# Experimental Design in Physics — ACT Science
The ACT Science section heavily tests your ability to understand experimental design and interpret results. Many physics passages present experiments with tables, graphs, and sometimes conflicting explanations. This topic covers the skills you need.
Key Concepts
Variables
- Independent variable: what the experimenter changes (e.g., ramp angle).
- Dependent variable: what is measured (e.g., distance traveled).
- Controlled variables: kept constant to ensure a fair test.
Controls
- A control group provides a baseline for comparison.
- Good experiments change only one variable at a time.
Reading Tables and Graphs
- Identify which variable is on each axis.
- Look for trends: increasing, decreasing, linear, exponential, no relationship.
- Note units and scales.
Data Interpretation Tips
- Direct relationship: as X increases, Y increases.
- Inverse relationship: as X increases, Y decreases.
- No relationship: Y doesn't change with X.
Conflicting Viewpoints
- Some passages present two or more scientists with different explanations.
- Identify: What do they agree on? What differs? What evidence supports each view?
Common Physics Experiment Types on ACT
- Measuring speed/acceleration on ramps.
- Heating/cooling experiments.
- Circuits with varying resistance.
- Projectile launches at different angles.
- Wave/sound frequency experiments.
Worked Example
Passage summary: Two students disagree about why a ball rolls farther on a smooth surface than a rough surface.
- Student 1: Less friction on smooth surfaces allows the ball to maintain its speed longer.
- Student 2: The smooth surface is slightly downhill, adding gravitational acceleration.
Q: What experiment would resolve the disagreement?
A: Repeat the experiment on a perfectly level surface (verified with a level tool). If the ball still goes farther on the smooth surface, Student 1 is correct.
Practice Questions
1. In an experiment testing how mass affects acceleration with constant force, what is the independent variable?
Mass.
2. A graph shows current vs. voltage for a resistor. The line is straight through the origin. What does this mean?
Current is directly proportional to voltage (Ohm's law behavior).
3. An experiment measures the period of a pendulum for different lengths but the students don't control the mass. Is this a problem?
Not really — pendulum period doesn't depend on mass (for small angles). But in principle, good experimental design should control variables even if theory says they don't matter.
Want to check your answers and get step-by-step solutions?
Summary
- Identify independent, dependent, and controlled variables.
- Look for trends in data: direct, inverse, or no relationship.
- In conflicting viewpoints, identify the specific claim that differs and what evidence could distinguish them.
- Good experiments change one variable at a time with proper controls.