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It’s all about Mass: Investigating inertia
So, what is that feeling of butterflies in the stomach?
It turns out that if an object (such as your stomach) is not moving, it tries to remain that way and resists any attempts to get it moving. It actually requires a force on the object in order to overcome its inertia and make it move. At the top of the roller coaster, your stomach (and all that is in it) is sitting fairly still. But as the coaster goes into a big drop downward, your body drops with it. The contents of your stomach attempts to remain in a fixed position and that’s why you feel that strange sensation we call "butterflies in the stomach".
How about in a car?
In a moving car, you are in motion right along with the car. If the brakes are applied very quickly to stop the car, your body attempts to resist that change of motion and you can sense your inertia. You actually feel your body resist stopping as you continue to move forward and it feels as if there’s a force pushing you forward. This is where your seatbelt comes in to stop you from continuing to move forward. Seatbelts help to "manage" your inertia, so that you don’t get hurt.
Here’s a warm-up exercise to demonstrate how most things behave when you change their motion. We’ll change the motion of an object by accelerating it. The more we accelerate it, the more it will resist that change in motion.
WHAT YOU NEED:
- o A pen or ball
- WHAT YOU DO:
1. Starting position: Put a ball or the pen on your outstretched flat hand and with your palm facing up, make the following motions with your hand – very quickly!
- Accelerate your hand forward (return to Starting position)
- Accelerate your hand backward (return to Starting position)
- Spin your whole body around!
WHAT HAPPENS:
You should notice that as you accelerate forward, the object stays put, appearing to roll backward on your hand.
As you accelerate backward, the object does not move and appears to roll forward on your hand. As you whirl around in a circle, the object remains fixed and appears to roll outward away from the circle’s center.
As you do the helium balloon experiment, compare what you just observed with the motions of the balloon.
Experiment 1: THE HELIUM BALLOON
WHAT YOU NEED:
- A helium filled balloon with a string attached to its end
- A moving vehicle (a car, van, bus….any moving vehicle will do). Make sure the windows are up and the fan is off.
Warning to the driver: Please note the driver should NOT be performing this experiment. It is further advised that whomever performs the experiment sit in the back seat to ensure the vision of the driver is not obstructed.
WHAT YOU DO:
Whenever you accelerate the vehicle, the helium balloon will lean in the direction of the acceleration.
1. Hold the string attached to the balloon, so that the balloon floats without touching the ceiling. Try the following 3 accelerations:
- Starting from a stationary position, move forward (acceleration is forward)
- In motion, apply the brakes (acceleration is backward)
- Go around a turn (acceleration is inward toward the center)
WHAT HAPPENS:
You should notice that the motion of the balloon (in the direction of acceleration) is exactly opposite the motion of the first experiment and also the motion of anything else free to move in the vehicle.
The leaning of the balloon is most easily observed as you go around a curve because in this case, as long as you are turning, you are also accelerating.
Let’s discuss how this works.
HOW IT WORKS:
To understand the physics behind this experiment, there are two main principles to discuss.
The first is how things float and the second is inertia of the air.
How things float:
Any object that floats, whether in air or in water, does so because there’s a force pushing it up. That force is called the buoyant force, which acts on all objects. If this force is greater than the objects weight, then the object floats. So where does this buoyant force come from?
Let’s investigate.
A helium balloon floats upward in a room filled with air. We learned in the "air pressure experiments" (in Segment 3) that air has weight. If you piled up a stack of bricks from the floor to the ceiling, where do you suppose the greatest pressure on the bricks would be due to its own weight?
Near the top or the bottom of the stack?
Definitely near the bottom since there’s so much more weight above it. In the same way, there is air from the floor of a room to the ceiling. There is also more air pressure near the floor than at the ceiling. Helium balloons (or anything that floats in air) float toward regions of lower pressure. Since the bottom of the balloon is at a higher pressure than the top, there’s a force on the balloon upward toward regions of lower pressure. This is where the buoyant force comes from.
Since a helium balloon is so light, it does not weigh very much. The buoyant force upward is greater than it’s weight downward, so the balloon rises.
To understand what happened in the moving vehicle, we need to add one more piece of knowledge to what we’ve learned so far.
Inertia of the air:
Since air has mass, it has inertia. That means when the brakes are suddenly applied to a moving car, the air continues to move forward while the car slows down, until the air comes in contact with the front of the vehicle to slow it down as well.
As this happens, the air pressure in the front of the vehicle increases and decreases toward the back. Now there’s a difference in air pressure, which pushes the balloon toward the back of the vehicle where the air pressure is lower. The balloon will lean in the direction of acceleration.
In the same way as you go around a turn, there’s a difference in the air pressure inside the vehicle. The air continues to move in a straight line while the vehicle turns. As a result, the air builds up on the far side of the vehicle creating a greater pressure on that side.
When the truck makes a left hand turn, the acceleration is inward toward the center of the turn. The balloon will also lean in this direction, contrary to anything else that’s free to move in the vehicle. There is greater air pressure on the right side of the trailer and less air pressure on the left side. Therefore, the balloon leans in the direction of less pressure!
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