Intro
This week introduces experimental design as a real physics skill. The point is not to memorize a lab template. The point is to decide what should be measured, what should be controlled, and what evidence would actually support a claim.
Core Lesson
A strong experiment starts with a sharp question. If you want to test how mass affects acceleration, you need to decide what quantity you will vary, what quantity you will measure, and what conditions must stay fixed so the result means anything.
Good experiments also respect uncertainty. Measurements are not exact. A useful design chooses tools with sensible precision, repeats trials when noise matters, and records data in a form that makes patterns visible.
Graphs are part of the design, not just the report. If you know what relationship you expect, you should already be thinking about which graph would reveal it clearly and what slope or shape would count as evidence.
AP Lift
The modern AP exam now has a standardized experimental design and analysis FRQ. Students need practice proposing a procedure, identifying variables, justifying measurement choices, and explaining how data would support or weaken a claim.
Must-Master Objectives
- Distinguish independent, dependent, and controlled variables.
- Propose a measurement plan that can actually test a claim.
- Explain why repeated trials or more precise tools matter.
- Describe what kind of graph would make a relationship visible.
Problem Set Prompts
- You want to test how the length of a pendulum affects its period. What is the independent variable? What is the dependent variable?
- In the pendulum experiment above, name two variables that should be controlled.
- A student measures the bounce height of a ball dropped from several heights. What graph would best show whether bounce height depends linearly on drop height?
- Why is it usually better to time ten pendulum swings than one swing?
- A group wants to test whether surface texture affects friction. What quantity should they measure directly?
- Describe one source of systematic error in timing a cart by hand with a stopwatch.
- If two groups get different slopes on the same graph, what are two plausible reasons besides “someone is bad at physics”?
- Stretch: Design a simple experiment to test whether spring extension depends linearly on hanging mass.
- Stretch: Explain why “measure everything and see what happens” is weaker than testing a specific hypothesis.
Reflection Prompt
- When you imagine an experiment, do you first think about the tools, the variables, or the graph?
- What feels less natural right now: controlling variables or deciding what counts as evidence?