We're going to experiment with Newton's Third law by blowing up balloons and letting them rocket, race, and zoom all over the place. When you first blow up a balloon, you're pressurizing the inside of the balloon by adding more air (from your lungs) into the balloon. Because the balloon is made of stretchy rubber (like a rubber band), the balloon wants to snap back into the smallest shape possible as soon as it gets the chance (which usually happens when the air escapes through the nozzle area). And you know what happens next - the air inside the balloon flows in one direction while the balloon zips off in the other.

Question: why does the balloon race all over the room? The answer is because of something called 'thrust vectoring', which means you can change the course of the balloon by angling the nozzle around. Think of the kick you'd feel if you tried to angle around a fire hose operating at full blast. That kick is what propels balloons and fighter aircraft into their aerobatic tricks.

We're going to perform several experiments here, each time watching what's happening so you get the feel for the Third Law. You will need to find:

  • balloons
  • string
  • wood skewer
  • two straws
  • four caps (like the tops of milk jugs, film canisters, or anything else round and plastic about the size of a quarter)
  • wooden clothespin
  • a piece of stiff cardboard (or four popsicle sticks)
  • hot glue gun
First, let's experiment with the balloon. Here's what you can do:

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11 Responses to “Newton’s Third Law of Motion”

  1. Its no trouble at all! Unit 21 – Advanced Physics helps provide the additional formulas. The advanced labs will be easier to tackle after the lessons in that unit.

  2. megan_northrup says:

    Yes thank you, we did problem 1 with no problem. And yes I think we are not using the right formula for #2. But there are no other formulas suggested in the lab or in the materials I have looked at for this lesson. Is there a reference sheet or someplace where I can look up formulas pertinent to this class? (My daughter is the student, and maybe she has seen the formulas needed in earlier lessons or videos, but forgotten. I have not had time to go through all the course videos with her. I am just trying to jump in and help when she needs it on the advanced labs, and a crib sheet of suggested formulas would be very useful. I find myself clicking around, guessing, not sure where to look to find formulas that we need to solve these types of problems.) Thank you & sorry for the trouble!

  3. Keep in mind that, in problem 1, you need to convert miles per hour (mph) into meters per second (m/s). Once you have the velocity in m/s, you can calculate the acceleration (which will be in m/s2). Once you have the acceleration, you can then use that value to solve problem 2. However, you need to use a formula that allows you to solve for distance when you know the acceleration and the distance travelled. I think you may be considering the wrong formula as you try to work backward.

  4. megan_northrup says:

    Thank you, we will go to unit 21! Sorry for the trouble.
    Since we have the answer key, we are trying to solve it in reverse. If the time is 5 seconds, the distance (answer) is 375m, then the velocity must be 75. But, why? Where does that come from? And how should we have known that in order to solve it correctly?
    Does that make sense?

  5. Yes if you are upper level students, you will want to work through Unit 21: Adv Physics which focuses on the problem-solving techniques for physics problems in detail.

  6. I am not sure I am understanding your question – please try again? Where are you seeing the “75” for velocity?

    For help with physics calculations (which has algebra all over it), please refer to Unit 21 sections 1 & 2 for complete worked-out problems done on video.

  7. megan_northrup says:

    *not squared, just 150 m/second

  8. megan_northrup says:

    Thanks. I don’t think it’s the algebra that’s the problem – it’s the physics concepts. I wish we could see a picture of the problems solved out. For instance – in # 2 – why is the velocity 75? Why isn’t the velocity the full 150 m/second(squared) that we calculated in #1?

  10. megan_northrup says:

    Do you have videos where you explain how to solve the problems in the advanced labs? We are trying to work through the advanced labs by ourselves and sometimes need assistance with the problems at the end. For instance, in this lab we did #1 but we are not sure how to do #2 and #3. It seems like they should be simple to solve, but without a live tutor or teacher to give tips, we just aren’t seeing it.

  11. emilyannejon says:

    I can see the fishing line in the first video 🙂
    Matilda