Mass and energy are conserved. This means you can’t create or destroy them, but you can change their location or form.
Most people don’t understand that the E energy term means all the energy transformations, not just the nuclear energy.
The energy could be burning gasoline, fusion reactions (like in the sun), metabolizing your lunch, elastic energy in a stretched rubber band… every kind of energy stored in the mass is what E stands for.
For example, if I were to stretch a rubber band and somehow weigh it in the stretched position, I would find it weighed slightly more than in the unstretched position.
Why? How can this be? I didn’t add any more particles to the system – I simply stretched the rubber band. I added energy to the system, which was stored in the electromagnetic forces inside the rubber band, which add to the mass of the object (albeit very slightly). Read more about this in Unit 7: Lesson 3.
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For plants, this means that energy from captured sunlight, combined with carbon dioxide and water, both of which have mass, make the plant heavier. Let’s find out how Einstein would have planted a garden while thinking about his big ideas.
Materials:
- scale for weighing your plant
- pot with soil
- plant (not potted yet)
- water
- time
- notebook and pencil to record your findings
Download Student Worksheet & Exercises
- Prepare a pot with dirt. Add a measured amount (like 1 cup) of water to dampen the soil. Weigh the pot filled with soil (but no plant).
- Add a plant to the pot and weigh the whole thing.
- Subtract the weight you found in step 1 from step 2 to find out how much the plant weighs.
- You’ll be weighing your pot each day. Weigh the plant before watering (water it the same amount each day) and write it down in your notebook . If you’re giving it water and sunlight, the plant should be getting heavier.
- Where does this mass come from? You can’t create mass, and yet the plant is getting heavier. How?
You and I get heavier when we eat food. You aren’t giving the plant food, but it is getting food. How? Where does its food come from? The energy from the sun is changed to sugars during photosynthesis, increasing the mass of the plant.
Exercises
- Where does this mass come from? You can’t create mass, and yet the plant is getting heavier. How?
- Can energy be created?
- Can energy be destroyed?
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