Newton’s First Law – Lesson 2 Answer Key
Understanding Newton’s First Law of Motion, often called the law of inertia, is a cornerstone of physics education. Now, lesson 2 typically builds on the introductory concepts by presenting scenarios, problem‑solving steps, and conceptual questions that test a student’s grasp of how objects behave when forces are applied—or not applied. Below is a comprehensive answer key that not only supplies the correct responses but also explains the reasoning behind each answer, helping learners solidify their knowledge and teachers to assess understanding efficiently.
1. Conceptual Questions
| # | Question | Correct Answer | Explanation |
|---|---|---|---|
| 1 | An object at rest will stay at rest unless what? | **A net external force acts on it.In real terms, ** | According to Newton’s First Law, a body will maintain its state of rest or uniform motion unless a resultant (net) force interferes. |
| 2 | A hockey puck slides across friction‑less ice at a constant speed. Which statement is true? | The puck experiences no net force. | With negligible friction, the only forces (gravity and normal) cancel, leaving zero net horizontal force, so the puck’s velocity remains unchanged. Worth adding: |
| 3 | Which of the following best describes inertia? | **The tendency of an object to resist changes in its state of motion.That said, ** | Inertia is a property of mass; the larger the mass, the greater the resistance to acceleration. |
| 4 | A book rests on a table. Also, the forces acting on the book are: | **Gravity downward and the normal force upward; they are equal in magnitude and opposite in direction. ** | These two forces constitute a balanced force system, resulting in zero net force, keeping the book at rest. |
| 5 | If a car traveling at 20 m s⁻¹ suddenly stops, what must happen to the passengers inside? | They will continue moving forward until acted upon by a force (e.g., seat belt). | Passengers possess inertia; without a restraining force, they maintain their original velocity. |
2. Multiple‑Choice Problems
Problem 1 – Inertia of Different Masses
Question: Two blocks, A (2 kg) and B (5 kg), are placed on a frictionless surface. A horizontal force of 10 N is applied to each block separately. Which block experiences a greater acceleration?
Answer: Block A (the lighter block) Easy to understand, harder to ignore. Simple as that..
Work‑through:
- Acceleration (a = \frac{F}{m}).
- For A: (a_A = \frac{10\ \text{N}}{2\ \text{kg}} = 5\ \text{m s}^{-2}).
- For B: (a_B = \frac{10\ \text{N}}{5\ \text{kg}} = 2\ \text{m s}^{-2}).
- The smaller mass yields a larger acceleration, illustrating that inertia (resistance to change) is directly proportional to mass.
Problem 2 – Balanced Forces
Question: A 12 kg crate is pulled forward with a 30 N force and experiences a backward friction force of 30 N. What is the crate’s acceleration?
Answer: 0 m s⁻¹ (the crate remains at rest or moves at constant velocity).
Explanation: The net force (F_{\text{net}} = 30\ \text{N} - 30\ \text{N} = 0\ \text{N}). According to Newton’s First Law, with zero net force the crate’s velocity does not change Surprisingly effective..
Problem 3 – Real‑World Application
Question: A passenger in a car feels a forward jolt when the car suddenly accelerates from rest. Which law explains this sensation?
Answer: Newton’s First Law (inertia).
Why: The passenger’s body tends to remain at rest; the car’s seat exerts a forward force on the passenger, creating the feeling of a jolt.
3. Numerical Exercises
Exercise 1 – Determining Net Force
A 3 kg sled slides on a frictionless ice surface. A child pushes it with a constant force of 9 N for 4 seconds.
-
Calculate the sled’s acceleration.
[ a = \frac{F}{m} = \frac{9\ \text{N}}{3\ \text{kg}} = 3\ \text{m s}^{-2} ] -
Find the final velocity after 4 s, assuming it started from rest.
[ v = u + at = 0 + (3\ \text{m s}^{-2})(4\ \text{s}) = 12\ \text{m s}^{-1} ] -
What is the net force acting on the sled?
Answer: 9 N (the only horizontal force, as friction is absent) Simple, but easy to overlook..
Exercise 2 – Inertia on a Conveyor Belt
A 0.5 kg ball sits on a moving conveyor belt traveling at 2 m s⁻¹. The belt suddenly stops.
-
What will happen to the ball?
The ball will continue moving forward at approximately 2 m s⁻¹ (ignoring friction) until another force—such as friction with the belt or a wall—acts on it. -
Why?
The ball’s inertia resists the change in motion. With no horizontal force applied, its state of motion persists.
4. Short‑Answer Prompts
-
Define “net external force.”
A net external force is the vector sum of all forces acting on an object from outside its system. If this sum is non‑zero, the object’s velocity will change. -
Explain why astronauts float inside the International Space Station.
Inside the ISS, both the station and the astronauts are in continuous free‑fall toward Earth, experiencing the same gravitational acceleration. No net external force acts on the astronauts relative to the station, so they remain in a state of weightlessness, illustrating Newton’s First Law. -
Describe a real‑life situation where neglecting inertia could cause injury.
Failing to wear a seat belt in a car crash: when the vehicle stops abruptly, the occupants’ bodies continue moving forward due to inertia, potentially colliding with interior objects or being ejected, leading to severe injuries.
5. Diagram‑Based Questions (Answers)
Figure 1: A block on a horizontal surface with forces labeled.
- Force A: 15 N to the right (applied push).
- Force B: 15 N to the left (friction).
Answer: The block experiences zero net force, so it either remains at rest or continues moving at constant velocity No workaround needed..
Figure 2: A space probe drifting in deep space with no forces shown.
Answer: The probe will maintain its current velocity (both speed and direction) indefinitely, as no external forces act upon it Nothing fancy..
6. Common Misconceptions – Teacher’s Guide
| Misconception | Correct Concept | How to Address |
|---|---|---|
| “If an object is moving, a force must be acting on it. | Use the frictionless ice puck example; ask students what forces are present. That said, | point out that inertia is resistance to acceleration, not a push or pull. Worth adding: |
| “Inertia is a force. ” | Motion does not require a continuous force; only a change in motion does. ” | In a vacuum, all objects accelerate at the same rate; inertia affects acceleration only when a net force is present. |
| “Heavier objects fall slower because of more inertia.Also, ” | Inertia is a property of mass, not a force. | Demonstrate with a feather and a hammer in a vacuum chamber video. |
7. Extension Activities
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Lab Experiment – Inertia on a Cart:
- Set up a low‑friction cart with a mass sensor. Apply a known horizontal force using a spring scale and record acceleration.
- Vary the cart’s mass and repeat. Students plot (F) vs. (a) and verify the linear relationship (F = ma).
-
Simulation – Spacecraft Maneuver:
- Use a free online physics simulator (e.g., PhET “Forces and Motion”). Have students apply thrust pulses and observe how the spacecraft’s velocity changes only when forces are applied.
-
Real‑World Research Project:
- Investigate how engineers design crash‑worthy cars that manage inertia during collisions (crumple zones, airbags, seat belts). Students present findings linking back to Newton’s First Law.
8. Frequently Asked Questions (FAQ)
Q1: Does Newton’s First Law apply in non‑inertial (accelerating) reference frames?
A: In accelerating frames, fictitious forces (e.g., centrifugal force) must be introduced to preserve the law’s form. The law strictly holds in inertial frames where no acceleration of the reference frame occurs And it works..
Q2: How is inertia related to mass?
A: Inertia is directly proportional to mass. The greater an object’s mass, the more force is required to change its state of motion.
Q3: Can friction be considered a force that “breaks” inertia?
A: Friction is a real external force that can create a net force, thereby changing an object’s motion. It does not break inertia; it simply provides the necessary force to overcome an object’s resistance to change.
Q4: Why do astronauts feel weightless even though Earth’s gravity is still acting on them?
A: They are in continuous free‑fall, so the only force acting on them is gravity, which is also acting on the spacecraft. Since both experience the same acceleration, there is no relative force between them, creating the sensation of weightlessness.
9. Summary & Take‑Away Points
- Newton’s First Law states that an object will remain at rest or move with constant velocity unless acted upon by a net external force.
- Inertia is the inherent resistance of an object to changes in its motion; it is quantified by the object's mass.
- Balanced forces (equal magnitude, opposite direction) produce zero net force, resulting in no change in velocity.
- Real‑world examples—seat belts, ice hockey, space travel—illustrate the law’s relevance and help students connect abstract concepts to everyday experiences.
- Mastery of this law provides the foundation for analyzing more complex dynamics, such as those described in Newton’s Second and Third Laws.
10. Answer Key Checklist for Teachers
- [ ] Verify that each numerical answer includes units and proper significant figures.
- [ ] Ensure conceptual explanations reference the law of inertia explicitly.
- [ ] Cross‑check diagrams with the corresponding force analysis.
- [ ] Use the FAQ section to address lingering doubts during class discussion.
- [ ] Assign the extension activities to reinforce the lesson’s objectives and promote higher‑order thinking.
By using this detailed answer key, educators can efficiently grade student work, provide targeted feedback, and deepen learners’ conceptual understanding of Newton’s First Law of Motion. The blend of quantitative problems, conceptual reasoning, and real‑world applications ensures that students not only memorize the law but also appreciate its pervasive influence across physics and everyday life Most people skip this — try not to..
