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

    The “why” is determined by observing “what happens if…”. For example, when you first drop a single ball, why does it bounce at all and not stick to the floor? It’s a collision between the ball and the floor. You can tell it’s an inelastic collision because the ball doesn’t bounce as high as the drop height, and gets lower and lower with every bounce. Some of the ball’s energy went was lost to heat and sound (did you hear it bounce?)

    Now watch what happens when you drop the second ball by itself – notice how high it goes with the first bounce.

    Now drop the two together, one on top of the other and notice the height of each one as it bounces. Did you notice one went a LOT further, and the other hardly at all? That’s the momentum being conserved and the energy transferring from one object to the other.

    The math involved with proving that the energy must transfer is actually quite involved, which is why we’re looking at actually WHAT happened and figuring out what’s going on through observation. You’d actually look at the forces involved before, during and after the collision in order to map out the entire system.

    Does that help?

  2. Koachkaren says:

    Thanks for the feedback! I’m still missing something as to the “why” of the transfer of energy. I see the transfer of energy and conservation of momentum. I understand the concept in general, but my missing link is understanding why the bottom ball has a need to transfer the energy. Why don’t both balls just bounce up? Does it have something to do with the bottom ball actually being stopped whereas the top ball only changes direction of motion?? It’s driving me crazy trying to figure out what I’m missing.

  3. Aurora says:

    Oh, my… it sounds like fun and educational! Yes, scientists DO need to wear goggles!!!

    I would do at least 3 trials so make sure you’ve got repeatable results, especially if one of them doesn’t make sense or is different than you expect. I would also weigh all three, because it’s the mass change you’re most interested in.

    If you want to take it a step further, you can calculate your momentum both before and after and see if they are about the same.

    Momentum = mass x velocity

    For calculating initial velocity of the top ball after collision, if you can get the top ball to go near-vertical in its flight, notice how high it went and that’s your “h” in the following equation (make sure your number for h is either in feet or meters and then choose the appropriate value for “g”):

    v = sqrt (2 x g x h)
    where g = 9.81 m/s^2 or 32.2 ft/s^2 (sqrt = square root)

    Now you can find out exactly how much momentum your system had both before and after, and you should have the same number!

    I like the idea of trying multiple balls – that’s exactly how this program was intended to be used – great job!!!

  4. Aurora says:

    Great question! Momentum conservation is really a statement of Newton’s 3rd law of motion. If you notice when the balls drop, they have a certain momentum (mass x velocity) which is equal to the momentum of the top ball after the collision (the mass doesn’t change, so you see a significant increase in the velocity after impact). Momentum conservation is one of the basic laws of physics.

  5. Koachkaren says:

    HA! Next observation and question: We are using 3 balls and observing the differences.
    Ball 1: small, solid, bouncy ball (~1inch diameter)
    Ball 2: medium, air filled bouncy ball (~4 inch diameter)
    Ball 3: Size 5 soccer ball (~10 inch diameter)
    Experiment 1: Ball 1 over Ball 2. Sweet spot is maybe 8 inches above ground so that Ball 1 doesn’t have time to roll to one side. Observation: Ball 2 bounces very slightly. Ball 1 shoots off. PERFECT!
    Experiment 2: Ball 2 over Ball 3. Same height. Ball 3 stops dead and Ball 2 shoots off way high. AWESOME!
    Experiment 3: Ball 1 over Ball 3. All same height. Ball 1 only bounces slightly higher than it would on it’s own. Ball 3 retains some of it’s own momentum and bounces more. We know it either has to do with materials of the balls or, more likely, the discrepancy in ball size. So, we thought maybe of the energy that is returned to the balls after colliding with the floor, only the energy that is in the direct spot that ball 1 is in is being transferred. All other energy is still dissipating through the Ball 3. ???
    Experiment 4: My husband and son decided to venture off into the great wide unknown and try the experiment with all 3 balls stacked. 1st time: great results. Ball one flew off! 2nd time: My 8 yo son had to help hold the balls in place before dropping and then had the painful lesson of learning why scientists usually wear goggles… Smacked him right in the eye! Poor kid is icing it now. :'( But, no worries. He looks fine.

  6. Koachkaren says:

    Hi Aurora,
    We are having fun experimenting with various heights from which to drop the balls in order to make the top ball bounce up rather than have time to roll to the side a bit and fly out in another direction. The thing that we’re wondering is, why is the momentum transferring in the first place? Why is it that the bottom ball isn’t bouncing and instead is transferring energy to the top ball?

  7. 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.

  8. mrsrohland says:

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

  9. Aurora says:

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

  10. 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 🙂

  11. Aurora says:

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

  12. 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!

  13. 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.

  14. 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.

  15. valerie faessel says:

    would you say that velocity = acceleration ? Thanks.

  16. 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:

  17. 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.

  18. 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?

  19. Talitha Mosley says:

    2nd video will not play.

  20. Shumaila Khan says:

    The first video is not playing at all.

  21. Aurora says:

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

  22. 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. 😛

  23. 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!

  24. 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?

  25. Cynthia Fillmore says:

    It worked!

  26. Aurora says:

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

  27. Cynthia Fillmore says:

    what would happen if had two small balls?

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