Today was the first homeschool physics class, and with most first classes in science we went over some basics.  We discussed scientific notation, metric units, significant figures and uncertainty.  I started by showing the two cartoons you see here.  I love the No Cussing! sign (found on this blog https://camarojones.wordpress.com/2008/10/21/the-metric-system/ – this blog has since been deleted)  because we don’t really have any rules in our house about cussing. Nobody gets in trouble for cussing… or I would be in time out all day.The USA is one of the last countries to keep using these archaic units and it drives me nuts.  We’ve had a meter stick in our house since the kids could walk, maybe even before then, and we have always measured things in meters or centimeters.  But when all the street signs are in miles not kilometers, its hard to understand what a kilometer means.    But we need to have a feel for these other units so we went over how to compare them with the ones used in USA.  We started with the meter stick, which is just a bit longer than a yard, and for first order estimates you can just remember a meter is approximately a yard or 3 feet.  So now you know a board that is 12 feet long is roughly 4 meters long.  Which is longer a kilometer (km) or a mile?  A mile is longer by quite a bit,  there’s roughly 1.6 km in every mile (1.609 to be exact), so if you’re driving in Europe and the sign says 15 km to the next rest area, you know you’ve got about 10 miles to go, you’re just looking for a rough estimate you can use 1.5 km ~ 1mi.

The other unit that causes a lot of confusion is the kilogram (kg) or gram (g), mainly because its a unit of MASS, not weight.  But if someone asks for your weight in Europe they are expecting you to give an answer in kg, so really, they are asking for your mass.  Mass is the amount of matter that makes up an object. Its the same amount of stuff, no matter where you are. Your mass is the same on Earth, the moon, Mars, everywhere.   But your WEIGHT is the force you are exerting on the scale when you step on it and that depends on the force of gravity, so that will be different if you step on the scale on the surface of the moon, or any other planet.   In science we make a very clear distinction between mass and weight but in everyday life, they are treated the same so its one of the concepts students can have a lot of trouble with.  We’ll get into mass versus weight, later in the class when we discuss gravity but today I just wanted them to know that their mass in kg is roughly half their weight in pounds.  1 lb = 0.454 kg, so if you weigh 120 lb then you have a mass of about 60 kg.  A 5 lb bag of sugar is roughly 2.5 kg, etc.
If you have these little math manipulatives around the house, they are 1 cm on a side and have a mass of 1 gram each, so you can actually use them in a balance to find the mass of other objects.  Frequently if we read something is 5 grams I’l remind the kids about the cubes and say that’s the same mass as 5 of those little cubes.  Just gives them a frame of reference.