Unit 1 sets up the whole course by explaining what physics studies, how physicists test ideas, and how they describe the world with numbers and units. After this unit you should be able to tell a hypothesis from a theory, design a fair test, convert units with dimensional analysis, and report answers with sensible significant figures.
Read the overview first, then work through each topic's notes and checkpoint question before checking the common mistakes at the bottom.
Later units build on the ideas and skills in this unit. The habits you build here, like tracking units through every step and asking what evidence supports a claim, show up in kinematics, forces, energy, and every lab report.
Honors Physics Unit 1 typically introduces physics as the study of matter, energy, and their interactions, including the split between classical and modern physics. It also covers scientific methods, such as testing hypotheses and using models, and measurement tools, such as SI units, dimensional analysis, vectors, and significant figures.
Noticing that a dropped ball falls is observation. Predicting that a ball dropped from rest falls about 4.9 m in one second near Earth's surface, if air resistance is negligible, is physics. The same core ideas, like conservation of energy, apply across branches from mechanics to electromagnetism.
A hypothesis makes a prediction, an experiment checks it, and other scientists try to replicate the result. Even well-tested models like Newton's laws have limits, which is why relativity and quantum mechanics exist.
Saying a car moves at 30 is useless until you know whether that's m/s or km/h. Significant figures help show the precision of a measurement. Whether to round a calculator readout of 7.8539 depends on the precision of the inputs and the operation used.
Every part of this unit supports that one goal. The branches of physics tell you what kinds of claims get made, scientific methods tell you how they get tested, and units and significant figures make sure the numbers in those tests are clear and honest. If you keep asking "what's the evidence, and what are the units?" you're already thinking like a physicist.
Physics is the study of matter, energy, and how they interact.
Scientific methods are the ways scientists test explanations against evidence.
Physics uses the SI system, which has seven base units.
Physics is the study of matter, energy, and how they interact. Matter is anything with mass that takes up space, and energy is the ability to cause change, such as making something move or heat up. Physics describes these interactions with mathematical models that predict what will happen.
The field splits into branches. Mechanics covers motion and forces, thermodynamics covers heat and temperature, electromagnetism covers charges, currents, and magnetic fields, and optics covers light. The branches overlap a lot. Optics is really part of electromagnetism because light is an electromagnetic wave, and conservation of energy links almost all of them.
Physics is also split by era and scale. Classical physics, built mostly before 1900 on the work of Galileo and Newton, works very well for everyday objects at everyday speeds. Modern physics began in the early 1900s when experiments showed classical ideas failing in extreme cases. Einstein's special relativity (1905) handles speeds near the speed of light, general relativity (1915) treats gravity as curved spacetime, and quantum mechanics handles atoms and smaller particles.
A common confusion is thinking modern physics replaced classical physics. It didn't. Newton's laws still land spacecraft on Mars. Relativity and quantum mechanics matter when classical predictions are not accurate enough. For example, GPS satellites must correct their clocks for relativity even though gravity near Earth is not extremely strong.
Physics shows up everywhere in daily life. Engineers use it to design bridges, doctors use it in MRI scanners, and phone screens depend on electromagnetism.
Check yourself: Would you need relativity to calculate how long a baseball takes to reach home plate? (Answer: No. A baseball moves at about 40 m/s, far below the speed of light, so classical mechanics gives an accurate answer.)
| Feature | Classical physics | Modern physics |
|---|---|---|
| When developed | Mostly 1600s-1800s | Early 1900s onward |
| Major names | Galileo, Newton, Maxwell | Einstein, Bohr, Heisenberg, Schrödinger |
| Works best for | Everyday sizes and speeds | Near light speed, strong gravity, atomic scales |
| Predictions | Often deterministic in an ideal model, but real predictions have uncertainty | Often probabilistic in quantum mechanics |
Scientific methods are the ways scientists test explanations against evidence. One common approach moves from observation and question to hypothesis and prediction, then tests the prediction with an experiment or further observations. Scientists analyze and share the results so others can check them. In practice scientists loop back often, which is why the topic says "methods" in the plural.
In a simple controlled experiment, you change one independent variable, measure the dependent variable, and keep other relevant conditions as consistent as possible. If you test whether mass affects falling speed, mass is independent, fall time is dependent, and drop height stays fixed.
Models are simplified versions of reality. A simple pendulum model ignores air resistance and string mass, which is fine until those effects get large. Every model has limits.
A hypothesis is a testable proposed explanation. A law describes a pattern, often as an equation. A theory explains why, backed by lots of evidence. Theories never "graduate" into laws because they do different jobs.
Worked example. Your hypothesis predicts that objects dropped near Earth's surface accelerate at about when air resistance is negligible. You drop a ball from rest through m and measure a fall time of s. Using , solve for : . The result is consistent with the prediction, but one trial does not prove the hypothesis. Repeated trials with different objects and careful control of air resistance would provide a stronger test.
Check yourself: You test how ramp angle affects a cart's speed at the bottom. Name the independent variable, the dependent variable, and one controlled variable. (Answer: Ramp angle is independent, final speed is dependent, and the cart's mass or the ramp's length should stay controlled.)
| Term | What it does | Physics example |
|---|---|---|
| Hypothesis | Proposes a testable explanation | Heavier objects fall at the same rate as light ones |
| Law | Describes a pattern, often as an equation | Newton's second law, F = ma |
| Theory | Explains why, backed by extensive evidence | Einstein's theory of general relativity |
Physics uses the SI system, which has seven base units. The ones you'll use most are the meter (m), kilogram (kg), and second (s). The others are the ampere, kelvin, mole, and candela. Derived units combine base units. Speed is m/s, and the newton is .
Metric prefixes scale units by powers of ten, so kilo means , centi means , and milli means . Dimensional analysis converts units by multiplying by fractions equal to 1 and canceling units. It also checks your work, because an answer with the wrong units is wrong.
Scalars have size only, like 5 kg. Vectors have size and direction, like 20 m/s north. Two 5 N forces add to 10 N only if they point the same way.
Significant figures show precision. When you multiply or divide, keep the fewest sig figs of any input. When you add or subtract, keep the fewest decimal places.
Worked example. Convert 72 km/h to m/s. . The km and h cancel, leaving m/s. Next, find the area of a board measuring 2.5 m by 3.14 m. The calculator gives , but 2.5 has only two sig figs, so report .
Check yourself: Add 12.1 m and 3.456 m and report the answer with correct precision. (Answer: The raw sum is 15.556 m, but 12.1 has only one decimal place, so the answer is 15.6 m.)
| Term | Definition |
|---|---|
| Matter | Anything that has mass and takes up space. |
| Energy | The ability to cause change, such as moving an object or raising its temperature, measured in joules. |
| Scientific law | A statement, often an equation, that describes a pattern seen consistently in nature without explaining its cause. |
| Scientific theory | A well-supported explanation of natural phenomena based on many tested hypotheses and repeated observations. |
| Model | A simplified physical, visual, or mathematical representation of a system that helps make predictions. |
| SI base unit | One of seven standard units, including the meter, kilogram, and second, from which all other SI units are built. |
| Metric prefix | A prefix such as kilo, centi, or milli that multiplies a unit by a power of ten. |
| Scalar | A quantity fully described by a number and a unit, with no direction, such as mass or temperature. |
In everyday talk, "just a theory" means a hunch. In science, a theory is a well-tested explanation supported by large amounts of evidence, like the atomic theory or general relativity. A hypothesis is the tentative idea, and a theory is far stronger than that.
Laws and theories do different jobs. A law describes what happens, often as an equation, and a theory explains why it happens. A theory never upgrades into a law, and neither one is ever proven beyond all possible revision.
If you multiply 5 km by (1 km / 1000 m), kilometers do not cancel and the result is not expressed in meters. Write every unit at every step and set up each fraction so the unit you want to remove cancels. Then check that only the units you want are left.
A calculator doesn't know how precise your ruler was. For multiplication or division, round the result to the fewest significant figures among the measured inputs. For addition or subtraction, round to the least precise decimal place. Round at the end of the calculation, not in the middle, so rounding errors don't pile up.