FORCES SPRINGS

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FORCES SPRINGS

• This section of work is also known as Hookes Law.

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When a force is exerted on a spring it will
either compress (push the spring together) or
stretch the spring if the weight is hung on it.
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Some objects like bridges will also behave like
springs. When a weight is placed on a bridge
parts will be stretched and under tension, other
parts will be be squashed together or compressed
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In compression the particles making up the
material are pushed closer together
When it is under tension the particles making the
material are pulled further apart
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In this experiment the extension of the spring is
measured with increasing weights being added 1N,
2N, 3N to 5N. The extension is a measure of how
much the spring stretched. (Not the total length
of the spring)
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The results of the Extension are plotted against
the Force added. The spring stretches in
proportion to the mass added but at 5N the spring
is stretched beyond its elastic limit and becomes
deformed.
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Because springs stretch proportionally we can use
them as a spring balance to measure a force.
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• The extension will double if the force doubles.
• It will treble if the force trebles.
• If the load exceeds the elastic limit of the
  spring then the law no longer applies and the
  spring will not return back to its original
  length.

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When the weight and the upward, restoring force
are equal the spring is said to be in equilibrium
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Read the questions carefully and answer these
questions on paper. When you have finished
answering them then go to the next slides to
check if you got the answers correct.
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Answer to Question A B

• In the first experiment the student tested three
  different diameter springs made with the same
  thickness of wire. To make it a fair test the
  springs would have to be the same length. 3N, 5N,
  7N, 9N 11N were added to each spring and the
  extension was recorded for each weight.
• Conclusion the greater the diameter of the
  spring the greater the extension. For every 0.5cm
  increase in diameter the spring extends an extra
  1cm.

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• In the Second experiment the pupil kept the
  diameter and the length of the spring constant
  but changed the thickness of the wire used for
  each spring.
• Conclusions the thicker the wire used the
  smaller the extension. If you double the
  thickness of the wire you halve the extension.
  This is because the restoring force from the
  thicker wire will be stronger so it will be more
  difficult to stretch it.

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Answer to C

• To make the springs less soft you need to make
  the spring with thicker wire and have as small a
  diameter as possible for the spring.

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SPRINGS IN SERIES

• Each spring extends as a result of the force. If
  there are identical springs the extension will be
  double that of a single spring.

Total 10 cm extension
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SPRINGS IN PARALLEL

• In parallel the load (weight) is shared by the
  springs. So if you have 2 identical springs the
  extension will half.

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SUMMARY

• When weights are added to a spring it stretches
  proportionally.
• When 2 springs are connected in series each
  spring will stretch proportionally so the
  extension will be double that of a single spring
  with the same load.
• When two springs are in parallel the load is
  shared so the extension will be the same as half
  the weight on a single spring.
• When the load is too great the spring will be
  stretched beyond its elastic limit and will not
  go back to its original length.
• We find the extension by subtracting the original
  length of the spring from the total length with
  the load on.