Calibrate a forcemeter by pulling a student

Demonstration

You can use the formula F = ma to calibrate a forcemeter without using gravity.

Apparatus and Materials

• Trolley, demonstration (or skateboard)
• Leightweight forcemeter, 0-10 N, with scale markings concealed by paper (this is to be written on)
• Bathroom scales, calibrated in kg (or N)
• Stopclock

Health & Safety and Technical Notes

A trolley runway requires two persons to carry it and set it up on the bench.

Ensure that a string is tied across the bottom of the runway to prevent the trolley falling onto anyone.

Read our standard health & safety guidance

Procedure

1. Measure the mass of a student and add it to the mass of the trolley.
2. Seat the student safely on the trolley. Attach the forcemeter to the trolley by a length of string.
3. With the forcemeter, apply a constant force to the trolley to accelerate it from rest.
4. Measure the time, t , for the motion over a measured distance, x .
5. Use the formula x = 1/2 at ² to calculate the acceleration, a.
6. Use F = ma, where m is the measured mass, to find force F .
7. Mark the paper over the forcemeter scale with this force. For the forcemeter, you also know where to mark zero.
8. Assume that the meter acts in a linear way. That is, that equal changes in force are represented on the scale by equal distances.
9. Make a complete scale for the forcemeter, on the paper.
10. Compare this with the scale provided by the forcemeter manufacturer.

Teaching Notes

• To allow for friction, first use the spring balance to pull the trolley with whatever force is needed to maintain constant speed. Mark the reading of the pointer for constant speed on the paper that covers the scale. Then do an acceleration experiment, keeping the pointer at another mark for the larger force that is used. Remove the paper and read off the places of both marks on the scale. Compare the force calculated by using F = ma with the difference between those scale readings.
• Newton's laws of motion define the concept of force. A scale of forces in newtons is essentially derived from measurements of mass and acceleration.
• Students often suppose that the manufacturer has access to some unknown means of providing a scale of absolute reliability. They may have little awareness that calibration is an important process of inherent uncertainty however it is done. Invite them to discuss which is more reliable, the scale made by themselves or by the manufacturer.
• It is essential to keep the logic straight: force is calculated from Newton's second law once the acceleration produced by the forcemeter has been found experimentally.

This experiment was safety-tested in March 2005