It has long been suspected by general relativists that physicists who do not specialize in general relativity (GR) believe that special relativity (SR) is incapable of modeling dynamics within accelerated reference frames. Consequently, many physicists may conclude that certain phenomena, such as time dilation due to acceleration, can only be…

We propose a method to measure the speed of sound in air using a motion detector. The experiment is based on the principle of a motion detector used to measure distances. This device measures the time of flight of alternating ultrasonic waves between the observer and the target. By assuming a fixed speed of sound, the time of flight can be…

The Sun does not return to the same position in the sky every 24 hours. At local noon, for example, the Sun will appear higher in the sky as we move from winter to summer solstice. In addition, and perhaps more surprisingly, solar days (the roughly 24 hours between subsequent local noons) vary in length, causing the Sun to be east or west of its…

Photogates play an important part of many of today's physics laboratory experiments. They signal when an infrared (IR) beam is interrupted, thus supplying the experimenter with important timing information. Currently photogates use an infrared photodiode (IR transmitter) and a phototransistor (IR receiver). User-built photogates have a weakness…

Einstein's special theory of relativity includes many non-intuitive and apparently paradoxical conclusions about space and time. One of these is time dilation, the fact that a clock moving relative to an observer runs slower than an identical clock at rest in the observer's reference frame. This is clearly exhibited in the extended mean lifetime…

During the early decades of the 18th century, Newtonian physics was still new. Much effort was expended in testing its validity. One arena in which evidence could be found was the shape of Earth. Was it perfectly spherical? On the observational side there were two hints. In 1671 Jean Richer had measured the rate of ticking of a pendulum clock near…

Exciting examples of physics principles illuminate the power and scope of our discipline. In this paper, we discuss a conservation of energy example for an introductory course. Energy conservation is applied to bird and dinosaur eggs to derive a method to predict the incubation period and the embryonic metabolism solely from the egg geometry. The…

The global positioning system (GPS) must be familiar to every user of a smartphone as a way to locate one's position on Earth to within several meters. The constellation of orbiting atomic clocks equipped with radio transmitters allows receivers on Earth to determine their own position by measuring the travel times of radio signals from multiple…

RC circuits are ubiquitous. They are the most common entities in modern electronics; introductory physics courses typically include a chapter and a lab on RC circuits. Several papers dealing with many aspects of RC circuits have been published in "The Physics Teacher" and other educational journals. An RC circuit contains a series…

In this paper we derive and discuss the time it takes for a force to turn a velocity. More precisely, we derive the formula for the time t it takes a constant force that makes an angle a with the initial velocity v(0) to have v(Tau) get within an angle ? < a of the force. We then show how the addition of a viscous force decreases Tau…

During every moment of our lives, we are surrounded by radiation from natural and manmade sources. Our bodies contain radionuclides from which we receive radiation. Radionuclides are found in the foods we eat and in the ground we walk on. We receive cosmic radiation from space. Radionuclides are also found in the airborne particles we breathe. The…

How far, in space, can we see? And can we see an object whose Hubble recessional velocity exceeds the speed of light? Maybe you've thought about these questions before, or perhaps you've seen them discussed in the literature or mentioned in the media. With the recent popularity of inflation and Big Bang cosmology, they're hard to avoid. The…

The observation that the shadows of objects change during the course of the day and also for a fixed time during a year led curious minds to realize that the Sun could be used as a timekeeper. However, the daily motion of the Sun has some subtleties, for example, with regards to the precise time at which it crosses the meridian near noon. When the…

In introductory physics laboratories, spring constants are traditionally measured using the static method. The dynamic method, via vertical spring-mass oscillator, that uses a stopwatch in order to measure the period of oscillations is also commonly employed. However, this time-measuring technique is prone to human errors and in this paper we…

The goal of this paper is to illustrate different ways that cardsorting activities (or "card stacks") can be implemented in the introductory physics classroom, along with various tips and resources for getting started. My first attempt at developing a card stack came about from simply wanting to try out a fun way to change student…