Notes
Scientific method and variables
- Scientists ask a question, form a testable hypothesis, run a controlled experiment, analyze data and draw a conclusion.
- The independent variable is changed on purpose, the dependent variable is measured, and controlled variables stay the same. A control group provides a baseline.
SI units and conversions
- SI base units: meter (length), kilogram (mass), second (time), kelvin (temperature). Volume: liter or $\text{cm}^3$.
- Metric conversions use powers of 10: kilo = 1,000, centi = 0.01, milli = 0.001. Going to a smaller unit, multiply; to a larger unit, divide.
Graphs and density
- Independent variable on the x-axis, dependent on the y-axis. Slope $\displaystyle = \frac{\Delta y}{\Delta x}$.
- Density $\displaystyle D = \frac{m}{V}$. Water is $1.0 \text{ g/cm}^3$; less dense objects float.
Accuracy and precision
- Accuracy is closeness to the true value; precision is closeness of repeated measurements. Percent error $\displaystyle = \frac{|\text{measured} - \text{accepted}|}{\text{accepted}} \times 100\%$.
Topics in this unit
Each topic has its own short notes and practice.
Practice
The independent variable is the factor the experimenter changes.
$0.75 \times 1000 = 750$.
Slope $\displaystyle = \frac{30}{5} = 6$ m/s.
$\displaystyle D = \frac{135}{50} = 2.7$.
Precision is about agreement between repeated measurements.
Objects denser than $1.0 \text{ g/cm}^3$ sink in water.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A student finds the density of a block that measures 5 cm by 4 cm by 2 cm. A balance shows the block's mass is 120 g.
(a) Find the volume of the block.
Show the answer and scoring guide
$V = 5 \times 4 \times 2 = 40 \text{ cm}^3$.
Check off the points you earned:
(b) Calculate the density and state whether the block floats or sinks in water.
Show the answer and scoring guide
$\displaystyle D = \frac{120}{40} = 3 \text{ g/cm}^3$. This is greater than $1.0 \text{ g/cm}^3$, so the block sinks.
Check off the points you earned:
(c) A piece of the same material has a volume of $25 \text{ cm}^3$. Find its mass.
Show the answer and scoring guide
Same material means same density. $m = D \times V = 3 \times 25 = 75$ g.
Check off the points you earned: