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Physics IB independent experiment - friction on an inclined plane

Discussion in 'Science' started by AshgarMary, Nov 25, 2011.

  1. AshgarMary

    AshgarMary New commenter

    PS I should point out that I am 'observing' not his actual teacher who will be assessing him, though we have been discussing this expt!
  2. Being simplistic and assuming he is not using a pulley system:
    the force causing the trolley to accelerate is due to the mass of the trolley (=mg sinA)
    the force causing the friction between the wheel and the slope is the normal reaction, which depends on the mass (=mgcosA).
    Doubling the mass results in both forces doubling, so will not produce much effect!
    If he does use a seperate mass to create the acceleration, linked via a pulley to the "variable" mass trolley, the above factors still play a part.
    What is the IB brief? If it relates to friction, I reckon a re-think may be called for and "I" would carry out a quick experiment (1 mass, 3 mass) before talking to him
    If he is keen to use that equipment, I would suggest using various frontal areas to investigate air resistance could be more productive.
  3. Just realised that if he uses a horizontal surface then frictional increases due to mass will not be accompanied by increased acceleration components: use a falling mass pulling a thread and look for variation which is not strictly related to increased inertial mass?
  4. AshgarMary

    AshgarMary New commenter

    Thanks Physics Suits You!

    You've confirmed our initial thoughts that there won't be much for him to see. All in all, I think he needs to deflect him away from this but it might be worth spending half an hour trying the experiment just to satisfy myself!

    As I mentioned, this is my first encounter with IB and I'm 'observing' so I'm trying to get to grips with the requirements.

    From what I understand, the teacher is not allowed to give him any help other than suggesting a general area (friction was the student's own choice). He is supposed to devise his own experiment, select his own independent variable, identify a dependent variable and obtain a linear relationship.

    It is also supposed to be an experiment no one else has done recently in the school which knocks out the most obvious ones.

  5. steve_cooke

    steve_cooke New commenter

    With wheels there is no kinematic friction - the tyres aren't sliding (for an accelerating car the static friction of the tyres acts forwards and if it is exceeded the tyres spin). There is rolling resistance. I've not tried it but at a guess rolling resistance will be fairly constant for a trolley on a slope, so if he plots measured acceleration against gsintheta he should get a straight line with a non zero crossing point from which rolling resistance can be extracted, but as it is not really friction is that fulfilling the brief?

    Might be an interesting practical though, maybe I'll find a reason to include it somewhere!
  6. steve_cooke

    steve_cooke New commenter

    sorry, ma against mgsintheta, else he can't actually plot the graph
  7. Carlsberg

    Carlsberg New commenter

    Evening PSU.

    Somehow I have to "do" a task on the determination of the coefficient of kinetic friction for (BTEC) Year 11. Clearly, I need to keep it fairly simple. Any ideas, please.
  8. AshgarMary

    AshgarMary New commenter

    Probably happily, the student has decided to do a more simple experiment for his IB not involving inclined planes at all.

    I am probably going to go ahead and do the experiment myself later in the week because we're all interested to see what happens!
  9. Amazingly, I did my PGCE project on kinetic friction almost 40y ago!
    I ended up with something like a record-player turntable (remember those?) and a load suspended from an arm attached horizontally from the central pivot (a bit like the tone arm, in some ways). I used a force meter tangentially to hold the mass steady, whilst the turntable rotated underneath it at a constant angular velocity. Thus I could measure the force at varying speeds.
    Both table and load were made from metal and I measured the contact resistance between them, as the theory suggested that the molecular area of contact (as opposed to the macroscopic area) varied. This was involved in the concept of vacuum-friction welding.
    Unfortunately, old age and a bed-time whisky means that I'm not fully conversant with the conclusions. [​IMG]

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