This class introduced the basics of measures in physics, focusing on:
-
The meaning and importance of physical quantities and units
-
The International System of Units (SI)
-
Derived units and their calculation
-
Dimensional analysis and its critical role in validating equations
-
Practical examples and critical thinking exercises
1. What Are Measures in Physics?
-
Measures refer to quantifying physical quantities (like length, mass, time) using standard units.
-
Understanding units is essential for solving physics problems correctly and for IMAT exams.
2. The Seven SI Base Units
| Quantity | SI Unit (Symbol) | Example |
|---|---|---|
| Length | meter (m) | Height of a door: 2 m |
| Mass | kilogram (kg) | Bag of rice: 5 kg |
| Time | second (s) | Stopwatch: 10 s |
| Electric Current | ampere (A) | Current in wire: 2 A |
| Temperature | kelvin (K) | Room temp: 293 K |
| Amount of Substance | mole (mol) | 1 mol of NaCl |
| Luminous Intensity | candela (cd) | Light bulb: 10 cd |
Note: In physics, always use kilograms for mass; in chemistry, use grams.
3. Derived Units
-
Derived units are combinations of base units, used for quantities like speed, force, and energy.
-
Examples:
-
Speed/Velocity: meters per second (m/s)
-
Formula: v=dt
-
-
Acceleration: meters per second squared (m/s²)
-
Formula: a=vt
-
-
Force: newton (N) = kg·m/s²
-
Formula: F=m⋅a
-
-
Energy: joule (J) = newton·meter (N·m) = kg·m²/s²
-
4. Dimensional Analysis and Critical Thinking
-
Key Rule: Only quantities with the same dimensions can be added or subtracted.
-
Why? This ensures physical equations make sense and are valid.
-
Critical Thinking Example:
-
Given: v=at+v0
-
v (velocity): m/s
-
a (acceleration): m/s²
-
t (time): s
-
v0 (initial velocity): m/s
-
-
Check dimensions:
-
at: (m/s²)·s = m/s
-
v0: m/s
-
Both terms have the same dimension (m/s), so the equation is valid.
-
-
Another Example:
-
x=12at3+v0t
-
x (distance): m
-
at3: (m/s²)·s³ = m·s
-
v0t: (m/s)·s = m
-
The first term has dimension m·s, which is not the same as m, so this formula is not dimensionally correct.
-
5. Practical Application
-
Always check units in calculations.
-
If a question gives a value in grams or centimeters, convert to SI units (kg, m) before using formulas.
-
Use dimensional analysis to validate equations and avoid mistakes.
6. Summary Table: Common SI Units and Examples
| Quantity | SI Unit | Example Value |
|---|---|---|
| Length | m | 1.75 m (height) |
| Mass | kg | 0.5 kg (fruit) |
| Time | s | 60 s (minute) |
| Speed | m/s | 20 m/s (car) |
| Force | N | 9.8 N (gravity) |
| Energy | J | 100 J (work) |
7. Key Takeaways and Critical Thinking Tips
-
Always use SI units in physics problems unless told otherwise.
-
Check that all terms in an equation have the same dimensions before adding or subtracting.
-
Convert units when necessary (e.g., grams to kilograms, cm to meters).
-
Apply dimensional analysis as a quick check for formula validity-a crucial skill for IMAT and beyond.