Hurray for Snow!

This morning, we were greeted by a fresh blanket of snow. Unfortunately, it doesn't look like this snow will last very long. Temperatures are predicted to be well above freezing today. I wonder how long this snow will manage to stay on the ground before it melts... ... Can we use some basic physics formulas to predict?

To get started, we need to make a few assumptions:

1. Let's look at 1 square meter of ground that is covered by 2 inches of wet snow. To determine the mass of this snow, use the conversion that 1 inch of wet snow is approximately 0.75 cm of rain. Furthermore, 1 cubic cm of rain has a mass of about 1 gram. What is the total mass of the snow on our 1 square meter of ground (give your answer in kg)?
2. Assume the ambient temperature is exactly freezing (0° C). As the sun shines down on the snow, some of the energy is absorbed to melt the ice into water. You should recall from chemistry that the amount of energy required to melt one kilogram of a substance is called the heat of fusion. The heat of fusion of ice is approximately 334 kJ/kg. Using our answer to question 1, how much heat is required to melt all of the snow?
3. The sun produces tremendous amounts of energy. Each second, approximately 1000 J of energy are delivered to each square meter of the Earth's surface. Assuming all of this energy works to melt the snow, how long will it take to completely melt all of the 2-inch-deep snow cover?
4. Does the answer to question 3 seem reasonable?

In our calculations, we have made quite a few assumptions and also omitted a few important factors.

1. Can you think of any assumptions or omissions we've made that might explain why our calculated value (problem 3) is higher than it should be? Explain!
2. Can you think of any assumptions or omissions we've made that might explain why our calculated value (problem 3) is lower than it should be? Explain!

Post your answers to any of the above questions in the comments section. Please be sure to indicate which question you are addressing.

ACT Test Prep: A Call for Feedback

Photo by Alberto G.

In Physics class, a typical day starts with us taking a look at the "ACT Question of the Day." This takes approximately 5 minutes of class time each day, and  I'm trying to evaluate its effectiveness.

1. Do you think you are learning any "tricks" or test taking strategies by answering these questions every day?
2. Do you think you will be able to perform better on the actual ACT because of our practice on these questions? Why or why not?
3. Do you actually give an honest effort to try to answer the question each day? If not, why? What could we do to encourage you to try harder?

Post your comments here and I'll consider making changes if it makes sense.

Info about Eris

Alexander Novati, Jet Propulsion Laboratory

Recently, astronomers were able to monitor dwarf planet Eris as is transited a star. This means that, from our (Earth's) perspective, Eris traveled directly in front of a distant star. This transit event was informative because it allowed us to measure Eris's size with unprecedented accuracy. The results were a bit of a surprise... ... Eris turns out to be a little smaller than previously thought. This seems to be because Eris's surface is more reflective than previously thought.

Besides astronomers, many people in the general public are interested in the size of Eris. Why does this relatively small object have such an important role to play in our solar system (specifically, the classification of objects within the Solar System)?

If you can't answer the previous question, let's start out with some basic facts about Eris. Where is it? When was it discovered? By whom? How big do astronomers think it is?

Super Cool Video!

Take a minute to watch this amazing video. Isn't this awesome!? Unfortunately, we can't get results like this with normal materials. The physicists involved refer to the disc as a "superconductor." What, exactly, is a superconductor, and why is it necessary for this effect to work?

Keep Sliding

Photo by kimfaires

Imagine giving a hockey puck a nice shove on a huge sheet of ice. We all know that the puck will slide for a long time before coming to rest. However, doesn't Newton's First Law predict that the puck should slide forever? After all, "an object in motion will stay in motion with a constant speed along a straight line path," right?

Why does the stopping puck not violate Newton's First Law? How can Newton's Second Law be used to predict the distance the puck will travel before stopping?