In physics class, you will make observations, do experiments, and create models or theories to try to explain your results or predict new answers, as shown in Figure 1-5. This is the essence of a scientific method. All scientists, including physicists, obtain data, make predictions, and create compelling explanations that quantitatively describe many different phenomena. The experiments and results must be reproducible; that is, other scientists must be able to recreate the experiment and obtain similar data. Written, oral, and mathematical communication skills are vital to every scientist. A scientist often works with an idea that can be worded as a hypothesis, which is an educated guess about how variables are related. How can the hypothesis be tested? Scientists conduct experiments, take measurements, and identify what variables are important and how they are related. For example, you might find that the speed of sound depends on the medium through which sound travels, but not on the loudness of the sound. You can then predict the speed of sound in a new medium and test your results.

Models, laws, and theories

An idea, equation, structure, or system can model the phenomenon you are trying to explain. Scientific models are based on experimentation. Recall from chemistry class the different models of the atom that were in use over time—new models were developed to explain new observations and measurements.

If new data do not fit a model, both are re-examined. Figure 1-6 shows a historical example. If a very well-established model is questioned, physicists might first look at the new data: can anyone reproduce the results? Were there other variables at work? If the new data are born out by subsequent experiments, the theories have to change to reflect the new findings. For example, in the nineteenth century it was believed that linear markings on Mars showed channels, as shown in Figure 1-7a. As telescopes improved, scientists realized that there were no such markings, as shown in Figure 1-7b. In recent times, again with better instruments, scientists have found features that suggest Mars once had running and standing water on its surface, as shown in Figure 1-7c. Each new discovery has raised new questions and areas for exploration.

A scientific law is a rule of nature that sums up related observations to describe a pattern in nature. For example, the law of conservation of charge states that in the various changes matter can undergo, the electric charge before and after stays the same. The law of reflection states that the angle of incidence for a light beam on a reflective surface equals the angle of reflection. Notice that the laws do not explain why these phenomena happen, they simply describe them.

Greek philosophers proposed that objects fall because they seek their natural places. The more massive the object, the faster it falls.

↓ Revision

Galileo showed that the speed at which an object falls depends on the amount of time it falls, not on its mass.

↓ Revision

Galileo’s statement is true, but Newton revised the reason why objects fall. Newton proposed that objects fall because the object and Earth are attracted by a force. Newton also stated that there is a force of attraction between any two objects with mass.

↓ Revision

Galileo’s and Newton’s statements still hold true. However, Einstein suggested that the force of attraction between two objects is due to mass causing the space around it to curve.

Figure 1-8 Theories are changed and modified as new experiments provide insight and new observations are made. The theory of falling objects has undergone many revisions.