Measuring the Acceleration of Free Fall with a Light Gate and Datalogger

A datalogger is a device which collects and processes raw data from sensors such as light gates. This method involves dropping a double interrupt card through a single light gate connected to the logger, as you can see. People sometimes ask about the advantages of light gates. All they are is a start/stop timer – a reduction in light intensity turns them on, a restoration of normal intensity turns them off again and software does all the rest to calculate velocities, accelerations and so on depending on the configuration. They have a very rapid response time so they start or stop in a matter of microseconds. Also, repeat determinations can be done quickly and efficiently.

we needn’t worry about dropping it from the same height each time, since a=constant

The logger is programmed with the length of each interrupt card ( I used 40mm), weighted a little bit to minimise air resistance and dropped vertically between the light gates.

The internal clock starts when the gate is interrupted and stops when it is not. It then calculates the average speed of each part of the double card as it falls. One speed is greater than the other because the card is accelerating. The internal clock in the logger records the time between the interrupt cards’ fall. Acceleration = change in speed / time.

The logger software can be used to display all speeds and times, or the calculated acceleration directly, which is simpler. Results were as shown.

These eight determinations can be output to a spreadsheet and their average calculated

As you see, all readings (should all be 9.8) are in error by approximately the same amount. This indicates a systematic error in the readings, perhaps because the weight of the card is unevenly distributed or the card lengths are not quite precise enough.

This is the calculation:

The Standard Model

The great Danish physicist, Niels Bohr, gave us a model of the atom which we still use today.

electrons in fixed orbitals around a tiny nucleus with Z protons and Z electrons. An electron decaying to a lower energy orbital emits a characteristic photon of energy proportional to frequency, ν 

But, protons neutrons and electrons are not the only particles…

If the protons and neutrons in this picture were 10cm across, the quarks and electrons would be less than 0.1mm and the whole atom would be 10km in size

The Standard Model explains almost everything about the fundamental structure of matter. Everything in the Universe is made from twelve basic building blocks called fundamental particles, governed by four fundamental forces. This post has been updated to include the Higgs boson Our best understanding of how these  particles and three of the forces are related to each other is encapsulated in the Standard Model. Terms can sometimes be confusing. A baryon is a composite subatomic particle made up of three quarks, example – protons and neutrons. Baryons and mesons belong to the hadron family, or quark-based particles. Mesons are composed of one quark and one antiquark and belong to the lepton family. Leptons (like electrons and neutrinos) do not experience the strong nuclear force.

The best way to understand the basics is to explore the interactive graphic by clicking on itScreen Shot 1.png

Developed in the early 1970s, it has successfully explained a host of experimental results and precisely predicted a wide variety of phenomena. Over time and through many experiments by many physicists, the Standard Model has become established as a well-tested physics theory.

Rutherford’s work  probed the atom. Deep inelastic scattering is the name given to extension of Rutherford scattering to much higher energies of the scattering particle and thus to much finer resolution, to identify the components of nuclei. It is used to probe the insides of hadrons (particularly  baryons, such as protons and neutrons), using electrons, muons and neutrinos. It provided the first convincing evidence of the reality of quarks, which up until that point had been considered by many to be a purely mathematical phenomenon. (IB question 2016)

As an example of an early application, in the early days the term beta ray was used for electrons in nuclear decays because people didn’t know they were electrons and it was wondered if beta decay violated energy conservation laws. Until the neutrino and antineutrino was discovered, that is…

Beta decay gives us this…

Screen Shot.png
Simplified Feynman diagram for beta decay


A neutron (udd) decays to a proton (uud), an electron, and an antineutrino. This is called neutron beta decay.

  • Frame 1: The neutron (charge = 0) made of up, down, down quarks.
  • Frame 2: One of the down quarks is transformed into an up quark. Since the down quark has a charge of -1/3 and the up quark has a charge of 2/3, it follows that this process is mediated (think of a handshake to seal a deal) by a virtual  W- particle, which carries away a (-1) charge (thus charge is conserved)
  • Frame 3: The new up quark rebounds away from the emitted W-. The neutron now has become a proton. A fermion is any particle that has an odd half-integer (like 1/2, 3/2, and so forth) spin. Quarks and leptons, as well as most composite particles, like protons, are fermions. Bosons are those particles which have an integer spin (0, 1, 2…).The nucleus of an atom is a fermion or boson depending on whether the total number of its protons and neutrons is odd or even, respectively
  • All the force carrier particles are bosons, as are those composite particles with an even number of fermion particles (like mesons).

  • Frame 4: An electron and antineutrino emerge from the virtual W- boson.
  • Frame 5: The proton, electron, and the antineutrino move away from one another.

The intermediate stages of this process occur in about 10-27s, and are not observable.

Here it all is…

This is good too…

Today, CERN announced after exhaustive experimentation that a muon neutrino – spectacularly hard to detect since it’s massless and without charge was clocked travelling  730 km from CERN in Switzerland to an Italian laboratory at Gran Sasso near Rome at speeds, impossibly, greater than the speed of light.

It can’t be right. If it is, Einstein was wrong and we all have to start again.