Introduction to forcemeters
Though we might claim to know, intuitively, the difference between a large force and small force, we resort to the extension of springs for actual measurement.
Apparatus and materials
For each student group
Mass hanger with slotted masses (10 g)
Unknown masses of between 0.5 and 1.0 kg
Sticky tape, "write on' variety
Forcemeters, selection of different ones
Health & Safety and Technical notes
The forcemeter scales need to be blank. You can achieve this by covering the scale of calibrated spring balances with ‘write on’ sticky strips.
a Hold the forcemeter vertically. There is no force pulling its spring (apart from the weight of the hook), so the reading must be zero. Make a mark on the blank strip that will be the 0 of your forcemeter scale.
b Pull on the spring with a force of 1 newton. On Earth, the weight of a 100-gramme mass is approximately 1 newton. So if you hang a 100-gramme mass from the forcemeter the Earth will pull it down with a force of about 1 newton. That is the force that stretches the spring. Make a mark on the blank strip that will be the 1 of the scale.
c Hang another mass from the forcemeter. The force pulling the spring of the forcemeter is now approximately 2 newtons. Make a 2 newton mark on your forcemeter scale.
d Repeat this up to 10 newtons. You have now ‘calibrated’ your forcemeter so that it has a scale for taking measurements.
e Take the masses off the forcemeter and hang the unknown mass from it. Record the force of gravity (weight) that acts on this mass. You can now use your forcemeter to measure any force, up to 10 newtons.
1 This important activity deals with fundamentals of the measurement of force. We have to use indirect means to measure many quantities. We can measure temperature, for example, by looking at the length of a thread of liquid in a glass tube. Here, students see that we measure force using the extension of a spring. To do so we must first calibrate the spring, as students have done.
2 To gain familiarity with this new instrument, students should choose an already calibrated meter to measure a range of forces such as open doors and lifting loads. Remember that the weight of an apple (choose the right one) is about 1 N.
3 Students could even make their own forcemeter from winding a spring using copper wire (26 SWG). It will not have a linear scale but it must not be extended so far so that it no longer returns to its original length. Give a warning about damaging forcemeters.
4 How Science Works Extension: This experiment provides an opportunity to consider various aspects of scientific instrumentation.
Calibration: Any instrument with a scale must have been calibrated, either by the manufacturer or by the user. Commercial instrument makers are expected to be able to trace their calibration back to a national (and hence international) standard. This would be an opportunity to discuss the work of national bodies such as the National Physical Laboratory (UK) or the Bureau international des poids et mesures (France/international). By considering the process of calibration which they have just carried out, students can assess the uncertainty in any measurement they make with their forcemeter.
Range and sensitivity: What are the largest and smallest values of force that can be measured using the forcemeter which the students have calibrated? How could they change the range of their forcemeter? If the meter has a greater range, what effect does this have on its sensitivity? (A stiffer spring will give a greater range, but the scale divisions will also be greater, so the meter will be less sensitive.)
See also the apparatus entry Forces and energy demonstration box.
This experiment was safety-checked in September 2004.