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Newton's Laws of Motion
Introduction
Newton's laws of motion are three physical laws that form the foundation for classical mechanics. They describe the relationship between the motion of an object and the forces acting on it. These laws were first published by Sir Isaac Newton in his 1687 work Philosophiæ Naturalis Principia Mathematica.
The three laws of motion, along with Newton's law of universal gravitation, provided a unified framework for understanding the physical world and revolutionized our understanding of physics.
First Law: Law of Inertia
An object at rest will remain at rest, and an object in motion will remain in motion with the same speed and direction, unless acted upon by an unbalanced force.
Key Concept
Inertia is the resistance of an object to any change in its state of motion or rest.
Everyday Examples:
- A passenger in a car continues moving forward when the car stops suddenly
- A tablecloth can be pulled from under dishes if pulled quickly enough
- Objects in space continue to move in the same direction indefinitely
Second Law: Law of Acceleration
The acceleration of an object depends on the mass of the object and the amount of force applied.
Mathematically, this is expressed as: F = ma (Force = mass × acceleration)
F = ma
Where F is force, m is mass, and a is acceleration
This law implies that:
- If the mass is constant, doubling the force will double the acceleration
- If the force is constant, doubling the mass will halve the acceleration
- The direction of acceleration is the same as the direction of the net force
Third Law: Law of Action-Reaction
For every action, there is an equal and opposite reaction.
This means that when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.
Important Note
The action and reaction forces act on different objects, which is why they don't cancel each other out.
Applications:
- Rocket propulsion: Gases expelled backward propel the rocket forward
- Swimming: Pushing water backward propels the swimmer forward
- Walking: Pushing against the ground moves your body forward
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Summary
Newton's three laws of motion provide a framework for understanding how forces affect the motion of objects:
- First Law (Inertia): Objects resist changes to their motion
- Second Law (F = ma): Force equals mass times acceleration
- Third Law (Action-Reaction): Forces always come in pairs equal in magnitude and opposite in direction
Pro Tip: When solving physics problems, always identify which law applies to the situation before attempting to solve.
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Practice Questions: Newton's Laws
Test your understanding of Newton's Laws of Motion with these adaptive practice questions.
A hockey puck of mass 0.16 kg is at rest on an ice rink. A player applies a force of 8 N to the puck for 0.2 seconds. What is the resulting speed of the puck?
Remember to use Newton's Second Law and the impulse-momentum relationship.
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