Lesson – Mass/Weight, Solids, Liquids, Gases, Density
Part 1 – Demonstration re: molecular structure of solids, liquids, and gases
- Arrange the desks in the room into a rectangular “corral” as shown below.
- Have each student report how much they weigh; write it down on the whiteboard. Have one student (or all, if they have calculators) add up all the weights, and calculate the average weight. Best if this is done in kilograms.
- Write the conversion factor: 1 pound = 0.45 kg on the whiteboard.
- Solid: Have the students all stand inside the corral (the circles in the illustration), and then close up the last table to complete the corral, and move the two end tables to tightly constrain the group of students. The students can “vibrate” in place but cannot move past their neighbors.
- Measure the area occupied by the students for the simulated solid; write it on the whiteboard. Take the height of the tallest student and use that as the height of the volume.
- Discussion: What are the physical characteristics of the molecules in a solid? What’s the difference between crystalline solids and amorphous solids?
- Liquid: I’m adding some heat to our simulated solid, so the molecules have more energy – they move around more. Back off the tables at one end to give the students a little room to move around, but just a little bit. Now they can slowly move past their neighbors.
- Measure the area occupied by the students for the simulated liquid; write it on the whiteboard. Same height as above.
- Discussion: What are the physical characteristics of the molecules in a liquid?
- Gas: I’m adding more heat to our simulated liquid, so the molecules have more energy – they move around more. Back off the tables on all sides sufficiently to give the students plenty of room to move around. Now they can move faster and can easily move past their neighbors.
- Measure the area occupied by the students for the simulated gas; write it on the whiteboard. Same height as above.
- Discussion: What are the physical characteristics of the molecules in a gas?
Students return to their seats.
- Have the students calculate the volume in cubic meters (1 foot = 0.3 meters) occupied by the solid, the liquid, and the gas.
- Have the students calculate the density of the solid, the liquid, and the gas using the total mass of the group. (kg / m3)
Part 2 – Mass and weight
- What is mass?
- Amount of stuff
- Number of molecules * mass of each molecule. For our demonstration, use the average mass of the students as calculated earlier.
- What is weight?
- Force exerted on an object due to gravity
- Changes with distance between the two objects (object being measured, and the center of the earth in our case)
- Weight on earth vs. weight on moon.
- What happens in weightlessness, or falling objects?
- How do we measure mass and weight?
- Demo with three scale types (spring scale, balance scale, triple beam balance scale).
- Why can’t I truly measure mass with the spring scale wherever I am (earth, moon, space)?
- Why does the balance scale accurately measure mass?
- Scientists use the metric system to define mass. Mention other units of mass (see conversion table below): ounces, pounds, stones (14 pounds, 6.35 kg), gram, kg (2.2 lbs), US ton (2000 lbs, 907 kg), metric ton (1000 kg, 2204 pounds), imperial (UK) ton (1016 kg, 2240 pounds) carat (0.2 gram).
- What units do scientists use to describe weight?
- Weight = mass * g (gravitational acceleration)
- 1 kg mass on earth has a weight of 9.8 newtons.
- Discuss weight on other planets (see table below). Does the mass change or just the weight? Why do things weigh more on the bigger planets?
Handout for the students