Key concepts
Have you ever noticed that loose objects tend to move around in a car as it accelerates or decelerates? Now imagine a helium-filled balloon floating in a car. All of the windows are rolled up and the vents are turned off. When the car accelerates, what, if anything, do you think happens to the position of the balloon? The answer may surprise you! Reproduce this classic physics scenario with your own car (or public transportation) to learn how fluid dynamics can cause tethered, floating objects such as balloons to behave in unexpected ways.
We all know what it feels like to be thrown forward into the seat belt when a car abruptly comes to a stop. Lurching backwards into the seat when the car accelerates and getting yanked in the opposite direction when a car makes a sharp turn should be familiar sensations, too. Where do these forces come from? The truth is, they are not actually forces. These sensations are better described as side effects of our bodies' inertia, the tendency of matter to resist changes in its motion. An object will either sit still or move in a straight line unless acted upon by an outside force. The more mass an object has­—in other words, the heavier it is—the more it will resist the forces acting on it, such as those trying to slow it down or accelerate it.
Let's take the example of a passenger riding in a car that accelerates from a stop to a steady speed of 30 miles per hour. During this acceleration, the car seat pushes on the passenger to move her forward. Her mass wants to stay put, however, so she feels like her body lurches back into the seat. What she's really feeling is simply her own inertia, which needs to be overcome by the pushing force of the car!
The explanation above may already feel intuitive to you (but don't worry if it doesn't). Without totally spoiling the surprise, the coolest thing about the activity you are about to explore is that your results will totally defy this intuition (and if you want a hint that might help you understand what you are about to see, remember that a balloon filled with helium is lighter than the air around it).

  • Balloon inflated with helium
  • Golf ball or other small, heavy object
  • Two 3-foot lengths of string, dental floss or fishing line
  • Tape
  • Car, bus or subway train
  • Adult driver (if using a car) or supervisor (if using public transportation)
  • A residential street with little or no traffic (if using a car)
If using a car:
  • Make sure that your climate control is turned off and your windows are rolled up. Otherwise, small air currents may interfere with the results of your experiment.
  • If your vehicle has a middle row of seats, it might help to fold them down or remove them to make the most room possible for objects.
  • First, we're going to observe what happens with a hanging weight. Tape one end of your string to your golf ball or other small weight.
  • Tape the other end of the string to the car's ceiling so that the weight can swing freely and not interfere with the driver. Be sure to get permission to attach tape to the car's ceiling first!
  • Before conducting the activity, make sure all passengers buckle up.
  • As the observer, it is your job to watch the objects you are testing; the driver will be watching the road and traffic.
  • Have the driver maneuver the car to a residential street that is free of traffic.
  • Steady the weight so that it is no longer swinging.
  • Have the driver put the car into drive and sharply (but safely!) accelerate from a dead stop to a steady speed. What happens to the position of the weight? Is this what you expected to happen? Why?
  • Now have the driver come to a full stop. What happens to the position of hanging weight when the car decelerates? Is this what you expected to happen? Why?
  • Now it's time to set up the helium-filled balloon. Remove the hanging weight. Tie your second length of string to the knot of your balloon. Tape the other end of the string to floor of the car (receiving permission first).
  • If the balloon comes into contact with the ceiling of the car, shorten the length of the string. Make sure that the balloon's string is taut and that the balloon has room to swing both forward and backward from where it is positioned but does not interfere with the driver's visibility.
  • Have the driver put the car into drive and sharply (but safely) accelerate from a dead stop to a steady speed. What happens to the position of the balloon? Is this what you expected to happen? Why?
  • Now have the driver slow to a full stop. What happens to the position of the balloon? Is this what you expected to happen? Why?
  • Extra: Mount your hanging weight and your floating balloon side-by-side so you can directly compare how they behave when they undergo different types of acceleration. How do their positions change during acceleration and deceleration?
  • Extra: Compare the motions of the hanging weight and floating balloon again. Does either object tilt on its tether when a steady speed is reached? Why do you think this is?
  • Extra: Have the driver of the car make a steady left or right turn around a corner (while staying in motion, rather than from a full stop). How does the balloon move as you turn a corner? How does the hanging weight move? How can you explain your observations? Remember, turning is another form of acceleration because you are changing an object's movement.
If using a bus or commuter train:
  • The same general procedure you would follow using a car applies, but you will not be able to control the acceleration and deceleration of the bus. These changes in movement may not be quite as sharp, either, so changes in the positions of your test objects might be subtle. Watch for them carefully!
  • If you can, try to sit away from any open windows or air vents (older school buses without air vents are perfect for this activity).
  • Tape the first length of string to your golf ball or small weight and dangle it from your hand, making sure it has room to swing safely and does not hit any objects or interfere with any other passengers. What happens to the position of the weight when the bus accelerates? Decelerates? Turns? Why?
  • Put your hanging weight away and tie the second length of string to the helium-filled balloon. Hold the end of your balloon's string in your lap, making sure the balloon is free to move unobstructed (adjust the height of the balloon as necessary) and does not interfere with any other passengers. What happens to the position of the balloon when the bus accelerates? Decelerates? Turns? Why?
  • Extra: If you have room and permission, use a heavy book (such as a textbook) to tether your balloon to the floor of the aisle. As you anticipate each move the bus driver will make, try to anticipate where the balloon will travel as a result.

Observations and results
The hanging weight should swing backward during a forward acceleration due to its inertia. Remember, when the car starts moving, the weight wants to stay put because of inertia. So why don't we see the same thing happen to the balloon? When the car accelerates, the balloon tilts toward the front of the car! What's going on here? Something is pushing the balloon forward. And it's not the car itself—the balloon isn't resting on anything.
Gases (including the air we breathe and pure helium) and liquids (such as liquid water) are both fluids—substances with no fixed shape. Low-density objects placed in high-density fluids are buoyant; they float, like a light rubber duck toy in a swimming pool. Let's review density: if one object or substance (a solid, liquid or gas) is denser than another, then it has more mass packed into the same three-dimensional space. Imagine you are holding a rubber duck against the bottom of a swimming pool. Because of gravity, both the water molecules in the pool and the air molecules inside the hollow rubber duck experience a downward pull. However, gravity exerts more force on objects or substances that have more mass, so the denser water wins out over the rubber duck's air in the fight to pack itself against the bottom of the pool. As a result, the rubber duck gets pushed up and out of the way by the heavier water, which causes the duck to float to the surface.
Now let's take a look at how this might help explain what was going on in the car. Like everything else with mass, the air around us has inertia, so when the vehicle accelerates, the air molecules don't just get pulled along with the car without putting up a fight. In fact, we can think of the air molecules as traveling in the same direction as the hanging weight did during the car's acceleration. When the air sloshes toward the back of the car, resisting the forward motion, the molecules force their way around and behind the less dense helium inside the balloon. As the air molecules crowd themselves against the back of the car, they create a zone of higher pressure, which pushes the balloon forward and out of the way! The same basic principle explains why a helium-filled balloon floats in the first place: gravity pulls the denser air down toward the floor of the car, creating a zone of higher pressure beneath the balloon which pushes the balloon up toward the car's ceiling. By accelerating and decelerating the car, you are just changing the location of that high-pressure zone—and the direction of the resulting push on the balloon.
You can also try the activity again using an air bubble trapped in a jar of water. Turn the closed jar on its side and watch what happens to the bubble when the jar is subjected to acceleration and deceleration. See if you can you explain the behavior of the bubble!
More to Explore
A Baffling Balloon Behavior, from SmarterEveryDay on YouTube
Balloon-in-Car Puzzler, from NOVA Online
Buoyancy, from HowStuffWorks
Science Activities and Experiments, from

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