Stretching springs and Hooke’s Law Learning Outcomes: Describe what is meant when an object is elastic Describe how to measure extension of a stretched object Accurately remember and use F = k x e Explain what is meant by ‘the limit of proportionality’

Elastic materials An object is elastic if it returns to its original shape when the forces deforming it have been removed.

Non elastic materials A polythene bag does not return to its original shape when stretched. It remains inelastically deformed .

Damping A car has lots of springs. Without dampers (shock absorbers) the car would be too bouncy and be unsafe!

Extension of specimen = present length – original length

Hooke’s law provided the elastic limit has not been exceeded.

Hooke’s law (Spring stretches in a non linear way beyond ‘P’) ‘P’ provided the has not been exceeded. proportional limit

Hooke’s law (Spring stretches in a non linear way beyond ‘P’) ‘P’ provided the elastic limit has not been exceeded. proportional limit

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Experiment to investigate Hooke’s Law

Experiment to investigate Hooke’s Law Load /N Original length /mm Final length /mm Extension /mm 20 20 1 20 40 20 2 20 3 20 4 20 5 20 Force /N Extension /mm

Experiment to investigate Hooke’s Law Load /N Original length /mm Final length /mm Extension /mm 20 20 1 20 40 20 2 20 60 40 3 20 80 60 4 20 100 80 5 20 120 100 Force /N Extension /mm

Experiment to investigate Hooke’s Law Load /N Original length /mm Final length /mm Extension /mm 20 20 1 20 40 20 2 20 60 40 3 20 80 60 4 20 100 80 5 20 120 100 Force /N These are sample plots ! Extension /mm

Conclusions: The graph shows……….. 1. 2. The anomalous results could be caused by………… 3. The investigation could be improved by ……………

Conclusions: The graph shows……….. 1. the extension increases in direct proportion to the force 2. The anomalous results could be caused by………… 3. The investigation could be improved by ……………

Conclusions: The graph shows……….. 1. the extension increases in direct proportion to the force because the straight line passes through the origin. 2. The anomalous results could be caused by………… 3. The investigation could be improved by ……………

Conclusions: The graph shows……….. 1. the extension increases in direct proportion to the force because the straight line passes through the origin. 2. The anomalous results could be caused by………… A parallax error in taking the reading from the scale. The spring moved on its support 3. The investigation could be improved by ……………

Conclusions: The graph shows……….. 1. the extension increases in direct proportion to the force because the straight line passes through the origin. 2. The anomalous results could be caused by………… A parallax error in taking the reading from the scale. The spring moved on its support 3. The investigation could be improved by …………… Repeating the readings ( eg unloading), and calculating the average. Increasing the range to see where Hooke’s law no longer applies.

Experiment to investigate Hooke’s Law A* work The spring constant (spring stiffness) is the gradient of a force extension graph : K = Δ F = 5.8 N = 0.071 N / mm Δ e 82 mm = 71 N / m Load /N Original length /mm Final length /mm Extension /mm 20 20 1 20 40 20 2 20 60 40 3 20 80 60 4 20 100 80 5 20 120 100 Force /N These are sample plots ! 5.8 N. Extension /mm 82 mm