Grade 8 · Science · 35 minutes
Unit 2 · Energy and Experimental Reasoning · Lesson 2 of 2
Audit a Force-and-Mass Investigation
Critique a flawed experimental report and redesign it for interpretable evidence.
Mr. S Teaches · Original starter mini-lesson · MS8-SCI-04Learning objective
Identify two design flaws and produce a controlled, repeated investigation plan with a measurable outcome.
Before you begin
- Distinguish applied force from net force and understand acceleration.
Gather your materials
- Paper or notebook
- Pencil and the passage, model directions or data printed in this lesson
Language to know
- operational definition
- A precise statement of how a variable will be measured or changed.
- uncertainty
- A limit to how precisely a quantity is known.
- replication
- Repeating a procedure to examine consistency.
Understand the concept
Flawed report: “A 1 kg cart pushed hard on a smooth track traveled 3 m. A 2 kg cart pushed gently on a carpet traveled 1 m. Therefore, doubling mass always cuts acceleration by two-thirds.” The report changes mass, push and surface at once; “hard” is not a calibrated force. It measures distance without stating elapsed time or starting velocity. A fair test of mass should control net force and starting conditions and measure how velocity changes over a defined time.
Teach it step by step
- Read the report and mark every changed or undefined condition.
- Explain why traveled distance alone, without time and initial velocity, does not establish acceleration.
- Design two mass conditions with the same measured net force, level track, starting velocity and observation interval.
- Specify at least three trials per condition and describe how you will compare variation before claiming a pattern.
Make the thinking visible
Worked example or model response
A stronger plan compares 1 kg and 2 kg carts from rest under the same known net force on the same track. Record velocity after equal intervals and calculate change in velocity per interval. Repeats reveal scatter; they do not automatically remove a consistently miscalibrated force sensor.
Guided practice
Together replace “pushed hard” with an operationally defined force condition and list which other conditions must stay consistent.
Independent practice
Write a reproducible investigation plan and predict qualitatively which cart should change velocity more under equal net force. Include one instrument check and a method of recording all trials, not only the preferred result.
Quick assessment
Would ten repeats of the original flawed comparison isolate the effect of mass?
Answer key & assessment criteria
Answers and teaching guidance
- Guided: Specify a measured net force, such as 2 N maintained for the defined interval, with the same track and starting conditions.
- Independent: The lower-mass cart should have the larger velocity change in the ideal model. Criteria: operational force and time measures, controlled surface/start, multiple recorded trials and an instrument check.
- Quick check: No. Repeating a confounded design does not separate mass from push or surface effects.
Look for: Look for a claim that answers this lesson’s question, accurate use of the supplied evidence and an explanation of a limit or alternative. Ask the learner to point to the detail supporting each inference.
Make it work for your learner
More support
Use a “changed/controlled/measured” planning table and discuss the flawed report one sentence at a time.
Extension challenge
Explain how friction measurements, a force sensor and a velocity sensor could distinguish systematic error from random scatter.
Selected contributions · Official sources
Standards connections
Selected NGSS band contribution; not complete performance expectation coverage.
Contributes to NGSS performance expectation
MS-PS2-2NGSS · Grades 6–8 band
Investigate how force and mass affect changing motion.
Read the official performance expectation ↗The lesson critiques and plans an investigation. Full performance requires carrying it out safely, recording motion data and supporting the force/mass relationship with empirical results.
Source and mapping checked 2026-11-04. This mini-lesson contributes selected skills; it does not claim full attainment.
Supporting Common Core literacy connection
RST.6-8.1Common Core · Grades 6–8
Support scientific-text analysis with specific evidence.
Read the official standard ↗The provided scientific passage/model is explicitly analyzed in the activity. This selected literacy practice does not establish subject-content alignment or complete coverage of the standard.
Source and mapping checked 2026-11-04. This mini-lesson contributes selected skills; it does not claim full attainment.