a tall, glass container. I used a glass vase, but you could also use a glass jar or the chimney from an oil lamp.
WARNING: This experiment uses a candle flame. Be sure to get permission, have an adult around, use common sense and take proper safety precautions.
Cut the candle so that it is at least 4 or 5 inches shorter than the glass container. Tear off a piece of foil and crumple it around the base of the candle to form a holder. This will need to be stable enough so that the candle will not fall over when it is moved. Place the candle in the holder on the plate and move the plate from side to side to be sure that the candle is stable enough and won't fall.
Next, light the candle. Let it burn for a few seconds, so that the flame can stabilize. Then move the plate quickly to one side. Watch the flame as you move the plate. If you slide the plate to the right, the flame leans to the left. If you slide the plate to the left, the flame will lean to the right.
Turn the glass container upside down and place it over the candle to form a cover. Be sure that the flame does not reach near the container. You don't want it to get hot and crack. If the flame is too close, blow out the candle and trim it to make it shorter.
Once the flame is the right height, place the container over the candle. Again, slide the plate quickly to one side. Watch the flame carefully. Wait a minute! This time, if you slide the plate to the right, the candle flame leans to the right. Instead of leaning in a direction opposite the movement of the candle, this time it leans in the same direction as the movement. Try this a couple of times and then take the cover off of the candle before it gets too hot.
What is going on? Why does covering the candle with the container make a difference? Without the cover, the air around the candle was sitting still. Because of inertia, the air stayed still until the candle pushed it. The definition of inertia tells us that an object that is sitting still will continue to stay in one place until something pushes hard enough to make it begin to move. This resistance of the air to movement bends the flame in the opposite direction from the movement.
When you put the cover over the flame, the cover pushes on the air to move it along with the candle. You might expect that the cover would cause the candle flame to stay upright, just as if you had not moved it. In fact, if you continue to move it a straight line, at the same speed, it will quickly return to the upright, just as if it was sitting still. It is only when the movement of the candle speeds up, slows down, or changes direction that it will lean. Again, inertia is the cause. Think about being in a car. If it is sitting still, say at a traffic light, and you suddenly step on the gas, the car will move forward. What do you feel then? The inertia of your body resists movement and you feel like you are being pushed backwards into the seat. The same thing happens to the air inside the container. But wait, doesn't that mean that the candle flame should lean in the other direction?
The heat of the flame causes the gases in the flame to expand. This means that they are less dense than the surrounding air. Less density means less inertia and less force is needed to move it. The denser air around the flame has more resistance to movement and so it is forced to the side of the container that is opposite the direction of movement. This pushes the lighter gases of the flame towards the front, in the direction of movement. As we have seen in a past experiment, you can observe the same thing with a helium balloon in a moving car. When you speed up and feel like your body is being pushed backwards into the seat, the balloon will move forward, just as the candle flame did.
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