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Physics
Newton's Laws of Motion
Newton's First, Second, and Third Laws in One Visual
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Isaac Newton published his three laws of motion in 1687, and they remain the foundation of classical mechanics. Together they explain why objects move the way they do - from a book sitting on a table to a rocket accelerating through space. Every force interaction in everyday life can be analyzed using these three laws.
The laws work as a system. The first law tells you when acceleration is zero. The second law tells you how much acceleration a net force produces. The third law reminds you that every force has a partner force acting on a different object. Mastering all three together - not in isolation - is the key to solving mechanics problems.
Key Facts
- First Law (Inertia): An object at rest stays at rest; an object in motion stays in motion - unless a net external force acts on it.
- Second Law: Net force equals mass times acceleration -
- Third Law: For every action force, there is an equal and opposite reaction force on a different object.
- Net force is the vector sum of all forces acting on an object.
- Mass measures inertia: heavier objects need more force for the same acceleration.
- Action-reaction pairs never cancel each other because they act on different objects.
Vocabulary
- Inertia
- The tendency of an object to resist changes in its state of motion.
- Net force
- The vector sum of all forces acting on a single object.
- Acceleration
- The rate of change of velocity, including changes in direction.
- Action-reaction pair
- Two forces that are equal in magnitude, opposite in direction, and act on different objects.
- Newton (N)
- The SI unit of force. .
Common Mistakes to Avoid
- Confusing action-reaction pairs with balanced forces. Balanced forces act on the same object and cancel; action-reaction pairs act on different objects and never cancel.
- Thinking a moving object needs a continuous force to keep moving. Per the first law, constant velocity requires zero net force.
- Applying to individual forces rather than the net (total) force. Always sum all forces first.
- Forgetting that forces are vectors - direction matters. A 10 N force left and a 10 N force right produce zero net force.
Practice Questions
- 1 A 5 kg box is pushed with 20 N to the right and 8 N of friction acts to the left. What is the box's acceleration?
- 2 You push a wall with 50 N. What force does the wall exert on you, and on which object does it act?
- 3 A car brakes to a stop. Identify all horizontal forces acting on the car and explain which law explains each.