This experiment shows that it is not possible to measure the weight of a body in free fall using a forcemeter.
Apparatus and materials
Demonstration forcemeter, reading at least 1 0N
Mass, 1 kg
Digital camera or videa camera
Health & Safety and Technical notes
If it is necessary for the student dropping the forcemeter to stand on something, a 'kick-stool' as used in libraries or store rooms would be safest. Do not allow other students to stand close to the stool.
It is a good idea to highlight the pointer so that it shows brightly in photographs. Paint it white or cover it in foil.
This activity is suitable for multiflash treatment. Or you could make a video and play it back frame by frame. If a jump from a diving board is possible, then the student could hold a balance.
a Hang the kilogram mass from the forcemeter, and get a student to drop it onto a cushion or a blanket held by other students.
b Take a photograph of the falling apparatus. Also, ask students whether they can see what the balance reads whilst dropping. Repeat this several times, so that students can look again, and so that you have a collection of photographs.
c Use the forcemeter readings in the photographs as the starting points for discussion.
d A simple demonstration of free fall is to offer a student a string on which a ring has been threaded. The string has loops at each end which can be put over the student's wrists. Without throwing the ring up into the air ask the student to bring his or her hands together so that the ring is trapped. This is clearly impossible. Ask the students for solutions to the problem before revealing that if the student jumps (safely with bent knees) off the bench then it is indeed possible to grasp the ring.
1 The forcemeter shows a reading, equal to the weight of the mass, when the mass simply hangs from it. The extended spring exerts an upwards force which balances the downwards weight.
The forcemeter reads zero during the fall (ignoring oscillations of the spring due to disturbance on release). Gravity has not been switched off, however, and the mass is still subject to a downwards force. It still has weight, which is the unbalanced force resulting in its acceleration throughout its fall.
2 When you release the system, oscillation of the spring can produce unwanted motion of the pointer. Try to take photographs during the later part of the fall, and take photographs of several falls. This will show that there is little pattern to these motions.
This experiment was safety-checked in March 2005