Weight.

One of my favorite things about teaching science is watching students realize how neat the world actually is. Teenagers, especially, like to think they know everything, and are prone to acting nonchalant about new information. "Duh. Everyone knows that...." Sometimes accompanied by an eye roll.

But every now and then I can surprise them with something they've never had to think about before. It happens a few times in physics. Good ol' cognitive dissonance. People often think that the math in physics is what's challenging, but it's really not. If you can do algebra, you can do all the math you need to for high school physics. The theory is what's tricky... any one can put a number into a formula and get an answer... but ask them to explain why the water in the bucket over my head doesn't fall out when I swing it?

One of the more challenging concepts for students is the idea of "weight". How much matter is in my body is called my mass, and I measure it in kilograms. We are brought up believing that this is our weight, but it's really not.

Weight is the force exerted on you by the Earth's gravity. When you step on a weigh scale, the scale is calibrated so that it takes the acceleration due to gravity into effect and tells you your mass, not your weight.

Fair enough, you might think. So what's the tricky part?

The tricky part comes when you ask a student why they feel like they have mass. (I once made the mistake of rhetorically asking a group of grade eleven's why I feel heavy, and one of them shouted out "'Cause you're fat!" much to the shock and horror of his classmates. I laughed.) What causes you to feel like you have "weight"? Most people will answer with gravity. But if I ask what they would feel if I threw them off a cliff, they all know that they would feel "weightless".

Feeling like you have mass is a condition of being in contact with a surface, namely the ground. My mass pushes down on the Earth, and in accordance with Newton's Third Law (for every action there is a equal and opposite reaction) the Earth pushes back. In physics this is called the normal force, or your apparent weight. The Earth pushing back on me is what I feel, what gives me the illusion of having weight. The force of gravity is still pulling on me when I fall off a cliff; what is missing is the normal force.

My apparent weight can change under different circumstances as well. Accelerate me up in an elevator and I feel heavier. Start going down and I will momentarily feel lighter as the floor falls out from underneath me and supplies less normal force. It's a mind-baffling concept, because it is quite different from our every day perceptions about the world around us.

Tomorrow, I am taking my Physics 20 class to Galaxyland at West Edmonton Mall to make calculations and observations about different amusement park rides. I'm looking forward to having them challenge their existing beliefs and experience some of these concepts first hand!

The following is the video I used to illustrate how we can artificially create a "zero gravity environment" (really, there is still gravity... just no normal force, because the plane is falling at the same rate you are!). I hope you enjoy it as much as my students did!