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Teaching moments - please help

Discussion in 'Science' started by brodiestar, Oct 27, 2010.

  1. Hi Science People

    Does anyone have any decent experiments for a lesson on moments? Physics really isn't my strong point and any help would be great.

  2. I have done a practical which involves creating a see-saw with a metre rule (or length of wood with marks at regularly spaced intervals) balanced on a pivot (triangular block of wood).
    Then place masses on the piece of wood eg, 50g placed 20cm from the pivot on one side will balance 50g placed 20cm from the pivot on the other side.
    If you leave one side the same it will balance 25g placed 40cm from the pivot on the other side etc
    ie moment = force x distance from pivot.
    This practical I find is great in theory, but may require a very careful explanation or even need to be given as as demo. The problem is that it is hard to balance the see-saw even without any masses on it and if you do get it balanced, when you begin to place the masses on if you are out by a millimetre then the see-saw topples! I let a lower ability group do this once and I don't think it did a lot to support the theory on the subject because of this problem. The more able could probably cope with it though.
    Sorry this is not a perfect solution, hope it helps a little!
  3. rachel_g41

    rachel_g41 Established commenter

    As the poster above says, when you're near balance, moving 1mm either way tips it. Point this out to them and tell them that if they get this close, not to worry about greater accuracy.
    I find this is very successful when done as a competition. Small groups, each with a seesaw and collection of known masses, and they all have to find the mass of a range of objects (I use whatever I can find handy - a stapler, a rubber etc) using moments.
    Then they all write their results on a the whiteboard (sometimes I think this is the bit they like best) and I award points for the closest. They get quite competitive about it.
  4. If you can do it with known masses on both sides, the children can then try to work out the rule - M1 x D1 = M2 x D2....
    ... which leads you into the definition of a moment
    ... and sets you up for M1 x D1 + M2 x D2 = M3 x D3 (demo?) - what's the rule now?
    ... but then I'm lucky and have a set of purpose built "see-saws"
    ... so I can then do the Big Bob challenge (Big Bob at 50g, has to work out where to sit to balance his children (Tiny Tim 20g and Little Eva 30g) on the other side of the see-saw
  5. blazer

    blazer Star commenter

    I have lots of little demos such so the smallest child in the class pushing the door open while I try to hold it shut.
  6. Thanks everyone, big problem is its an interview so I'm not sure how good the "see~saws" that I get will be, but it seems the way to go for the practical side of things. Thanks again.
  7. blazer

    blazer Star commenter

    Metre rule and 1kg mass. get a kid out and get them to hold the rule horizontally using one hand at the end of the rule. Slide the mass alon the rule till it is next to their hand. Ask them to keep the rule horizontal. Then move the mass a few cm ata time further from their hand, eventually they won't be able to hold the rule horizontal because the 'effect' the mass is having has increased.
    A good starter is to find a really long cardboard box, fill one end with a heavy mass (a few bricks). Then position the box sticking out over the end of the bench with the mass end on the bench, most of the box should be sticking out in this air. As the class come in you should stand holding the end of the box as if you are supporting it, talk to them, welcome them and then when they are all watching you just walk away from the box, usually they gasp because they expect the box to fall but it doesn't!
    Get a strong plank, choose the largest kid in the room. Put the plank across a brick on the bench with the brick close to one end of the plank. Get the kid on the bench and standing on the short end of the plank, get the smallest kid (or you can use one finger) to press down on the long end and the heavy kid goes up!
  8. blazer

    blazer Star commenter

    Check if the Maths dept have any balance beams that they use for multiplication. We borrowed then so often that eventually we bought a set of our own.

    Ours look like this but the masses are made from steel
  9. Thanks, they look like they would do the trick. I'll definitely be asking if they have any.
  10. There's a good activity on eChalk if you have access to it.

    I also did moments for my interview lesson, and I got the job. Since then I've added a competition element to the lesson. Have teams with a see-saw each. Give one member of the team a set of arrangements for the masses on one side, and the other members of the team have to balance it using only one mass.
  11. aer


    Balancing seesaws is definitely the way to go but, get the pupils to feel a moment:
    they can hold their own 30cm ruler at one end and place a mass of plasticine at different distances from their hand (the pivot). The further away, the plasticine from their hand, the bigger the moment. They will see the ruler bending more and feel the bigger moment too.
    Another way is to use them as the see saw. Have masses tied to thick string or rope and hang them off their arms. It always goes down well in the lesson and they can really feel the moments turning clockwise and anti clockwise.

    Just remember, always use the words mass and force correctly and Moment = force x perpendicular distance from pivot (important for teaching the higher ability or if its GCSE).
    Good luck!
  12. Hi everyone,

    I just want to say a huge thanks to you all, I had the interview and I got the job. They loved the lesson (the kids and the observer) so a really big Thank You to everyone who posted, it would definitely not have become the lesson it did without all your help.
  13. Great to hear that you were successful - and really appreciate the fact you came back on and thanked people.

    Too many - including I - fail to do this.
  14. m.mouse

    m.mouse New commenter

    Hi! I know this is really cheeky but I'm teaching moments this week during observations of the department and I'm struggling! I found your post and would really appreciate some help from you about what you did. Sarah, NQT.
  15. Henriettawasp

    Henriettawasp New commenter

    Part of a lesson? That's one of my classroom management techniques!


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