Grade: Grade 11 Subject: Science Unit: Physics Lesson: 5 of 6 SAT: ProblemSolving+DataAnalysis ACT: Science

CER Writing

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The Claim-Evidence-Reasoning (CER) framework is a structured approach to scientific writing that helps you construct clear, logical arguments based on data and scientific principles.

What is CER?

CER stands for:

  • Claim: A statement that answers a scientific question
  • Evidence: Data from observations or experiments that supports your claim
  • Reasoning: The scientific explanation of how the evidence supports the claim

Writing a Strong Claim

A good claim should:

  • Directly answer the question asked
  • Be a complete sentence
  • Be specific and testable
  • Not include words like "I think" or "I believe"

Weak claim: "I think the ball fell faster."

Strong claim: "The heavier ball experienced the same acceleration as the lighter ball when dropped from the same height."

Selecting Appropriate Evidence

Strong evidence should:

  • Come from your data or observations
  • Include specific numbers, measurements, or observations
  • Be relevant to the claim you are making
  • Include units and reference to where data came from

Weak evidence: "The graph showed they were similar."

Strong evidence: "According to the velocity-time graph, the 100 g ball reached 4.9 m/s after 0.5 s, while the 200 g ball reached 4.8 m/s after the same time interval."

Developing Clear Reasoning

Effective reasoning should:

  • Connect the evidence to the claim using scientific principles
  • Reference specific laws, theories, or equations
  • Explain why the evidence supports the claim
  • Address potential counterarguments or limitations

Weak reasoning: "This shows they fell the same."

Strong reasoning: "This data supports the claim because, according to Galileo's principle and Newton's Second Law, all objects in free fall experience the same gravitational acceleration (g = 9.8 m/s^2) regardless of mass. The equation a = F/m shows that while the gravitational force (F = mg) is greater for the heavier object, so is its mass, resulting in the same acceleration for both objects."

CER Structure Template

Paragraph 1 - Claim:

[State your claim clearly, directly answering the question.]

Paragraph 2 - Evidence:

[Present specific data from your experiment. Include numbers, units, and reference to data sources like tables or graphs.]

Paragraph 3 - Reasoning:

[Explain the scientific principles that connect your evidence to your claim. Reference relevant equations, laws, or theories.]

Common CER Mistakes to Avoid

  • Vague claims: Make sure your claim is specific and measurable
  • Missing data: Always include specific numbers and units in evidence
  • Restating evidence: Reasoning should explain WHY, not just repeat what you observed
  • Skipping scientific principles: Always connect to physics concepts, laws, or equations
  • Ignoring uncertainty: Acknowledge limitations and sources of error

Examples

Example 1: Complete CER Response

Question: Does the mass of a cart affect its acceleration when pulled by a constant force?

Claim: As the mass of the cart increases, the acceleration decreases when the applied force remains constant.

Evidence: In our experiment, a 0.5 kg cart pulled by a 2.0 N force accelerated at 3.9 m/s^2. When the mass was doubled to 1.0 kg with the same 2.0 N force, the acceleration decreased to 2.0 m/s^2. With a 1.5 kg cart, the acceleration further decreased to 1.3 m/s^2.

Reasoning: This data supports the claim because it demonstrates the inverse relationship between mass and acceleration described by Newton's Second Law: F = ma, or rearranged, a = F/m. When force is held constant and mass increases, acceleration must decrease proportionally. The theoretical accelerations would be: a = 2.0/0.5 = 4.0 m/s^2, a = 2.0/1.0 = 2.0 m/s^2, and a = 2.0/1.5 = 1.33 m/s^2. Our experimental values closely match these predictions, with small differences likely due to friction.

Example 2: Evaluating a CER Response

Student Response: "The spring stretched more when we added more mass. We added 100g and it stretched 5 cm, then 200g stretched it 10 cm. This proves Hooke's Law."

Evaluation:

  • Claim: Missing a clear claim statement
  • Evidence: Good specific data but needs units for mass (should be 0.1 kg and 0.2 kg or specify grams)
  • Reasoning: Mentions Hooke's Law but doesn't explain it or show how the evidence supports it

Improved Response: "The extension of a spring is directly proportional to the applied force (Claim). When a 0.100 kg mass (F = 0.98 N) was hung from the spring, it extended 5.0 cm. A 0.200 kg mass (F = 1.96 N) caused a 10.0 cm extension (Evidence). This linear relationship supports Hooke's Law, F = kx, where the spring constant k = F/x = 0.98 N / 0.05 m = 19.6 N/m. The doubling of force resulted in doubling of extension, confirming the direct proportionality (Reasoning)."

Practice

Complete these practice problems to develop your CER writing skills.

1. Write a claim that answers: "How does the height of a ramp affect the speed of a ball at the bottom?"

2. A student conducts a pendulum experiment and records: Trial 1: 1.98 s, Trial 2: 2.01 s, Trial 3: 1.99 s, Trial 4: 2.02 s, Trial 5: 2.00 s. Write an evidence statement using this data.

3. Write a reasoning paragraph that connects the concept of conservation of energy to an experiment where a ball is dropped from different heights.

4. Identify the problems with this claim: "I believe the experiment showed that friction makes things slow down or something like that."

5. An experiment measures the force needed to accelerate a 2.0 kg cart at different rates: (1.0 m/s^2, 2.0 N), (2.0 m/s^2, 4.1 N), (3.0 m/s^2, 5.9 N). Write a complete CER response explaining what this data demonstrates about Newton's Second Law.

6. What scientific principle would you include in your reasoning for an experiment about the relationship between voltage and current in a circuit?

7. A student writes: "The ball bounced lower each time because energy was lost." Improve this reasoning by adding specific physics concepts and explaining the energy transformation.

8. Write a complete CER response for the question: "Does the angle of a ramp affect the acceleration of an object sliding down it?" Use the following data: 15 degrees = 2.4 m/s^2, 30 degrees = 4.9 m/s^2, 45 degrees = 6.9 m/s^2.

9. Explain why including specific numerical data in your evidence makes your argument stronger than using words like "more," "less," or "about the same."

10. A student's CER response for a momentum experiment states: "Momentum was conserved because the total before equaled the total after." What is missing from this evidence statement?

Check Your Understanding

Answer these questions to test your knowledge of CER writing.

Question 1: Which component of CER includes specific numbers and measurements from an experiment?

A) Claim
B) Evidence
C) Reasoning
D) Hypothesis

Question 2: The reasoning section should:

A) Restate the claim
B) Present new data
C) Explain how evidence supports the claim using scientific principles
D) Ask a new question

Question 3: Why is it important to include scientific laws or principles in your reasoning?

Next Steps

  • Practice writing CER responses for different physics experiments
  • Review scientific principles to strengthen your reasoning
  • Move on to the next lesson: Unit Checkpoint