CER Writing: Scientific Arguments About Forces
📖 Learn
Scientists communicate their findings through written explanations that include evidence and reasoning. The CER framework helps you write clear, logical scientific arguments.
What is CER?
CER stands for Claim - Evidence - Reasoning. It is a framework for constructing scientific explanations and arguments.
C - Claim
A statement that answers a scientific question. Your claim should:
- Directly answer the question being asked
- Be specific and clear
- Not include data or explanations (save those for E and R)
Sentence starters: "The data shows that..." / "Based on my investigation..." / "[Variable A] affects [Variable B] by..."
E - Evidence
Data or observations from your experiment that support your claim. Good evidence:
- Comes directly from your data or reliable sources
- Includes specific numbers with units
- Is relevant to your claim
- May reference data tables or graphs
Sentence starters: "In the experiment, when..." / "The data table shows that..." / "According to the graph..."
R - Reasoning
The scientific explanation of WHY your evidence supports your claim. Reasoning:
- Connects evidence to the claim using scientific principles
- References scientific laws, theories, or concepts
- Explains the relationship between variables
- Shows your understanding of the science behind the results
Sentence starters: "This occurs because..." / "According to Newton's [First/Second/Third] Law..." / "This relationship can be explained by..."
Common CER Mistakes to Avoid
- Vague claims: "The experiment worked" vs. "Increasing force increased acceleration"
- No numbers in evidence: "The acceleration was higher" vs. "The acceleration increased from 2 m/s2 to 6 m/s2"
- Missing reasoning: Just restating the evidence instead of explaining WHY it happened
- Reasoning without science: "It happened because that's what we expected" vs. "This confirms Newton's Second Law, F = ma"
💡 Examples
Example 1: Newton's Second Law CER
Question: How does the mass of an object affect its acceleration when the same force is applied?
Claim: Increasing the mass of an object decreases its acceleration when the same force is applied.
Evidence: In our experiment, when we applied a constant force of 5 N to a cart, the 1 kg cart accelerated at 5.0 m/s2, the 2 kg cart accelerated at 2.5 m/s2, and the 5 kg cart accelerated at 1.0 m/s2. As shown in the data table, when mass doubled from 1 kg to 2 kg, the acceleration was cut in half.
Reasoning: This inverse relationship between mass and acceleration is explained by Newton's Second Law, which states that F = ma, or rearranged, a = F/m. When force is constant, acceleration is inversely proportional to mass. The cart with more mass has more inertia (resistance to changes in motion), so it takes more force to achieve the same acceleration. Since we kept the force constant, increasing mass resulted in proportionally lower acceleration.
Example 2: Friction CER
Question: How does surface texture affect the force of friction?
Claim: Rougher surfaces produce greater friction forces than smoother surfaces.
Evidence: When pulling a 500 g wooden block across different surfaces with a spring scale, the force required varied significantly. On ice, only 0.5 N was needed to maintain motion. On tile, 2.1 N was required. On wood, 4.2 N was needed, and on sandpaper, 8.5 N was required. The bar graph clearly shows that friction force increases with surface roughness.
Reasoning: Friction is caused by the microscopic bumps and irregularities on surfaces interlocking with each other. Rougher surfaces have larger and more numerous irregularities, which creates more contact points and requires more force to slide past each other. Smoother surfaces like ice have fewer irregularities, so there are fewer contact points, resulting in less friction. This explains why ice has the lowest friction (0.5 N) while sandpaper has the highest (8.5 N).
Example 3: Gravity CER
Question: Do objects of different masses fall at the same rate?
Claim: In the absence of significant air resistance, objects of different masses fall at the same rate.
Evidence: When we dropped a 100 g ball and a 500 g ball from a height of 2 meters, both hit the ground at the same time (0.64 seconds). We repeated this trial five times, and in each trial, the balls landed simultaneously. The average fall time for both objects was 0.64 s with a variation of only 0.02 s.
Reasoning: While heavier objects experience a greater gravitational force (F = mg), they also have more mass to accelerate. According to Newton's Second Law (a = F/m), the acceleration equals the gravitational force divided by mass. Since both F and m increase proportionally, the acceleration due to gravity remains constant at approximately 9.8 m/s2 for all objects. This is why both the 100 g and 500 g balls fell at the same rate, regardless of their mass difference.
✏️ Practice
1. Identify whether each statement is a Claim (C), Evidence (E), or Reasoning (R):
- a) "The cart accelerated at 4 m/s2 when pushed with 8 N of force."
- b) "Greater force causes greater acceleration."
- c) "This relationship is described by Newton's Second Law, F = ma."
2. What is wrong with this claim? "My hypothesis was correct and the experiment worked well."
3. Improve this evidence statement: "The heavier objects moved slower."
4. A student wrote: "The ball fell because gravity pulled it down." Is this good reasoning? Explain what is missing.
5. Write a claim to answer this question: "How does the angle of a ramp affect the acceleration of a rolling ball?"
6. Given this data, write an evidence statement:
| Applied Force (N) | Acceleration (m/s2) |
|---|---|
| 2 | 1.0 |
| 4 | 2.0 |
| 6 | 3.0 |
| 8 | 4.0 |
7. Write a reasoning statement that explains why doubling the force doubles the acceleration. Reference the appropriate scientific law.
8. Complete this CER for the question "Does mass affect the gravitational force on an object?"
- Claim: _______________
- Evidence: A 1 kg mass weighed 9.8 N on a scale, while a 2 kg mass weighed 19.6 N, and a 5 kg mass weighed 49 N.
- Reasoning: _______________
9. A student pushes a box across a rough floor. At first the box doesn't move, but after applying more force, it starts sliding. The student then notices it takes less force to keep it moving than it took to start it moving. Write a complete CER to explain this observation.
10. Read this CER and identify at least two ways it could be improved:
"Claim: Force affects motion. Evidence: When I pushed harder, things moved faster. Reasoning: This happened because pushing harder makes things go faster."
✅ Check Your Understanding
Question 1: What is the difference between evidence and reasoning in a CER?
Show Answer
Evidence is the data or observations from an experiment (the "what"), while reasoning explains WHY the evidence supports the claim using scientific principles (the "why"). Evidence describes what happened; reasoning explains the science behind it.
Question 2: Why is it important to include specific numbers and units in your evidence?
Show Answer
Specific numbers and units make your evidence precise and verifiable. Saying "the acceleration was 3 m/s2" is much stronger than "the acceleration was fast" because it can be checked and compared to other measurements.
Question 3: How should you reference scientific laws or principles in your reasoning?
Show Answer
You should name the law or principle, explain what it states, and show how it connects to your specific evidence and claim. For example: "According to Newton's Second Law (F = ma), acceleration is directly proportional to force when mass is constant, which explains why doubling the force from 4 N to 8 N doubled the acceleration from 2 m/s2 to 4 m/s2."
🚀 Next Steps
- Practice writing CER responses for your own experiments
- Review Newton's Laws to strengthen your reasoning
- Move on to the Unit Checkpoint to test your understanding
- Read scientific articles and identify their claims, evidence, and reasoning