Friction

Friction was our topic this week. We started out class predicting what would happen if the students pushed a textbook across the table. We then talked about what made the book stop. The students knew that friction acted on the book, causing it to slow down and eventually stop. I took this opportunity to review Newton's First Law of Motion (inertia) with the students.

I moved on to ask if the class had ever seen gymnastics on TV. I asked if they had seen the gymnasts rub a white powder on their hands. Ballet dancers will often rub the same powder on the bottom of their shoes before dancing on a smooth floor. I asked the students why they do this. Gymnasts and dancers use rosin to create friction between their skin or shoes and the floor or gym equipment. This creates more grip for them.

Friction is a force that opposes motion between two surfaces that are touching each other. In other words, friction is what happens when two things rub or roll against each other. This could be your two hands rubbing together, skis rubbing on the snow, or a hammer hitting a nail. When two objects are touching, their surfaces tend to stick together like the tiny loops and hooks of Velcro.

Would you expect more friction between an oily floor and a slick leather shoe sole or between a rough sidewalk and the bottom of a tennis shoe?
The amount of friction depends on two factors: The kinds of surfaces and the force pressing the surfaces together.

Sliding Friction: This is what we observed when we pushed the textbook across the table. Sliding friction is caused by two objects touching each other and sliding past one another.

Rolling Friction: This uses wheels. Only the bottom of each wheel is in contact with the ground/road so rolling friction is less than sliding friction. The students each rolled a toy car across the table and observed how quickly it was able to move.

Fluid Friction: When an object comes in contact with a fluid (in the form of a liquid or gas), it is considered fluid friction. Airplanes and race cars are streamlined to reduce fluid friction. They have smooth, curved surfaces to reduce the friction, known as drag, with the air.

We completed three experiments to demonstrate rolling and sliding friction as well as the effect of weight on friction.

1) Roller lab.

For this experiment, the students each had two books. They tied a piece of string around one of the books and then tied a rubber band onto the end of the string. The students placed a second book on top and then moved the books by pulling on the rubber band and measured how far the rubber band stretches.

Each student was then given ten round markers. They used the markers to make "wheels" for the books and tried the experiment again. New measurements were taken and we compared the results.
The rubber band stretched more when the bottom book was placed flat against the table than when it was placed on the pens. This showed that things that roll cause less friction than things that slide.

2) Farther lab.

This lab showed the effect of weight on the energy of a moving object.
The students were given two books, 1 small round jar, and 1 large round jar. They set the books up with the edge of one book on the second book in order to form an incline. The students then placed the small jar at the top of the incline and let it roll down. We used a tape measure to see how far the small jar rolled then repeated the experiment with the larger jar.
The larger, heavier jar rolled farther than the smaller, lighter jar. The friction of the books, air, and floor remains constant during the two experiments. The major difference in this experiment, then, is the weight of the two jars. As the weight of a rolling object increases, its energy increases. This means the heavier jar has more energy and will roll farther than the smaller jar.

3) Wobbler lab.

This lab also involved a jar and books that we used to form an incline. Before we started, I had the students predict if an empty jar or a jar with water would roll farther. They predicted the heavier jar with water would go farther. The students started out letting the empty jar roll down the incline. They then measured how far the jar rolled. I then filled the jar about three-quarters full with water. After securing the lid, the students then let the jar roll down the incline and measured the distance it rolled.
Contrary to what they learned in their last lab, the heavier jar in this experiment did not roll as far. :)
In this lab, the friction of the jar did not remain constant. The water in the jar sloshed around, increasing the friction inside the jar. It takes more energy to move the jar with the water swishing around inside.

We wrapped up our discussion of friction by talking about the advantages and disadvantages of friction and then completed one last friction-related lab.

How Far?

This lab showed how the texture of a surface affects motion.
The students were each given a piece of poster board, a piece of wax paper, and a piece of sand paper. They also each had a bottle of glue, a rubber band, some string, and a paper clip. I had pre-cut the poster board with a small horizontal slit to hold the paper clip and then attached the rubber band to the paper clip. The students looped the string through the rubber band and placed the poster board on the table. They placed the glue bottle at the end of the poster board and then pulled the string just enough to straighten the rubber band.
They then pulled the poster board along three different surfaces (the table, the sand paper, and the wax paper) and measured how far the rubber band moved each time.
The wax paper had the least amount of friction so the rubber band stretched the least when the poster board was placed on this surface. The sand paper had the greatest amount of friction, causing the rubber band to stretch a lot.

Baby carrot submarines.

This was a lab we didn't get to last week so I'd made myself a note to do this one at the beginning of class today. I got all excited about teaching friction that I completely forgot! Luckily, we had a few minutes at the end of class today to complete this buoyancy experiment.
Parents may remember those little plastic submarines that used to come in cereal boxes (I know my siblings and I had quite a few!) This is a really fun experiment to try at home.
Cut a baby carrot in half then use a flat-head screwdriver to make a hole in the cut side of the carrot. Break two toothpicks in half and insert the pieces in the rounded part of the carrot (2 on each end).
Fill the hole in the carrot with baking powder and then place your sub in a bowl of room-temperature water.

 At first, the carrot submarine is more dense than the water. However, when the baking powder and water react, they produce a gas (carbon dioxide) that is less dense than the water. This causes the carrot to rise to the surface. When the bubbles dissipate, the carrot will sink back to the bottom. This will continue until all the baking powder is gone.

Reminder: There will be no class next Thursday, November 11th, in honor of the Veterans Day holiday.

To look forward to on November 18th: Kinetic and Potential Energy; The Law of Conservation of Energy

The labs "Roller," "Farther," "Wobbler," and "How Far?" 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.

I found the baby carrot submarine directions on this website:
http://www.coolscienceclub.tripod.com/baking_powder_submarine.html

October 28th - Buoyancy

We continued our discussion of buoyancy today. We started out reviewing displacement and then seeing how displacement, density, and buoyancy are related.

Density can be described as a way of comparing the "heaviness" of materials. I gave the students an example of a basketball and a rock that weigh the same. The rock is smaller than the basketball but it is heavier. Since the rock has its weight held in a smaller package, the rock is denser than the basketball. 

The first activity today was a demonstration lab - "Mover." For this lab, we had a plastic water bottle that I cut the top off to make a funnel. The bottom of the bottle was filled with cold tap water that we added four ice cubes to. We then filled a measuring cup with warm tap water and added blue food coloring to the warm water. Using the funnel, I poured the warm blue water into the ice cold water while the students observed what happened. We noticed the blue water rise to the top of the bottle.
This happens because cold water is denser than warm water. This is due to the cold water contracting while the warm water expands. A drop of cold water, then, is more dense than a drop of warm water. The dense cold water settled in the bottom of the bottle while the less dense warm water rises. This also explains why the surface of the ocean feels warmer than deeper water in the same section of ocean.

I then dropped a nail in a glass of water and asked the students why, if the small nail sank, can large cruise ships and aircraft carriers float? The students provided some suggestions and we completed the lab, "Floating Boat," to find out.

For the "Floating Boat" lab, the students were each given two 12-inch squares of aluminum foil and 20 paper clips. The students placed 10 paper clips on one of the pieces of foil and scrunched the foil into a tight ball. They then created a square boat with the other piece of foil and placed the remaining 10 paper clips into the foil boat.

The students each placed their boats and foil balls into a bucket of water. The boats floated as they were supposed to. The balls were supposed to sink but ours all floated!

Even though the ball and boat have the same weight, the ball takes up a smaller space making it the denser object. The ball pushes less water out of the way than the boat so there is not (should not be) enough upward force to cause the ball to float.
Large ships are able to float, despite their great weight, because they have hollow compartments that are filled with air. This air increases their buoyancy.

Continuing our discussion of buoyancy and buoyant force, we moved on to discuss differences in salt water and fresh water. I asked the students if there is a difference in the buoyant force of fresh water and salt water. I also asked which one they thought had a greater buoyant force. To test their hypotheses, the students completed a hydrometer lab. For this, we filled a pen cap with modeling clay and then dropped it in a jar of tap (fresh) water. The students then added salt (1 tablespoon at a time) and observed the changes in the pen cap. We found that adding salt caused the pen cap to gradually float to the top of the jar.

The hydrometer lab led into a discussion of the Dead Sea. The Dead Sea is the lowest point on earth. Its elevation is 1,300 feet below sea level! We compared this to the elevation of Vista which is 563 feet above sea level. The Dead Sea is also the saltiest body of water on earth - it is almost 10 times saltier than any of earth's oceans! The minerals in the Dead Sea make the water so dense that people are able to bob on the surface like a piece of cork.

We finished the class session with two last lab projects: "Risers" and "Subs."

For "Risers" the students were each given a clear plastic cup, club soda, and some modeling clay. They divided the modeling clay into five rice-sized pieces and added club soda to the cups. The students immediately dropped the pieces of clay into the cups of soda and watched what happened. The carbon dioxide bubbles in the soda stuck to the clay causing the balls of clay to be light enough to rise to the surface of the cup. Once the clay reaches the top, the bubbles are knocked off and the clay sinks back to the bottom of the cup.

To complete the "Subs" lab, each student was given a plastic cup and a flexible drinking straw. Each student had a turn placing the cup on its side and pushing it beneath the surface of a bowl full of water. They then turned the cup so it sat upside down on the bottom of the bowl. Each student slipped the end of the straw under the rim of the glass and blew through the straw while supporting the glass with one hand (while trying not to restrict the movement of the glass). The students observed that the glass tried to rise to the surface of the water while they were blowing into the straw.

 

In order for submarines to sink, special tanks on the subs are filled with water. This is similar to the students filling their cups with water. To allow the sub to surface, the water in the tanks is replaced with air. We demonstrated that by blowing into the cups. The air makes the submarines more buoyant, allowing them to float to the surface.

To look forward to next week: Friction!

The labs Floating Boat and Risers 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 Mover, Hydrometer, and Subs are from 200 Gooey, Slippery, Slimy, Weird, and Fun Experiments.
VanCleave, J. (1993). 200 Gooey, Slippery, Slimy, Weird, and Fun Experiments. New York: John Wiley and Sons, Inc.