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OK physics teachers- a moment of your time please

Discussion in 'Science' started by toedipper, Dec 12, 2010.

  1. Hello. I just heard an email read out on radio 4 from a teacher saying that objects fall at a speed independent of their mass, and this was proved by Galileo dropping things off leaning tower of piza, and you could show this in the classroom by dropping rolled up paper and tennis balls. I am a biology teacher who also teachs physics so I spent some time on this before I taught terminal velocity as it is out of my comfort zone so-
    Can I just confirm that gravity does accelerate things independent of their mass but this is only properly seen in a vacuum because the minute you drop stuff in air, air resistance come into play and DOES effect speed depending on mass? I am thinking that a cannon balll and a beach ball of same size and shape dropped from a plane would have very different terminal velocities. Therefore the only reason you can get away with this in a classroom is that you are dropping things from a relatively low height and air resistance hasnt had time to affect speed significantly?
    I keep hearing people say things the same shape but different weights fall at the same speed but this seems to be wrong in real life - in air. Is this right or do I have to start again?

     
  2. Hello. I just heard an email read out on radio 4 from a teacher saying that objects fall at a speed independent of their mass, and this was proved by Galileo dropping things off leaning tower of piza, and you could show this in the classroom by dropping rolled up paper and tennis balls. I am a biology teacher who also teachs physics so I spent some time on this before I taught terminal velocity as it is out of my comfort zone so-
    Can I just confirm that gravity does accelerate things independent of their mass but this is only properly seen in a vacuum because the minute you drop stuff in air, air resistance come into play and DOES effect speed depending on mass? I am thinking that a cannon balll and a beach ball of same size and shape dropped from a plane would have very different terminal velocities. Therefore the only reason you can get away with this in a classroom is that you are dropping things from a relatively low height and air resistance hasnt had time to affect speed significantly?
    I keep hearing people say things the same shape but different weights fall at the same speed but this seems to be wrong in real life - in air. Is this right or do I have to start again?

     
  3. Brand new on this forum but thinking of becoming a physics teacher so let's have a go.....
    Hmmm. A interesting one to get your head round. Go back to the first principles. F = m*a
    Now F (force on the object) is proportional to the mass of the object however you'll also notice that mass appears on the rhs of the equation too. So in this respect the acceleration is independent of the mass and is entirely dependent on the magnitude of the gravitational force. All this is ok if you are in a vacuum. Thankfully we aren't but what it means is that as the object speeds up in the air, the upwards force on the object due to air resistance increases (in proportion to the square of the downwards velocity) and this effectively reduces the net acceleration force to zero after a while. The more fluffy an object is, the quicker this happens.
    So then, a less-dense more resistant object will stop acccelerating after less time than a dense streamlined object. Using v = u + at, it's simple to deduce that the terminal velocity will be lower because "a*t" is acting for less time. Any skydiving contributers out there to verify this?
    Galileo is quite correct that mass is irrelevant - but only in a vacuum or frictionless fluid.
     
  4. A quick clarification on that last sentance - Galileo is quite correct - and correct in a vacuum and anywhere else you care to name. Density is an issue as this influences air/fluid resistance in comparison to gravitational force. Density is irrelevent in a vacuum as the object will accelerate independent of mass and also independent of air resistance. There - that's enough for a Sunday morning. I can see that I'll have to be more careful when I try and explain something!
     
  5. blazer

    blazer Star commenter

  6. Thanks for the replies, (and the clip- there was another nice one to the left filmed in a bell jar) I feel calmer now!
     
  7. hcb

    hcb New commenter

    I have two sheets of A4 paper - one flat and one rolled up into a ball. If dropped, the flat one floats down slowly - air resistance.
    I then drop the screwed up one at the same time as a squash ball (medium mass) and a ball of plasticene (large mass) which all have approx the same cross section. If you drop them from shoulder height they fall together and land at the same time. I get the class to guess which will fall fastest.
     
  8. MarkS

    MarkS New commenter

    A very good answer given Tonydon...except to clarify, the drag is proportional to velocity, not its square. See Stokes' Law: http://en.wikipedia.org/wiki/Stokes'_law
    Mark
     
  9. I've had several disbelieving A-level students complaining about this one! The way I explain it to them is that current scientific thinking suggests that gravity acts at an atomic level, so each individual atom in something will be accelerated at 9.8 m/s/s no matter how many atoms there are in total.
     
  10. MarkS

    MarkS New commenter

    Not sure what you mean Sianio...what were they thinking? How does that differ from the original post?
    Interesting thread!!!
    Mark
     
  11. The reason one equation only has one v is because the units of velocity are built into the viscosity. Viscosity is measured in kgm^-1s^-1. So, you could write your viscosity with units of kg if you multiplies the formula by velocity. So yes, drag is proportional to velocity squared.
     
  12. MarkS

    MarkS New commenter

    That doesn't contain units for velocity. The units of velocity are m.s^-1, not m^-1.s^-1. For fixed conditions, the viscosity of a fluid is a constant...it doesn't depend on the speed of an object moving through it.
    No you couldn't!
    As already discussed, only under certain conditions. I'm afraid your physics here is wrong!
     
  13. Wow, I truly am stupid sometimes. Hands up, fair cop, it's been a long day!
     
  14. With drag, the power to which velocity is raised depends very much on the degree of turbulence.
    n = 4 is not unknown.
     
  15. MarkS

    MarkS New commenter

    Yes, fair point Piper. Stokes' Law is very much for low speeds - I guess it makes the Physics quite difficult to predict without some very clear empirical data!
     
  16. blazer

    blazer Star commenter

  17. blazer

    blazer Star commenter

  18. Great thread!!
    I loved dusting off my old Uni thinking cap!!
    Sorry for dulling it down but I thought I might take it in another direction. Most of us are teaching GCSE and or A-level. It can be summed up (in terms of how science works) as follows:
    In a vacuum the answer is simple; everything falls at the same rate like in the famous video from the lunar landings when the astronaut dropped the feather and hammer.
    On earth in the atmosphere it gets more complicated, the air resists the progression of objects for different reasons:

    <font face="Comic Sans MS">The density of the atmosphere, Joseph Kittinger who still holds the record for the highest freefall parachute jump reached a terminal velocity of nearly 615mph where as a jumper from 15,000 feet in similar equipment will only reach speeds of about 120mph.</font><font face="Comic Sans MS">The density of an object will also have an effect but this is really cross sectional area in disguise!! Think of a lead ball the same size as a ping pong ball. (the ping pong ball being far less dense) will have the same cross sectional area presented to the direction of travel but one will fall faster because it has more mass (higher density)</font><font face="Comic Sans MS">The skin of the object, a peregrine falcon (the world&rsquo;s fastest animal) has a small muscle at the base of every feather that pulls it tight in against its body so there is as little drag as possible. Where as a pigeon (its prey) does not have this little muscle and is almost as fast but not quite enough! (Lucky for the Peregrine!)</font><font size="3">Right I&rsquo;ll shut up now, </font>
     
  19. Love this - there is a great misconception in here that's repeated too often. And that is that Galileo dropped things off the leaning tower of piza. No he didn't! He was correct in his surmise but he never actually did the experiment himself.
    Just to be really helpful I can't even tell you who did - it's in John Gribbin's History of Science, that I have left at school helpfully.
    Sorry, spending too much time looking at how science works!
     
  20. blazer

    blazer Star commenter

    They will in a vacuum.

    On Earth both balls will accellerate at the same rate for a split second. However to achieve terminal velocity the force of air resistance must equal the downward weight of the ball. Thus the ping pong ball will reach its terminal velocity rapidly as the required air resistance will be acheived at a very low downward velocity. However due to the greater weight of the lead ball it will not reach its terminal velocity until its speed is sufficient to produce an air resistance force equal and opposite to its weight. So the lead ball will fall faster (or accellerate for longer if you prefer).
     

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