🥋 Newton’s First Law – White to Black Belt Mastery

“An object in motion stays in motion, unless acted on by a force.”

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White Belt Level – Key Concept

Newton’s First Law is also called the Law of Inertia.
It says that objects keep doing what they are doing—staying at rest or moving at constant velocity—unless a net force acts on them.

In simple words: No force → No change in motion.

🎮 Explore: PhET Simulation – Forces and Motion: Basics

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Core Principles

1. Inertia is the resistance of an object to changes in motion.
2. A net force is required to change velocity (speed or direction).
3. If velocity is constant (including at rest), net force = 0.
4. Everyday “stopping” (like a ball rolling to rest) happens because of hidden forces like friction.

📖 Learn more: OpenStax University Physics, Ch. 4

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Key Equations (SI Units)

• Net Force: ΣF⃗ = 0 → constant velocity
• Mass (m): measured in kilograms (kg)
• Velocity (v⃗): meters per second (m/s)

📄 Reference: OpenStax – Physics Formula Appendix

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Common Mistakes and Pitfalls

• Thinking motion always needs a force (wrong—only changes in motion need force).
• Confusing velocity = 0 with force = 0 (an object can be at rest and still have forces acting, balanced).
• Forgetting friction and air resistance are forces that stop objects.

🔎 See also: HyperPhysics – Newton’s First Law

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Sensei’s Shortcuts

📦 Box Trick: Imagine sliding a box on ice. With almost no friction, the box keeps moving. This is Newton’s First Law in action.

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Worked Example – Step by Step (White Belt)

Problem: A hockey puck slides on frictionless ice with velocity 5.00 m/s. No forces act on it. What will its velocity be after 10.0 s?

Step 1. Formula:
No net force → velocity stays constant.

Step 2. Substitution:
v = 5.00 m/s (constant)

Step 3. Final Answer:
v = 5.00 m/s after 10.0 s

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Practice Drill (White Belt)

1. A ball is floating in space with velocity 2.50 m/s. What is its velocity after 1 hour?
   Answer: 2.50 m/s
2. A car is parked on a frictionless surface with no forces. What happens to it?
   Answer: It stays at rest.

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Yellow Belt Extension – Deeper Skills

Problem: A 1,200 kg car is moving at constant velocity of 15.0 m/s. What is the net force acting on the car?

Solution Outline:

• Newton’s First Law: constant velocity → ΣF⃗ = 0
• Substitution: velocity constant, so no acceleration
• Final Answer: ΣF⃗ = 0 N

Extra Practice:
A 0.145 kg baseball moves at constant speed through space. Net force?
Answer: 0 N

🎮 Test this yourself: Use the Friction tab in PhET Forces and Motion: Basics

📖 Extra reading: Khan Academy – Newton’s First Law

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Black Belt Mastery – Exam Strategy and Challenge

Challenge Problem: A 1,000 kg spacecraft drifts in space at velocity 120 m/s. Its engines fire briefly, adding a force of 600 N forward for 10.0 s.

a) What is the change in velocity during thrust?
b) What happens to velocity after the engines stop?

Strategy Notes:

• Use F = m × a
• Acceleration only exists while force is applied
• After thrust, velocity becomes constant again

Solution:
a)
F = 600 N
m = 1,000 kg
a = F / m = 600 / 1,000 = 0.600 m/s²
Δv = a × t = 0.600 × 10.0 = 6.00 m/s

b)
Final velocity after thrust = 120 + 6.00 = 126 m/s
After engines stop, no force → velocity remains 126 m/s

Final Answer:
a) Δv = 6.00 m/s
b) v = 126 m/s (constant)

🚀 Related: NASA – Newton’s Laws in Spaceflight

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Sensei’s Final Words

Inertia is the quiet power of physics. Mastering Newton’s First Law means you see motion not as magic, but as balance and resistance. Train your mind to always ask: Is there a net force?