The Effect of Weight on the Amount a Fixed Dowel Deflects

The Experiment:

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.

Controlled variables:

=====================

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:

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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

fulcrum.

If the distance will be kept constant, then I can rewrite the equation

into:

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

3rd edition.

Obtaining Evidence

Procedure:

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

deflection.

Safety measures:

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.

Results:

Weight (N)

1st Reading

(mm)

2nd Reading

(mm)

3rd Reading

(mm)

Average Reading

(mm)

0

1

2

1

1

0.5

12

11

12

12

1

42

43

43

43

1.5

65

67

66

66

2

86

84

86

85

2.5

110

112

114

112

3

133

131

133

132

3.5

146

150

150

149

4

170

175

177

174

4.5

190

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... 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 ...

185

191

189

5

212

215

216

214

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

onto it.

Evaluating Evidence

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

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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

conclusion?

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.