things fell faster than lighter things. Well, you could see how he would think that. Try dropping a piece of paper and a book. The book hit the floor first, right? But not because of gravity and not because it is heavier.

Discuss.

Here is where Galileo comes in. He carefully dropped balls from towers and rolled them down inclines. He stated that the reason your book drops faster than your paper is not due to their difference in mass, but rather because air resistance (friction) affects the falls differently.

Let's explore Galileo’s idea.

Step 1: Go to the moon where there is no air to affect the way things fall.

Step 2: Drop a feather and a hammer at the same time.

Oh, going to moon is not in your budget? Well, watch this video of an astronaut who tried it out in 1971:

Read and discuss the passage to the right with the class.

View and discuss NASA video.

Read and discuss the passage to the right with the class.

As great as Galileo was, Isaac Newton was the one who really hit it out of the park with this idea: he said that gravity is not just about the Earth pulling things, but rather that everything that has mass creates a gravitational force.

Consider an apple and the planet Earth.

Newton said that the apple is pulling on the Earth in exactly the same way as the Earth is pulling on the apple.

Huh?

Then, you might ask, “Why doesn’t the Earth fall up to the apple?” Well, the answer to that is...

It does.

Huh??

Just not very much. Actually, it’s an immeasurably small distance.

So, how can the apple and the Earth react to equal forces so differently?

Show the images to the right.

Paraphrase:

• Think about flicking a paper clip with your fingers.
• Think about flicking a parked car with your fingers.

Ask:

• Why wouldn’t the car move as much as the paper clip??

After an appropriate amount of discussion, present the "ANSWER" but have them address the associated question.

Finally, for continued study of this subject assign students the following reading. It includes everything discussed thus far and continues on to introduce inertia, acceleration, and velocity.

​