Physics and Nature Physics studies the fundamental laws of nature. It is based on observations and experimentation, which lead to laws and theories. We can then use these laws and theories to predict and explain everyday phenomena. You can check what you predict based on physics laws. Physics is the oldest and most fundamental of all the sciences and is applied and specialized in many other branches. In daily life we use human languages such as English. For computers we use computer languages such as BASIC or C++. The language of physics is mathematics because the nature laws are precisely described and natural phenomena calculated. Physics 141 is a calculus based physics course.

Mechanics is the oldest and most fundamental part of physics. The modern era started as Newton discovered the Newton’s laws of mechanics. Mechanics is the study of forces and motion. Whenever anything moves, be it a galaxy or an elementary particle, it must obey the laws of mechanics. If you want to know whether an asteroid will hit the earth, the only knowledge you need to have is mechanics. What we are going to study in Physics 141 are Newton’s laws. Newton formulated mechanics in terms of three very simple laws. You don’t have to just accept these laws. They can stand the test by experiments, observations, and measurements. We will study these laws and their applications. Mechanics

Measurements and Units The need of units. Most quantities we encounter in daily life need to be measured in some unit. Distance, cost, time, etc. Basic units. The basic units we use in mechanics are Time, Length, and Mass. Proper combinations of these basic units give us derived units, such as velocity, acceleration, force, torque, etc. Example: [unit of area] = [unit of length] x [unit of length] International systems (SI) SI is also known as the metric system. Most countries in the world are in fact using SI. In SI the basic units are: Time in second (s) Mass in kilogram (kg) Length in meter (m)

Check and Balance Check the validity of an equation The first step to check the validity of an equation or an expression in physics is to look at the unit. Example: If Velocity of a car = A x B / C We must have [unit of A] x [unit of B] / [unit of C] = [length]/[time] Because of this, we can see immediately that the expression log(A/B) makes no sense if [unit of A] ≠ [unit of B], since A/B will not be a pure number as required by the logarithmic function.

Conversion of Units Standard systems. A few countries are still using different systems of units. In the US we are using the standard system, second, pound (lb), foot (ft). 1 kg = lb 1 m = ft = yard We live in the US and occasionally we need to convert the quantities in the standard units to SI Example: 1 mi = km ( 1=1.609km/mi) 1 hr = 3600 s

Necessity of High Standard It’s extremely important to have high standards for the units. There used to be a government agency called National Bureau of Standards (NBS), now renamed as National Institute of Science and Technology (NIST), whose in charge of the standards of units in the US. Unit of time. In the old days the standard of time is a Day, and 1 second = 1/(24x3600) day = 1/86400 day. Now we have much higher standard. According to the NIST, 1 second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom. As technology improves, there can be even better clocks to serve as the standard unit of time. See for more details.

Unit of Length The meter is used to be defined as of the distance from the north pole to the equator of the Earth. Now it is defined as the distance traveled by light in the vacuum in 1/299,792,458 second. That the velocity of light in a vacuum is a constant is in fact a basic law of physics, and Einstein discovered special relativity from this basic law!

Definition of Mass Mass is not as obvious a physical quantity as length or time. The mass of a body is a measure of the amount of matter contained in it. The SI mass standard is a platinum-iridium cylinder, defined as 1 kg, kept at the International Bureau of Weights and Measure near Paris. A more accurate definition is using a carbon atom of isotope C 12, which has a mass of x kg.

Mass or Mess “Lost weight” or “lost mass”When someone says he/she has “lost weight,” what he/she really means is he/she has lost mass. Weight is the force by the Earth acting on a mass and is proportional to the mass, but if you go to the moon, you will loose weight by 5/6 without doing any dieting! Gravity mass or acceleration mass?Gravity mass is related to the attraction between different objects, and acceleration mass is from the change of motion of a mass by a force. Can you see any natural relations between the two? In fact it is still an unsolved problem in physics, and the latest news is that with the best equipment and techniques, no difference between the two can be observed.

Orders of Magnitude Big numbers, small numbers In reality we often encounter big numbers or small numbers, for example, the electron mass is m e = kg or, m e = x kg. Usually we only need 3 significant digits, or m e = 9.11 x kg In our CAPA homework, you are typically required to supply only 3-4 significant digits.

Checking Orders of Magnitude Writing quantities in the form of ?.?? x 10 ?? has another added advantage: it allows us to check if the order of magnitude of an answer makes sense. Example: x x x ~ ~10 -9 Without going through the actual calculation you should see if the order of magnitude is reasonable. If you find a formula to calculate your time to spend in college to be 10 2 s, then you know either the formula is wrong or you’ve put in numbers in wrong units!