Grade: Grade 7 Subject: Science Unit: Forces Lesson: 6 of 6 ACT: Science

Unit Checkpoint: Forces

📖 Review

This checkpoint covers all the key concepts from the Forces unit. Before beginning, review these essential ideas:

Newton's Three Laws of Motion

  • First Law (Inertia): An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by a net force.
  • Second Law (F = ma): The acceleration of an object is directly proportional to the net force and inversely proportional to its mass.
  • Third Law (Action-Reaction): For every action, there is an equal and opposite reaction.

Gravity and Weight

  • Gravity is the attractive force between objects with mass
  • Weight = mass x gravitational acceleration (W = mg)
  • On Earth, g = 9.8 m/s2
  • Mass is constant; weight changes with gravitational field strength

Scientific Investigation

  • Independent variable: what you change
  • Dependent variable: what you measure
  • Controlled variables: what you keep constant
  • Multiple trials improve reliability

Data and Graphs

  • Tables should include units and averages
  • Line graphs show continuous relationships
  • Bar graphs compare categories
  • Best-fit lines show trends

CER Writing

  • Claim: Answer to the question
  • Evidence: Data with numbers and units
  • Reasoning: Scientific explanation using laws/principles

💡 Key Formulas and Concepts

Essential Equations

  • Force: F = ma (Force = mass x acceleration)
  • Weight: W = mg (Weight = mass x gravitational acceleration)
  • Acceleration: a = F/m (Acceleration = Force / mass)
  • Average: Sum of values / Number of values

Units to Remember

  • Force: Newtons (N)
  • Mass: kilograms (kg)
  • Acceleration: meters per second squared (m/s2)
  • Weight: Newtons (N)
  • Distance: meters (m)
  • Time: seconds (s)

✏️ Checkpoint Questions

Answer these questions to check your understanding of the Forces unit. Try to answer without looking back at previous lessons first.

Part A: Newton's Laws (Questions 1-4)

1. A hockey puck slides across the ice and gradually slows down. Which of Newton's Laws explains why it slows down? What force is responsible?

2. A 5 kg object is pushed with a force of 20 N. Calculate the acceleration.

3. When you jump off a small boat onto a dock, the boat moves backward. Which of Newton's Laws explains this? Describe the action and reaction forces.

4. Two carts have masses of 2 kg and 6 kg. If the same force is applied to both, which cart will have greater acceleration and by how much?

Part B: Gravity and Forces (Questions 5-7)

5. Calculate the weight of a 70 kg person on Earth (g = 9.8 m/s2).

6. An astronaut has a mass of 80 kg. On the Moon, where g = 1.6 m/s2, what would be their weight? How does this compare to their weight on Earth?

7. A 2 kg ball and a 10 kg ball are dropped from the same height at the same time. Ignoring air resistance, which hits the ground first? Explain using Newton's Second Law.

Part C: Scientific Investigation (Questions 8-10)

8. A student wants to investigate how the height of a ramp affects the speed of a ball at the bottom. Identify:

  • a) Independent variable
  • b) Dependent variable
  • c) Two controlled variables

9. Why is it important to repeat trials in an experiment and calculate averages?

10. A student collected this data. Calculate the averages and describe the pattern:

Force (N)Trial 1 (m/s2)Trial 2 (m/s2)Trial 3 (m/s2)Average
52.42.62.5?
104.95.15.0?
157.47.67.5?

Part D: Data and Graphs (Questions 11-12)

11. Based on the data in Question 10:

  • a) What type of graph would best display this data?
  • b) What would you label on each axis?
  • c) What pattern would the graph show?

12. A graph of mass vs. acceleration (with constant force) shows a curved line that starts high on the left and decreases toward the right. What type of relationship does this show? Give a real-world example.

Part E: CER Writing (Questions 13-14)

13. Write a complete CER response to answer this question: "How does increasing the mass of a cart affect its acceleration when pulled with a constant force?"

Use this data: 1 kg cart accelerated at 6 m/s2, 2 kg cart at 3 m/s2, 3 kg cart at 2 m/s2 (force = 6 N for all trials).

14. Identify what is wrong with this CER and rewrite it correctly:

"Claim: The experiment proved Newton was right. Evidence: Things moved when we pushed them. Reasoning: This happened because force makes things move, which everyone knows."

✅ Answer Key

Question 1:

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Newton's First Law (Inertia). The puck would continue moving at constant velocity, but friction between the puck and ice exerts a force that slows it down.

Question 2:

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a = F/m = 20 N / 5 kg = 4 m/s2

Question 3:

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Newton's Third Law. Action: You push backward on the boat. Reaction: The boat pushes forward on you (propelling you toward the dock).

Question 4:

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The 2 kg cart will have 3 times greater acceleration. Since a = F/m, with the same force, the cart with 1/3 the mass (2 kg vs 6 kg) will have 3 times the acceleration.

Question 5:

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W = mg = 70 kg x 9.8 m/s2 = 686 N

Question 6:

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Moon weight: W = 80 kg x 1.6 m/s2 = 128 N. Earth weight: W = 80 kg x 9.8 m/s2 = 784 N. The astronaut weighs about 1/6 as much on the Moon.

Question 7:

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They hit at the same time. Although the 10 kg ball experiences more gravitational force (F = mg), it also has more mass to accelerate. By Newton's Second Law, a = F/m = mg/m = g. The mass cancels, so both accelerate at g = 9.8 m/s2.

Question 8:

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a) Independent: height of the ramp. b) Dependent: speed of the ball at the bottom. c) Controlled: same ball, same starting position on ramp, same ramp surface, same measurement method.

Question 9:

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Repeating trials helps account for random errors and variations in measurement. Averaging multiple trials gives a more accurate and reliable result than a single measurement.

Question 10:

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Averages: 2.5 m/s2, 5.0 m/s2, 7.5 m/s2. Pattern: Direct relationship - when force doubles, acceleration doubles. When force triples, acceleration triples.

Question 11:

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a) Line graph. b) X-axis: Force (N), Y-axis: Acceleration (m/s2). c) A straight line passing through or near the origin, showing a direct (linear) relationship.

Question 12:

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An inverse relationship (as one increases, the other decreases). Example: Pushing a shopping cart - an empty cart accelerates quickly, but as you add more groceries (mass), it accelerates less with the same push.

Question 13:

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Claim: Increasing the mass of a cart decreases its acceleration when pulled with a constant force.

Evidence: When a constant force of 6 N was applied, the 1 kg cart accelerated at 6 m/s2, the 2 kg cart accelerated at 3 m/s2, and the 3 kg cart accelerated at 2 m/s2. Doubling the mass from 1 kg to 2 kg cut the acceleration in half.

Reasoning: This inverse relationship is explained by Newton's Second Law, F = ma, or rearranged as a = F/m. When force is constant, acceleration is inversely proportional to mass. More mass means more inertia (resistance to acceleration), so the same force produces less acceleration.

Question 14:

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Problems: Claim is vague and doesn't answer a specific question. Evidence lacks numbers and units. Reasoning doesn't reference scientific principles.

Improved CER:

Claim: Applying force to an object causes it to accelerate in the direction of the force.

Evidence: When we applied a 10 N force to a 2 kg cart, it accelerated at 5 m/s2. When we applied 20 N, it accelerated at 10 m/s2.

Reasoning: This confirms Newton's Second Law (F = ma), which states that acceleration is directly proportional to the net force applied. Doubling the force doubled the acceleration because a = F/m, and mass remained constant.

🚀 Next Steps

  • Review any questions you found challenging
  • Revisit specific lessons for topics that need more practice
  • Congratulations on completing the Forces unit!
  • Move on to the next Science unit when ready