Kindergarten: Unit 1 Gravity. gravity mass distance attract

Transcription

1 Kindergarten: Unit 1 Gravity Vocabulary: gravity mass distance attract Background Knowledge for the Teacher: Gravity is one of the four fundamental forces of nature. (The other three natural forces are electromagnetic, which is the force of electricity, magnetism, and light; and strong and weak nuclear forces, which hold atoms together.) Gravity is the weakest of these four forces, but it is important to life on Earth and the structure of the universe. Everything that has matter, having mass, generates gravity, from the smallest grain of sand to the largest objects in the universe. That gravity pulls things together. Gravity is described as the force which attracts all objects to each other. We tend to think of gravity as pulling everything toward the center of the Earth, but that is only true because we are on the planet. We feel gravity as a force that pulls all materials toward the center of the Earth because the Earth s mass in comparison to ours makes it by far the stronger gravitational force. However, although the Sun has far greater mass than the Earth, and thus far greater gravitational force, the distance we are from the Sun makes its force less than the gravitational pull we feel from the planet we are standing on. The closer two objects are to each other, the stronger the gravitational attraction between them. The greater the distance between two objects, the less the gravitational attraction. When it comes to gravity, the larger an object is, the stronger its force. A person creates gravity but not enough to pull objects toward him or cause things to go into orbit around him. A planet, being much larger and with greater mass, has enough gravity to pull objects toward it and keep them in orbit around it. The Earth s gravity keeps our moon in orbit. The moon has less mass and therefore less gravitational force so it is pulled toward the Earth

2 rather than the Earth being pulled toward it. If not for gravity between the Earth and moon, the moon would simply float away into space. A star makes enough gravity to pull whole solar systems into its orbit. Our Sun generates enough gravity to hold all of the eight planets, their moons, and all other matter in our solar system in orbits around it. Our Sun's gravity is so strong that it keeps an object Pluto that is roughly 3.7 billion miles away in orbit. Gravity also causes the tides of the oceans. Because it is made of matter, the Moon also has a gravitational pull, but it is not strong enough to move Earth. It pulls on the soft parts of the Earth s surface, the oceans, and that pull is observed as tides. As the Earth rotates on its axis, the moon pulls from different angles and locations, causing high and low tides. Whenever the water on the beach recedes and comes back with the ebb and flow of the tide, the ocean is reacting to the pull of the Moon's gravity. The sun s gravitational force causes some tides as well. All life needs gravity to survive. Gravity not only holds us in place, but it holds our atmosphere close to the surface, providing us with the air we breathe. Without gravity, people and all other objects would go floating off into space. Gravity also keeps Earth close enough and far enough in orbit around the Sun that we are not freezing or burning. Therefore, life would not have begun on Earth were it not for the force of gravity. Gravity has been around since the very beginning of the universe and it works the same way everywhere in the universe, on all kinds of different objects, and on objects of all different sizes larger than atoms. (Atoms are held together by strong and weak atomic forces.) In the very beginning of the universe, after the Big Bang, gravity pulled atoms together to form the stars and planets. Once the stars and planets had formed, gravity kept the planets in orbit around the larger stars, and moons orbiting around the planets. And on each planet that is large enough, gravity keeps an atmosphere around the planet just like a blanket is held on a bed. Without gravity, our atmosphere would disappear into space and life on Earth could not exist. On Earth, gravity keeps the air around us and everything else from drifting off into space. When on the moon, astronauts had to have enough energy in their lunar

3 module to overcome the force of the Moon s gravity to lift off and out of the moon s atmosphere. Once out of the pull of the Moon s gravitational force, the spacecraft was at freefall until the Earth s gravity took over and pulled the spacecraft back through the atmosphere and to the surface. Gravity also causes lighter hot air to rise while more dense colder air falls; the convection currents that in turn cause wind. Nobody fully understands how gravity works, or even why gravity exists. One way of looking at gravity is to think of it not as a force like magnetism but instead as a natural result of the way mass bends space. Think of space as a thick rubber sheet. A heavy object placed on the center of the sheet will stretch or bend the fabric with its weight. Now picture smaller objects along the outside corners of the sheet. Heavier objects will cause the lighter or less dense objects to move toward them. Any object with mass, like a star, pushes on space and bends it, so that other objects, like planets, that are moving in a straight line are also going around the star. It looks to us like the star is pulling on the planet, but really the star is bending space. Another way of thinking about gravity is to think that the star and the planet are exchanging tiny particles called gravitons that help to pull the star and the planet together. It's as if they were tossing tiny balls back and forth between them. But if the planet gets further away from the star, more of the balls get dropped and lost, and so the gravity between them is weaker. It's likely that both of these ideas are at least partly right maybe the gravitons bend space, or maybe bending space produces gravitons. Most of the time, when people need to figure out some problem involving gravity, like how fast a ball will fall when you drop it from a tall building, they forget about why it works and just use a good enough approximation of what gravity does. This approximation tells us that gravity at the surface of the Earth is approximately 9.8 meters per second. That means that, not counting friction with the air, if you drop a ball from a building, it will gain speed at the rate of by 9.8 meters/second for every second that it falls. If it begins at 0 meters/second, after one second the ball will be going 9.8 meters/second, and after two seconds it will be going 19.6 meters/second, and after three seconds it will be going 29.4 meters/second, and so on. The closer the ball gets to Earth, the faster it is falling.

4 Activity 1: Show the following picture on a dry erase board or white paper screen. Ask students to look at the picture and describe what they see. Describe the objects in the picture and their location in the room. Ask the students questions like: Where is the floor of the room? Where is the ceiling? How is this different from most rooms in our houses? Could this happen in our house? Why or why not? Are some things closer to the ceiling? Are some things closer to the floor? What is the heaviest thing in the room? The lightest? How does the heaviest object compare in location to the lightest object? Ask the students to consider the present position of each object. Have them make surmises as to the original location of each object. For example, where

5 might the bed have been positioned on the floor? Where might the chairs have been? What objects might have been on other objects? Using a dry erase or other marker (depending on medium the picture is projected on), have students trace the motions the objects may have made traveling from their original location to their present position. Draw arrows or lines to show the movement. Lines might be waving left to right, up and down, or forward and backward. Remind the students of the motions they have learned about in previous units. Once the motions of the objects have been determined, explain to the students that they are going to construct a story in the style of a journal entry. It may begin, Dear Diary, This morning the most extraordinary things began happening as I was waking up Have students create a sequence of events that shows how each object may have moved and in what order. Did the lighter objects float easier and faster than the heavy objects? Where would the person in bed end up? Encourage students to describe the emotions of feeling a bed float and objects begin to move around them. Challenge students to explain why they think these occurrences are happening.

6 Activity 2: Begin this lesson by showing students the picture below. Ask them to describe what is happening to Mr. Newton in the picture. Ask them if they can explain why it is happening. Introduce the word GRAVITY to the students. Explain that it is another type of force, like push and pull. A force acting on objects causes motion. What motion is occurring in the picture? Why did the apple drop? What movement might the man in the picture make in reaction to the motion of the apple? Ask students to discuss the statements on the chart below. Be sure to record their responses and save the chart for later use. Statement Yes No Why Gravity holds the air and clouds around us. Gravity moves water. A bowling ball will drop to Earth faster than a wiffle ball. I weigh more on Earth than I would on Mars. If there was no gravity, I could throw a baseball and it would keep going straight and never slow down. Students should be aware that objects that are dropped will fall to the Earth s surface but because gravity is an invisible force it is an abstract concept for them to explain. Gravity is also a non contact force, meaning it does not touch the objects it acts upon. Explain to students that gravity cannot be seen, but

7 we can observe the results of it acting on objects. Point out that although they can see the apple apparently falling on Newton s head, gravity is all around in this picture. As the students discuss the statements, it is more important that they generate ideas about gravity than have right answers. Listen to the WHY reasoning and guide them with further information. Allow students to amend or change their responses as they listen to the discussion. YES: Gravity is the force that keeps our atmosphere in place around the planet. Without gravity, the air and clouds would lift up into space and scatter. YES: Gravity moves water downhill, across lower regions, and as ocean tides. Water flows from the mountains of Georgia, across the Piedmont, and onto the coastal areas. It is this movement of water from higher elevations to lower that causes erosion and moves freshwater to the oceans. NO: Because gravitational force varies only slightly at different locations on Earth, the bowling ball and wiffle ball will drop at the same speed. Although the bowling ball has greater mass, it takes greater force to move, thus equaling out the speed of the fall. All matter falls to Earth at the rate of 9.8 meters per second. (that s a little over 32 feet per second) YES: Because Mars gravitational pull is so much less than Earth s, a 100 pound individual will weigh 38 pounds on Mars. Jupiter has the strongest gravitational force of all the planets due to its mass. A 100 pound person would weigh 31,800 pounds on Jupiter. YES: If no gravity was present, all objects in motion would stay in motion, traveling in a straight line until acted on by another force. Gravity continually acts on objects in motion, causing them to travel at a descending arc toward the surface of the Earth. Without gravity, the ball would fly through the air without any force acting upon it.

8 Activity 3: Student Handout 1 or as Teacher Demonstration Materials needed: Student Handout (copy this on cardstock if done as teacher demonstration) tape scissors paper clips Ask students to watch the following video, Chicken Little. After they have watched it through once, replay it and have the students read along with the words on the screen. Discuss with students why Chicken Little thought the sky was falling. What was it that hit Chicken Little on the head? Where did it come from? What other things seem to fall through air? Ask students to generate a list of everything they can think of that falls down to Earth. Be creative and consider animal products, plant products, things people send into the air, things that may fall from space (meteors, falling stars ) and things the wind might carry from place to place. (paper, sand, dust, trash.) Draw arrows or lines beside each object on the list to show the movement of the object in air. It is important that students grasp the concept that gravity is attraction BETWEEN objects, and that objects can travel in different directions in the air than straight up and down. Many forces, like wind, temporarily overcome the force of gravity, but as the force lessens or is withdrawn, like the wind dying down, gravity will pull the object toward Earth. Use Student Handout 1 as either a teacher demonstration or allow each student to make their own gravity floater. One model is leaves and the other feathers.

9 Follow these instructions for construction: 1. Cut out each model on the solid dark lines. 2. Fold the side flaps back on the dotted lines. 3. Tape the folded flaps back with a single piece of tape. 4. Place a paper clip on the bottom of the model. 5. Bend the top blades in any direction. 6. Toss the Gravity Floater up into the air. Observe the floaters as they move. Allow students to experiment with folding the top blades in different directions. Have them describe the movement of the Gravity Floater as it comes down. Try the Floaters without the paperclip on the bottom. How does that affect movement? (the paperclip adds mass and thus more gravitational pull) Ask students to compare the mass of the Floater to the air around it. Does the Floater go up or down? If it moves down, it has more mass than the air around it. Ask the students why the object moves downward. They should be able to offer a good explanation of gravitational pull and gravity.

10 Activity 4: Review the list and motion generated in Activity 3, then ask the students to observe the following pictures. Ask students to describe what is happening in each picture. Encourage them to determine where the most gravitational force is in each. Point out directions of motion in the pictures. Ask them to explain how gravity is working in each. What has mass? In what direction will gravity attract the objects? What would happen in each picture if there were no gravity?

11 Activity 5: Student Handout 2 Show the students the picture used in Activity 1 and reread the diary entry created in Activity 1. Discuss how these events would be affected by gravity. Could this picture be real? Why or why not? Encourage students to work through the Concept Map below. Ask them to use the picture to offer examples they can use on the Map. Be sure to use words like mass, attraction, gravity, and distance from Earth. I can show how the picture is not real here. I can show how gravity attracts me here. Gravity I can write a sentence about gravity here. I can draw three different masses shown in the picture here.

12

13 Student Handout 2 I can show how the picture is not real here. I can show how gravity attracts me here. I can write a sentence about gravity here. Gravity I can three different masses shown in the picture here.