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GCSE Physics CA experiment not working?

For our controlled assessment experiment, we had to use different grades of sandpaper, and measure the force required to move a wooden block across it. However, many groups in our class found that the 'finest' grades of sandpaper needed more force than the 'course' ones, despite the fact that it should supposedly have less friction?! Does anyone have an explanation for why this has happened, or have we all made a mistake?

Thank you! :smile:


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Original post by Lainathiel
For our controlled assessment experiment, we had to use different grades of sandpaper, and measure the force required to move a wooden block across it. However, many groups in our class found that the 'finest' grades of sandpaper needed more force than the 'course' ones, despite the fact that it should supposedly have less friction?! Does anyone have an explanation for why this has happened, or have we all made a mistake?

Thank you! :smile:


Posted from TSR Mobile


The force needed also depends on the weight of the block. Were all the blocks the same mass? Same shape? Same type of wood?
"Less friction" isn't very meaningful.
Were you measuring the coefficient of friction?
(edited 10 years ago)
Original post by Lainathiel
For our controlled assessment experiment, we had to use different grades of sandpaper, and measure the force required to move a wooden block across it. However, many groups in our class found that the 'finest' grades of sandpaper needed more force than the 'course' ones, despite the fact that it should supposedly have less friction?! Does anyone have an explanation for why this has happened, or have we all made a mistake?

Thank you! :smile:


Posted from TSR Mobile


I don't know, but perhaps there is a greater total contact surface area with the fine sandpaper than with the coarse sand paper?
Original post by Chlorophile
I don't know, but perhaps there is a greater total contact surface area with the fine sandpaper than with the coarse sand paper?


Were they all the same type of sandpaper using the same abrasive materials?

Glass paper (coarse) will have a different friction coefficient to fine-paper which may be silicon carbide?

Greater contact surface area can increase friction. i.e. The contact pressure caused by the weight of the block together with it's surface area.

Were all the blocks exactly the same wood?

Was the surface of the block smooth or rough etc?
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Lainathiel
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Sorry for the late reply: I've had internet issues and loads of other work to catch up on. So, the block, the mass of the additional weights and the side of the block that was used was kept constant throughout, in order to try to keep the experiment as fair and accurate as possible. Our teacher, when many groups found a similar to result to my own, suggested that there many have been a greater contact surface area when using the finer paper, but was unhelpfully unsure about what the problem was. The point about the different abrasive materials is interesting: we weren't given any information about each grade of paper, only that I'm presuming they were of the same brand. I'm going to have a look into the different materials used, as you say, and see if I can find anything. :smile: Thank you everyone!

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