Newton’s Second Law of Motion

Second Law of Motion: Momentum is conserved. Momentum can be defined as mass in motion. Something must be moving to have momentum. Momentum is how hard it is to get something to stop or to change directions. A moving train has a whole lot of momentum. A moving ping pong ball does not. You can easily stop a ping pong ball, even at high speeds. It is difficult, however, to stop a train even at low speeds.

Materials: garden hose connected to a water faucet

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21 Responses to “Newton’s Second Law of Motion”
  1. Aurora says:

    Yes, you’re right! The energy went somewhere, and with a squishy foam ball, it goes into the squeezing the ball. Try something harder, like a bouncy ball.

  2. mrsrohland says:

    It works! We try a foam ball—and it didn’t work. We think it is absorbing the momentum???

  3. Aurora says:

    I would give it a try… anything with wheels like a skateboard or roller skates can work also!

  4. Tatiana Spencer says:

    ilana, 11 here! I don’t have a wagon and I wondered if I could make a Lego friends version but my mother thinks it won’t be heavy enough to measure results. What do you think Aurora? Thank you 🙂

  5. Aurora says:

    Oops – you’re right. That should read mass in motion. Sorry about that!

  6. Meg Peery says:

    Just curious — is “inertia in motion” a typo? Should it be “mass in motion”? If not, can you help me understand how what “inertia in motion” means? Thanks in advance!

  7. Rosalind Hitchcock says:

    Minor note: in the advanced student lab worksheet linked from this page (, answers are not provided for questions 1-5 (for 1-5, the “solutions” just repeat the questions) – maybe a copy/paste or versioning glitch in the document?

    Not having answers probably makes sense for 1-3, since it will depend on the specific items we choose, but it would be great to show at least how the problems should be worked, using variables. For 4 and 5, we know the answers :), but again, it would be great to have those included in the solutions section.

  8. Aurora says:

    No, acceleration is the rate of CHANGE of the velocity. When your car goes from zero to forty mph, you can do it slowly (like on a bike) or quickly (like in a sports car). The sports car has a faster acceleration than the bike. The velocity is the speed and direction, so 10 mph northwest is your velocity.

  9. valerie faessel says:

    would you say that velocity = acceleration ? Thanks.

  10. Aurora says:

    This unit doesn’t give a full description of Newton’s Laws or terms like inertia. It’s meant to just be an overview of what’s coming. I am sorry if this only caused more confusion for you!

    For inertia, check this page out:
    and here;s a neat experiment with inertia:

    For Newton’s second law, go here:

    For Momentum, here’s a couple of cool experiments you can do:

    Hope this helps!

    For older students, there’s an entire section on Newton’s Laws, momentum, inertia, and more here:

  11. Nicole Robinson says:

    I’m having a hard time teaching the second law to my kids because I don’t understand it. Could you help me understand this? You say that momentum is conserved, but what does that mean? How does that relate to a train and ping pong balls? Could you also define inertia for me? You state the law, but what you then show doesn’t seem to connect to the law.

  12. Aurora says:

    What happens if you try a different computer? The videos play fine over here, but that’s also why we’re updating all the players starting next week. What kind of computer and browser are you using?

  13. Talitha Mosley says:

    2nd video will not play.

  14. Shumaila Khan says:

    The first video is not playing at all.

  15. Aurora says:

    You need one to be a lot more massive that the other in order for this to work right. 🙂

  16. Marcia Urgino says:

    We tried doing it with 2 tennis balls. Neither of them bounced up; instead, they both went sideways with what seemed like equal velocity. We’re not quite sure what happened. 😛

  17. Aurora says:

    Yes – we’re currently working on implementing a new player that will work on all devices and need your feedback on it. We’re making changes today – so please check back tomorrow!

  18. Deborah Crowe says:

    Any video in Unit Zero with that particular video player (on the top side “Newtons second law of motion”) is not working sadly, however the second player on the bottom is working fine. It’s a mix of both video players throughout the unit, half working and half not working. Fix any time soon?

  19. Cynthia Fillmore says:

    It worked!

  20. Aurora says:

    Great idea! Try it and let me know how it goes! 🙂

  21. Cynthia Fillmore says:

    what would happen if had two small balls?

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