The Effect of Weight on the Amount a Fixed Dowel Deflects
How weight affects the amount a fixed dowel deflects.
Labelled diagram of proposed set up:
When reading off the ruler, it is necessary to crouch otherwise
parallax will occur and readings will be inaccurate.
There are 4 independent variables in this experiment. The thickness of
the dowel, the weight being hung from the dowel, the distance the
weight is being hung from the dowel and the amount of over-hang of the
dowel. These will produce a dependent variable, the bend/deflection of
the dowel. I will use the weight being hung as my independent
variable, and I will keep the others constant.
Expected independent variable range:
I will take readings between 0-5N (0-500g) inclusive. From zero, I
will take down readings in 0.5N (50g) increments. Each reading will be
taken down 3 times with a margin of error of 2mm either way and to 0
decimal places. Once all 3 readings have been noted, an average
reading will be calculated.
Prediction, including scientific explanation:
I believe that the deflection will be directly proportional to the
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weight being added onto it.
This investigation appears to test the understanding of moments. The
equation for moments is: moment = force x perpendicular distance to
If the distance will be kept constant, then I can rewrite the equation
Moment = force x X, where moment =1, force = 1 and X is constant.
If the force were to be doubled, then force = 2, thus on the right
side of the equation, force multiplied by distance gives 2X, so moment
must equal 2X too.
Sources of information, including results of preliminary trials:
I conducted some preliminary trials, where I used a 1cm thick dowel. I
found that the deflection was too small to measure accurately, so a
decision to use a thinner dowel was made.
The equation for moment was cited from Tom Duncan’s GCSE Physics book
I did the experiment exactly how I planned out earlier, with the
exception of adding some double-sided adhesive tape to the rod so the
weight would not slip down the rod whilst reading the amount of
I made sure that all unused weights were placed away from the table
edge so they would not fall onto my foot. I also made sure that no
more than 500g was put onto the rod in case it were to snap and a
splinter were to come into my eye.
... and test the results. Procedure: A) Put the weights necessary for each of the vector forces on each hook. ... B) Set the wheels of the force table ... that the student will not put on more weight than necessary. Another error that can occur while ... this, you should ask for help while putting weights on. Finally, it is important to make sure ...
Analysis and Conclusion
Conclusion (eg relationship between variables as given by graph)
I have to come the conclusion that the more weight added to the dowel,
the more it will deflect. Looking at the graph, it appears that the
relationship between the two variables is linearly proportional,
seeing as the line of best fit does not pass through the origin.
Scientific explanation of results (including comparison with
prediction, where appropriate):
In the planning stage, I predicted that the relationship between the
two variables would be directly proportional, but given that the
conclusion that the relationship is linearly proportional, it shows
that my prediction was wrong.
If you consider the dowel to be like a spring, it is easier to see why
the results are proportional. If the top half of the length of the
dowel is under tension when being bent, and the bottom half is under
compression, like Hooke’s Law, a spring will extend or shorten if
pressure is applied. In this case, the things that are being extended
and shortened are the molecules of wood within the dowel, so they will
change size directly proportionally until a certain point, rather like
the elastic limit of a spring when it will start to stretch/compress
out of proportion.
The above law however, should be obeyed by the results. At a closer
inspection of the graph, the results are very nearly directly
proportional. Knowing that no experiment can be perfect, and there
will be slight flaws, I can fairly confidently say that I was correct
with my prediction after all and that the conclusion that I came up
with is wrong too.
The conclusion should state that the amount a dowel bends is directly
proportional (not linearly proportional) to the amount of weight added
In what way was the method used good/bad? Was the procedure suitable?
This method was good because it was simple to set up and easily
repeatable. The measurements were also quite easy to read. The
experiment was bad, however, because when no weight was added to the
... , it is the results that count. Analysing evidence and drawing conclusions: From the graphs we saw that the heavier the weight attached to the ... a wide range of results that will help our conclusion be accurate. To record our results, we will make graphs (as seen in evaluating ...
dowel, it bent under its own weight, thus making my graph not pass
through the origin, which, in theory, it should do
Reliability of Evidence – are the results good enough to support a
I believe that the results are easily good enough to support a
conclusion because if you look at the graph in the obtaining evidence
stage, the points all lie very close to the line of best fit – thus
showing a very strong correlation, enough to support my conclusion.
Improvements and Complementary work:
Believe that the experiment could be improved by a few ways:
· Use a dowel which wouldn’t sag under its own weight
· Attach a drawing pin onto the end of the dowel, thus making readings
a lot easier and more accurate
In addition, because the rule may have been inaccurate, I could use
rules manufactured by several different companies, and measure using
those, then taking another average to make sure that a fair experiment
would be conducted.