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Weight and the speed of a falling object

Discussion in 'Science' started by eatmygoal, Jan 23, 2011.

  1. Hi,

    Have tried googling this, but have yet to find the definitive answer.

    In Year 6 we conduct the classic 'spinners' experiment where the children alter a variable and see how this affects the time it takes for a paper spinner to reach the ground when dropped from a set height. Typically the children alter the width of the spinner blades, type of material etc.

    One of the variables that always get picked is weight of the spinner, altered by adjusting the number of paperclips attached to the base of the spinner. Invariably every year the children's results show that the heavier the spinner, the faster it falls (all other variables being the same). This fits with the intuitive idea that heavier objects fall faster. However my understanding is that this should NOT be the case, as two objects that generate the same amount of air resistance should fall at the same rate, regardless of their weight. Is this not what Galileo demonstrated? If this is the case then what is going on with our spinners?

    It tends to make it doubly difficult to convince the kids that Galileo was correct when their own scientific experiment seems to prove the exact opposite!

    Grateful for any advice.
     
  2. In a vacuum they would land at same time, however, in air, weight and air resistance must balance to achieve terminal velocity (a steady speed). Terminal velocity would be achieved more quickly for a lighter object- a heavier object would need to accelerate for longer beore reaching terminal velocity - hence the difference in landing times. Depending on where the paperclips are there may also be an air resistance effect.
    This might be difficult to explain to year 6 but you could show clip of Neil Armstrong dropping hammer and feather on the moon to illustrate the point.
     
  3. blazer

    blazer Star commenter

    OK.

    An object dropped in a gravitational field will accellerate downwards. However in an atmosphere that downward force is opposed by the air resistance or drag. As an object accelerates the size of the drag force increases until it is equal but opposite to the weight of the object. At this point the object will fall at a steady speed (terminal velocity).
    So two identical spinners which have different weights will, if dropped together initially (for a split second) accelerate at the same rate but the lighter spinner will reach the point where the drag equals its weight first. The heavier spinner will accelerate longer until it too reaches its terminal velocity as the drag equals its weight. Other factors also play as the wings on the faster falling spinner will be pulled back more thus lowereing their drag so the heavier spinner will have to fall even faster to achieve terminal velocity.

    Newtons laws on falling objects only works in a vacuum.

    See http://www.youtube.com/watch?v=5C5_dOEyAfk

    Hope this helps
     
  4. blazer

    blazer Star commenter

    It wasn't neil Armstrong but I think Apollo 14 or 15 where they did this

    http://www.youtube.com/watch?v=5C5_dOEyAfk
     
  5. I stand corrected!
     
  6. blazer

    blazer Star commenter

    No problem, you are probably to young to have seen it live!
     
  7. blazer

    blazer Star commenter

    Just checked and it was Apollo 15. July 1971. The astronaut was David Scott.
     
  8. Spinners are exceedingly difficult to theorise about. The mechanics of rotating bodies will not be studied until A level and even then, not in anything like sufficient detail to enable students to understand what is going on here. They can, however, have a link to seed dispersal and this may be of interest to younger children.
    The effect of altering the width or length MAY affect the weight (if done by cutting - presumably you have discussed this with them to establish "fair testing") but it also affects the rotational dynamics of the object, thus affecting the resistive effect in a complex manner.
    For a simple "air resistance" type experiment, you are much better off (IMHO) using drogues (upside down parachutes). These are easily obtained in the guise of bun cases. The simplest factor to change is weight (1 case, 2 cases, 3 ... etc, by stacking them inside each other).
    As blazer stated, the object will fall at its teminal velocity once weight and air resistance are equal. This will be in a very short time for these objects and it is easy to get reliable results showing an inverse relationship between weight and time to fall. With the appropriate understanding, pupils can convert to a relationship between weight and average speed, but I would suggest this may be beyond Y6.
    Is it part of your SoW to demonstrate that heavy and light objects fall at the same rate? If so, I would suggest dropping 2 A4 sheets, one flat & one crumpled, from the same height: they clearly differ (air resistance). Then drop a 2p coin and a 2p-sized piece of card: they also differ (terminal velocity when air resistance = weight). Then place the card on top of the coin (thus removing the effect of air resistance): when dropped they fall as one object, but usually seperate on hitting the ground, thus proving they were not stuck together. My arguments are NOT scientifically robust but are quite good enough for anyone upto the end of KS3, I believe.
     
  9. Please, can we stop doing this practical?
    The fall of the spinner is incredibly complicated; you are unable to control the variables as you would like. The moment you increase the weight, the rate of spin and angle of blades change automatically. So much for fair testing!!
    If you are desperate, try a parachute, which, although not perfect, is far better.

     
  10. blazer

    blazer Star commenter

    Why stop it? It is a simple practical that gives repeatable results, can be explained in simple terms and is fun! OK, if you are in your 3rd year of a BSc in Physics it gets complicated but if you are in year 7 it is brilliant. Perhaps this practical will lead to some kids taking Physics post school!
     
  11. Thank you all for your advice.

    I can see the argument for not doing this particular experiment, or at least doing it in a substantially different way. In fact I was having just this discussion with my Year 6 colleague yesterday and we both commented that the rotor blades made this a potentially incredibly complex experiment to evaluate!

    Unfortunately, planning being what it is, we are too far down the road this time around to change the experiment entirely, but a note has been made and we will revisit the choice of experiment next year. Rest assured our classes are having a robust discussion as to the flaws in the experiment! Although, in my experience, it is is rare that any experiment in a primary school could every truly be described as 'fair', all we can do is make efforts to ensure our tests are 'fairer'.

    Thanks again for the help.
     
  12. Blazer, the problem is that every time this practical crops up, people start asking how to explain what is happening (as, again is the case here), and, in the end, it becomes a fudge. It cannot 'be explained in simple terms'.
    If you must use it, use it as a skills based piece of work and forget the Physics. However, I believe that would be a great pity.
    The parachute is far better can be explained in quite simple terms, with some correctness, and is fun, espcially as the Lego men/women become larger.
     
  13. An emphatic NO! to this statement, blazer. Your post seems to indicate some knowledge of the subject, but this statement is flat dead WRONG.
    Newton's laws ALWAYS work. Period. That's why they're called "laws". It's the situation that makes applying them more complicated, but in the end they always give the right answers (if done correctly).

     
  14. Onthe first evening in my current house I had a discussion wiuth my housemate about this, he insisted that a heavyer object dropped more quickly.

    We ended up with cushons on the floor to protect it droppin 1Kg weighs vs the cat's plastic ball and various other objects.

    Surely you could fit something like that in, get a set of random objects and get the kids to put them in order which would fall fastest.

    If you then get out a couple of sheets of paper you could demonstrate a ball of paper against a flat piece of paper and change the orientation of the paper - this would show the effect of air resistance. But asyou have aleady shown that objects hit the floor at the same time they should be able to see that it is the air resistance not the weight that causes the difference.
     
  15. blazer

    blazer Star commenter

    OK I stand corrected but not in science but in my use of English. Newton's laws are only observably working in a vacuum!
     
  16. blazer

    blazer Star commenter

    OK English still not so good!

    You are right, I am not a Physicist (I am a Chemist).

    However gone are the days where kids wil accept things they can't observe. I recall my schools day physics where statements were often prefaced with 'In perfect system' or 'In a perfect world' or some other such get out clause.
     
  17. You could try an experiment with two coins with different weight, say 10 p and £2. They have the same air resistance and it works - they land at the same time.
     
  18. MarkS

    MarkS New commenter

    But they shouldn't!!!! Having different weights, if their cross-section is the same, the lighter one will have a lower terminal velocity and take longer to hit the ground.
    Mark
     
  19. This rather depends on the conditions under which you are working.
    An object will fall at its terminal velocity when the resitive forces balance the motion-producing forces. Coins of identical size but different weights will achieve different terminal velocities, but only after quite a long fall. Until then they will accelerate according to "ma = mg-R" where R is the resistive force, which will be velocity dependent.
    PURELY on guesswork, I'm reasonably confident that there will be no observable difference for drops of less than 10m and I would suggest 100m could give similar results. Few schools are equipped to carry out this experiment, but next time you are at a deep wishing well (preferably organised by a charity), please drop 10p and £2 in and describe the time difference in the splashes. Perhaps someone could compile a spreadsheet of "depth of well" versus "time difference" as part of an action-research project.[​IMG]
    I would, however, point out that as a coin falls, small perturbations of the air will start it flipping over and at that point any hope of simple analysis, or comparison, goes out of the window. That is another reason for using drogues, as they are remarkably stable and fall in a reproducible manner.
     
  20. MarkS

    MarkS New commenter

    Fair point PSY, but it would still be wrong to say that the coins hit the ground at the same time because they have the same air resistance, as stated by Julia...that isn't the reason - as you say, it's because they don't have a long enough distance to fall.
    A £2 is almost twice the mass of a 2p coin, so it's acceleration will be greater than that of the 2p for the whole descent (except at t=0), but they won't be falling for long enough to have a perceptible difference in descent time.
    Mark
     

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