RP6: Force and Extension

Question 1

What is the aim of this experiment?

Answer 1

• Determine the correlation between the mass placed on a spring and the spring’s extension,
  by measuring resultant spring lengths.

Question 2

Describe how you can determine the correlation between the mass & extension by measuring spring lengths (method)

Answer 2

1. Set up your equipment, ensuring the spring will return to its original dimensions if
   stretched within its elastic limit.
2. Attach the pointer to the base of the spring, ensuring that it isn’t angled (parallel to
   the workbench) and perpendicular to the metre ruler. Align the top of the ruler with
   the top of the spring.
3. Measure the initial length of the spring without any weights attached.
4. Add a 10g mass to the base of the spring and record the length of the spring.
5. Repeat and continue to add masses, ensuring that the spring doesn’t oscillate after
   each weight has been added.
6. Calculate the extension of the spring for each mass by subtracting the initial length of
   the spring from each different length of the spring.
7. Convert all masses to weights using the equation:
   Weight (N) = mass (kg) x 9.81 (N/kg)
   where 9.81N/kg is the gravitational field strength on Earth.
8. Plot the graph of force (y-axis) against extension (x-axis). Calculate the gradient.
   If the spring obeys Hooke’s Law, the graph of force against extension will be linear and
   pass through the origin. The gradient will be equal to 1 / k, where k is the Spring Constant
   measured in N/m.

Question 3

List some safety precautions

Answer 3

• Ensure the stand holding the spring and weights is secure, either by clamping the
  base of the stand to the desk or by putting additional weight on the base of the stand.
• Don’t stand directly beneath the weights, in case they fall off.
• Take care when adding the maximum weight to the spring. If too weak, the spring
  may snap. If the weight exceeds the elastic limit, the spring may extend suddenly.
• Wear safety glasses in case the spring snaps.
• Place weights on gently and avoid oscillating the spring

Question 4

What piece of apparatus is used to secure the clamp stand to the desk and why?

Answer 4

• A G-Clamp - so that the clamp doesn’t tip over whilst adding masses
• The clamp produces a moment which counteracts the moment caused by the masses so that the stand remains in equilibrium

Question 5

How do you calculate the extension of the spring when you add masses?

Answer 5

• Subtract the extended length of the spring from the original stretched length

Question 6

What can you add to the spring to ensure that the extension measurements are accurate?

Answer 6

• A pointer (e.g. a splint) attached horizontally to the base of the spring and extending to the metre rule
• Make reading the length easier

Question 7

What graph should you plot with your results? What would you expect it to look like?

Answer 7

• Extension against force (weight)
• Expect it to be a straight line passing through the origin - variables should be directionally proportional

Question 8

What is the name of the relationship between extension and force?

Answer 8

• Hooke’s Law
• Force = Spring Constant x Extension

Question 9

How can the elastic potential stored in the spring be worked out graphically?

Answer 9

• The stored energy is equal to the area under the force/extension graph