Force and extension

Investigating the relationship between force and the extension of a spring.

In this required practical, students are expected to carry out an investigation into the extension of a spring under different masses. Their use of appropriate apparatus to safely observe and measure forces and extension is also assessed. Students will be able to explain the relationship between force and extension and will be able to calculate the spring constant of the spring.

Equipment (per participant)

• Safety glasses
• Expendable springs,
• Metre ruler
• Set of newton hanging masses
• Retort stand and 2 x clamps
• Wooden splint (or similar – for making a pointer)
• Tape

Method

Students should set up a retort stand with two clamps attached. The retort stand could be secured to the desk using a G-Clamp, a large mass or similar (so that the equipment does not topple over).
The metre ruler is attached to the lower of the two clamps so that it hangs vertically over the side of the desk.
A section of wooden splint is then attached horizontally to one end of a spring to function as a pointer and students then attach the spring to the free clamp (they may need to re-position the ruler as the zero mark needs to be at the same level as the top of the spring).
Students take a reading from the ruler, which will act as their value for the unstretched spring. They should record this value in a suitable table.
An empty 1N mass hanger is then hooked on to the bottom of the spring. The reading on the ruler is taken using the pointer, which is the length of the spring when 1N of force is applied.
Further weights are then added, measuring the length of the spring each time. Students can then calculate the extension for each addition of masses and record it in their table.
Their results are then plotted on a graph, plotting the values for weight in Newtons along the x-axis, and the extension of the spring in cm on the y-axis. Students should now be able to draw a conclusion as to the relationship between force and extension.

Technician tips

Using Newton masses will negate the need for students to convert a mass in grams to weight in Newtons. Masses in grams could be used for more advanced students.
It should be made clear to students that extension is the total increase from the original length of the spring.
The measured length = initial unstretched length + extension length