October 21 - Center of Gravity, Buoyancy

The students learned about center of gravity this week. We also began an introduction to buoyancy.

I gave the students three kinesthetic tasks to begin the lesson. See if you can do these at home!

1) The Impossible Leap

Bend over and hold your toes with your hands while keeping your knees slightly bent. Now try to jump forward while remaining in that position. Don't unbend your knees or take your hands away from your toes. Can you jump?

2) The Super Glue Chair
Sit in a straight-backed, armless chair with your feet planted flat on the floor and your arms folded across your chest. Now try to stand up while keeping your feet flat and your back straight. Can you do it?

3) Pick Up Trick

Place a pencil (or other object) on the floor about 20 inches from the wall. Stand with your back and heels flat against the wall. Keep your feet together. Try to pick up the object without moving your feet or bending your knees. Can you do it?

These experiments all demonstrate center of gravity. Center of gravity can be described as the point at which the entire weight of a person or object is concentrated or held. In symmetrical objects, the center of gravity is the geometric center of that object. In non-symmetrical objects, such as the human body, the center of gravity changes with every movement we make. In the experiments above, the students' centers of gravity changed each time. Here are explanations of what happened in each task:
The impossible leap. If you were to re-do this but jumped backward you would have no problem. When a person jumps, their center of gravity shifts in the direction they want to jump. To prevent falling over, the person must move their base of support in that same direction. The students were unable to jump forward because they would need to use their toes (base of support) to do so.
The super glue chair. When a person sits down, their center of gravity is at the base of their spine. If you try to stand up while keeping your back straight, your center of gravity is unable to move to a position above your feet (where you need support to stand).
Pick up trick. When standing straight against the wall a person's center of gravity is over their feet. Normally, when a person bends over, their center of gravity moves forward. In order to maintain their balance in this trick, the students would have had to move their feet. Since they weren't able to do this, they weren't able to pick up the pencils.

After discussing the center of gravity movement activities, the students completed a lab ("Shake Up") to show how shape affects speed. They had three objects - a roll of masking tape, a marble, and two jar lids that were taped together. The students predicted which they thought would roll to the end of a tilted table first. The students then let the objects go and observed what happened.

The marble reached the end of the ramp first. This is due to the marble's small size. Because of this, the marble's center of gravity is closest to its overall weight. The slowest object was the roll of tape since it was the largest and its center of gravity is furthest from its overall weight. We decided that this would be more obvious if we had a larger roll of tape. :)

We moved on to begin a lesson on buoyancy that we will continue next week. I placed 1 cup of water into a glass measuring jug and asked the students what they thought would happen to the water level if I added an ice cube. They correctly guessed that the water level would rise. I then asked them how many ice cubes I would need to add to raise the water level by 1/4 cup, 1/2 cup, and 1 cup. The students made predictions for each and we tried each one out.
The raising of the water level when something is added to the water is called displacement. The ice cube displaced or pushed away some of the water causing the level in the jug to rise. We then spoke a little about Archimedes and Archimedes' Principle (displacement and the idea that the weight of the water displaced by an object is equal to the buoyant force in the water).
We also talked a little about buoyant force. Buoyancy is a pushing force while gravity is a pulling force. When we put the ice cube in the jug of water, we noticed that only part of the ice cube stayed above water. This is because gravity is trying to pull the ice cube down to the bottom of the jug while buoyancy is pushing it up to the top of the water/jug. 

The experiments "The Impossible Leap," "The Super Glue Chair," and "Pick Up Trick" are all from this website: http://www.escapadedirect.com/plwigr.html
The "Shake Up" lab is from Physics for Every Kid.
VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.

October 14 - Gravity

We began class by discussing what the students already know about gravity. I also asked them how gravity affects human beings.

Each student was then given a sheet of notebook paper and a hardcover textbook. The students were asked to predict what would happen if they dropped the book and paper separately. Would the book or paper fall fastest? We tried this and then predicted what would happen if the paper was placed on top of the book. Again, the students tried the experiment and discussed their observations.

Gravity pulled equally on the book and the paper. So, even though the book hit the ground before the paper when we dropped them separately, gravity's pull on each object was equal. The paper fell at a slower speed due to air resistance. The book's weight overcame the force of the air but the paper, being so much lighter, had little effect on the air's push. This made it fall at a much slower rate.

Gravity is a force that attracts all objects to each other. This attraction is called Gravitational Pull. On Earth, gravity keeps an atmosphere around the planet. It also causes things to fall to the ground, causes the ocean's tides, and causes hot air to rise while cool air falls (which leads to winds).

Gravity pulls things to the center of the Earth. Gravity also gives people and objects weight. Since the force of gravity is different on other planets, people and objects can weigh more or less than they do on Earth on other planets.
Here is a link to a fun website you can use to discover weights of people, pets, or objects on other planets: http://www.exploratorium.edu/ronh/weight/

We completed several other experiments about gravity this week. Here are the details.
*Bigger - In this lab, students made two parachutes each to determine if size affects the speed of a falling parachute.
Each student was given a 12-inch square of plastic (cut from a plastic bag), a 24-inch square of plastic, 8 20-inch pieces of string, 2 washers, and 2 4-inch pieces of string. They used these materials to construct parachutes and then predicted which they thought would hit the ground first (similar to the book and paper experiment from the beginning of the class session). We found that the parachute made from the larger bag fell to the ground more slowly than the smaller parachute. Just like in the book and paper experiment, this is due to air resistance. The larger parachute has a larger surface area and a small weight so it has more air resistance. 

*Gravity Won - The students learned about water's surface tension in a previous lesson. This lab demonstrated the effect of gravity on weak surface tension.
We filled a baby food jar with rubbing alcohol then colored it with food coloring. A straw was placed into the jar of alcohol and held in place with a small piece of modeling clay. When we tipped the jar upside down, the alcohol flowed out of the jar and the straw.
Alcohol has a weaker surface tension than water meaning the attraction between the alcohol molecules is not very strong. The air pressure inside the straw was not enough to hold liquid in the straw so the pull of gravity caused the liquid to flow out of the straw.

*Anti-Gravity - This was very similar to the last lab except we used water instead of rubbing alcohol. Since water has a greater surface tension than alcohol, the air pressure inside the straw pushed up on the water when the jar was tipped upside down while the water molecules pulled from side to side. These pushing and pulling forces were greater than the pull of gravity so the water remained in the straw.

The labs "Same Speed" (book and paper) and "Bigger" are from Physics for Every Kid.

VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.

The labs "Gravity Won" and "Anti-Gravity" are from Chemistry for Every Kid.
VanCleave, J. (1989). Chemistry for Every Kid: 101 Easy Experiments That Really Work. San Francisco: Jossey-Bass. 

October 7 - Motion: Inertia; Newton's Laws of Motion

We continued our study of motion this week by covering inertia (Newton's First Law of Motion) as well as Newton's Second and Third Laws of Motion.

Inertia is the tendency of an object to resist any change in its motion. If an object is at rest, it will remain at rest until it is acted upon by a force. If the object is moving, it will continue to move until acted upon.

We started our session with a lab. This lab was "Crash!" from Janice VanCleave's Physics for Every Kid. Each student had a small piece of modeling clay, 2 rulers, a toy car, a pencil, and 2 books. For the first part of the lab, the students set up a ramp using one book and a ruler. They taped the pencil to their desk about 2 car lengths from the ruler. The students then placed the modeling clay onto the hood of their toy cars and allowed the car to roll down the ruler. The students observed the modeling clay fly off the hood and used the second ruler to measure how far from the car the clay fell. We kept track of the measurements.
After three rolls, the students added a second book to the stack. They made predictions about whether or not the clay would now fly further from the car. Again, they had three rolls and we tracked the measurements.

Once we completed both sets of rolls we discussed how all this related to inertia: when the clay flew off the front of the car, it continued to move even though the car had stopped. As the car rolled down the ruler, its speed increased. The clay was moving at the same speed as the car so, even though the pencil stopped the car, the clay continued to move until gravity acted upon it causing it to fall to the table.

After completing the work with two books, we decided to try it with four.

We moved on to talk about mass and how it relates to inertia. I asked the students if they thought objects with different weights have the same inertia. I used the example of a ping-pong ball, a tennis ball, a basketball, and a bowling ball. The greater an object's mass, the greater its inertia. I also asked them if they would be able to change the velocity of a bowling ball by swatting at it with a ping-pong paddle. We discussed the fact that a greater force would be needed to change the velocity of the bowling ball than would be needed to change that of the ping-pong ball.

The students then completed the lab "More." This lab demonstrated the effect of weight on inertia. The students were given a pair of plastic soda bottles. One bottle was empty; the other was about half full of water. We tied a 12-inch piece of string to a rubber band and then slipped the rubber band onto the bottom of the empty soda bottle. Before we began, I had the students predict which one would have a greater inertia or a greater resistance to motion. The students each pulled on the string to drag the empty bottle across the table. We measured how far the rubber band pulled away from the bottle and recorded our results. We then did the same with the half full bottle. The students discussed that since the bottle with water was heavier than the empty bottle, it had a greater inertia that caused the rubber band to pull away further from this bottle.

We then discussed Newton's Second and Third Laws. I used the example of hitting a baseball versus a bowling ball to describe Newton's Second Law: The Law of Force (Force = Mass x Acceleration). They understood that since the bowling ball has a greater mass, it would require a greater force to hit the bowling ball with a baseball bat.

Newton's Third Law is the law of action and reaction. Whenever an object pushes on another object, the first object gets pushed back in the opposite direction equally hard. We used two labs to demonstrate this concept.
"Balloon Rockets" - Students were given a long piece of string that the threaded a piece of drinking straw onto. They then tied the string to two chairs. Each student then inflated a balloon and taped the balloon onto the straw. The students observed what happened when the balloon was released.
When the balloon was released, it pushed the air out. However, that air pushed back on the balloon (action-reaction), causing the balloon to move forward.

"Paddle Boat" - The students used cardboard to create a simple boat shape. They cut a paddle out of the cardboard and used a rubber band to attach it to the boat. The students then wound up the paddle and placed the boat in a container of water.
When the students wound up the paddle, it turned and hit against the water. The paddle, then, pushed against the water and the water pushed back. This action and reaction caused the boat to move.

We closed up class with one more lab that demonstrated inertia. For this one, I placed five textbooks on the edge of a rolling chair. I pushed the chair across the classroom and then stopped abruptly. Due to inertia, the books remained in motion when the chair stopped. 

To look forward to next week:

We will learn more about Sir Isaac Newton by studying gravity.
We'll find out what some common objects would weight on different planets and we'll complete several experiments about the force of gravity.

The labs "Crash!", "More," "Balloon Rockets," "Plop!" and "Paddle Boat" are from Physics for Every Kid.

VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.

September 30 - Motion (Pushes and Pulls), Velocity

We're going to start out the semester learning about forces. Our class on September 30th focused on motion.

We discussed how a force can be a push or a pull and then spent a little time talking about examples of pushes and pulls in our daily lives (pushing or pulling a door open or closed, bending over to pick something up, stretching a rubber band, squeezing a tube of toothpaste, etc).

The students completed three hands-on experiments to observe how things can pull on each other. Our first project, "Floating Sticks," involved the students placing two toothpicks side-by-side in a bowl of water. The students dipped a third toothpick into dish washing detergent and then carefully dipped this third stick into the water between the two floating toothpicks. This lab showed how dish washing detergent breaks up the attraction of water molecules. This causes some water molecules to pull on others, taking the toothpicks with them.

Our second project for the day was "Tug of War." For this one, each student was given a sheet of aluminum foil, 1/2 cup of water that was mixed with blue food coloring, and 1/4 cup of rubbing alcohol. The students placed the foil on the table and then poured a thin layer of the blue water onto the foil. They then used an eyedropper to add a drop of rubbing alcohol to the center of the layer of water.
The water molecules on the surface of the water normally pull equally in all directions. However, when we added the rubbing alcohol, the two liquids immediately separate. The alcohol pulls away from the water while the water pulls away from the alcohol.

We also tried a lab called "Powder Dunk." This one didn't have the desired effect so we may re-try this one at some point during the semester. Each student had two soup bowls filled with water. We sprinkled a thin layer of talcum powder over the surface of the water in each bowl. The students dipped one of their toothpicks in shampoo and the other in the dish soap. They then touched the soapy end of each toothpick to the center of the powder in each bowl. We should have seen the shampoo cause the talcum powder to break up into large floating blocks and the dish soap to cause it to rush to the sides of the bowl and then sink.

Since the talcum powder is water resistant, the pieces of talc will float on top of the water. As we know from the last experiment, water molecules normally pull equally in all directions. Soap or shampoo break up the attraction of the molecules causing the water to move outward. The powder should have followed! Since the liquid dish soap will dissolve in water, it should have caused the little pieces of talcum powder to sink since the water will quickly cover the talc.

We moved on to discuss velocity. Velocity describes the speed and direction of an object. We were going to go outside to move around at different speeds and in different directions to demonstrate changes in velocity but the thunder and rain kept us in the classroom. Instead, I used the example of cars traveling on a road to explain this concept. If two objects (cars) are moving at the same speed and in the same direction, we can say they have the same velocity. However, if one car slows down or speeds up, the velocity of that car has changed. We also discussed cars traveling at the same speed in opposite directions (even though the speed is the same, their velocity is different).

After discussing velocity, we spent a little time talking about balanced and unbalanced forces. Forces on an object that are equal in size but opposite in direction are balanced forces. To explain this, I used the example of playing tug-of-war with a dog. When you plant yourself on the ground, the ground actually pushes back up on you. If you do not move backward or forward, the force of the dog pulling must be equal to the force of the ground pushing on you. The forces of the dog and the ground, then, are equal or balanced.

Now, imagine you are playing tug-of-war with the dog but your foot hits a slippery spot on the ground. The ground is now unable to exert as much force back on you but the dog is still able to pull just as much. Since the pull of the dog is now greater than the push of the ground, the forces acting against you are unbalanced. We also tied in velocity. When the forces are balanced and you are not changing either speed or direction, your velocity remains constant. When you hit the slippery patch of ground and the dog is able to pull you over, you accelerate in the direction of the greater force (in this case, the dog). Therefore, your velocity has changed.

There was one more quick lab to close out the class session. This one, "Fly Away," involved the movement of air due to unequal pressure. Each student was given a small glass soda bottle which they laid on its side on the table. They then squeezed a small piece of notebook paper into a small ball. The students placed the wad of paper just inside the opening of the bottle and then crouched down in front of the bottle to try to blow the paper into the bottle. We had mixed results (probably due to the pieces of paper being too big) but we should have observed the paper ball flying out of the bottle. This happens due to changes in air pressure. Before the students blew into the bottles, the amount of air inside and outside of the bottle are the same. When they blew into the bottles, the amount of air inside the bottle increased, thus increasing the air pressure. That extra air has to go somewhere so it exits through the neck of the bottle, pushing the paper ball out with it.

To look forward to next week:

Inertia and Newton's Laws of Motion
We will be making paddle boats and balloon rockets!

The labs "Floating Sticks," "Tug of War," and "Powder Dunk" are all from Janice VanCleave's Chemistry for Every Kid.
VanCleave, J. (1989). Chemistry for Every Kid: 101 Easy Experiments That Really Work. San Francisco: Jossey-Bass.

The lab "Fly Away" is from Physics for Every Kid.
VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.

Here's a list of topics we'll cover this semester.

Week 1 (September 23) - Introduction to physical science; the scientific method

Week 2 (September 30) - Motion - balanced and unbalanced forces; velocity

Week 3 (October 7) - Motion - Inertia; Newton's Laws of Motion

Week 4 (October 14) - Gravity

Week 5 (October 21) - Gravity - Archimedes' Principle; center of gravity

Week 6 (October 28) - Buoyancy

Week 7 (November 4) - Friction

*No class November 11 - Veterans' Day Holiday*

Week 8 (November 18) - Kinetic and potential energy; Law of Conservation of Energy

*No class November 25 - Thanksgiving Holiday*

Week 9 (December 2) - Temperature and heat

Week 10 (December 9) - Temperature and heat

Week 11 (December 16) - Simple machines

*No class December 23 or 30 - Winter Break*

Week 12 (January 6) - Simple and compound machines