I started class with the following videos on fluids, density, pressure and buoyancy.
Before showing the last video, I crushed a few soda cans using air pressure, then showed the Mythbusters using air pressure to crush a steel tanker car! For the demo, I put just a little bit of water in the soda can and then place it on a hot plate.
When I see steam coming out of the top of the can, I grab it with tongs and flip it upside into a bowl of ice water. The ice water cools the can so that the water vaper (steam) inside the can condenses into a liquid leaving the air pressure inside the can very low compared to the air pressure outside the can which is what crushes the can. The Mythbusters do the same but used a vacuum pump to lower the air pressure inside the tanker car.
The first lab the students did was “Pressure Differences” from Unit 1, lesson 5 in Science Fusion Module I. Students had two straws, one placed in a beaker with water (and a few drops of food coloring) and the other is held horizontal so that they can blow air across the top of the first straw. As they blow across the straw their lab partner watches the colored water move in the clear straw.
When the student blows across the top of the straw, the pressure inside the clear straw decreases and the higher air pressure in the beaker pushes water up the straw. This is how a straw works when you use it to drink as well, you suck out the air in the straw which causes low pressure and the high outside pressure pushes down on the liquid in the cup forcing it up the straw.
The second lab was “Finding the Buoyant Force” , also from Unit 1, lesson 5. For this lab students took a bit of clay and squished onto a string and found its weight in Newtons by hanging it from a spring scale. They also filled a graduated cylinder about half way with water and recorded the volume of water. Then they lowered the clay into the water, still hanging from the spring scale and observed that the force measured by the spring scale decreased as the clay was submerged in water. The force decreased because there is now a buoyant force acting on the clay, pushing up on it. When the clay is completely submerged, students measured the force on the spring scale and the water level in the graduated cylinder (volume of original water + clay). Students then calculated the buoyant force by finding the difference in the spring scale readings (F above water – F in water) and they calculated the volume of the clay.
Since the density of water is 1 g/ml we can actually calculate the buoyant force since its equal to the weight of the water displaced by the clay. Say the volume of the clay was 10 ml, that means 10 ml of water was displaced and that much water has a mass of 10 grams (density = mass/volume = 1.0 g/ml for water so every ml of water has a mass of a 1 g). The weight of that displaced water is 9.8 m/s2 times its mass (10g = 0.010 kg) which equals 0.098 N. This matches the buoyant force they found by using the spring scales.
We ended class by crushing the rest of the soda cans with air pressure…. because physics is fun. Here’s a video of people crushing a 55 gallon steel drum with the same method I used for the can.