Newton's Laws of Motion
State and apply the three laws of motion with calculations.
Newton's laws connect force, mass and motion.
- State each of Newton's laws.
- Calculate resultant force, mass or acceleration.
- Explain action-reaction pairs.
Law 1: Inertia
A body remains at rest or in uniform motion unless acted on by a resultant force. Mass is a measure of inertia.
Law 2: F = ma
Resultant force equals mass times acceleration. Doubling the force doubles acceleration; doubling the mass halves it for the same force.
Law 3: Action and Reaction
Every action has an equal and opposite reaction. The two forces act on different bodies, so they never cancel each other.
Mass 5 kg, acceleration 4 m/s²: F = 5 × 4 = 20 N.
Resultant force 30 N on 6 kg: a = 30 ÷ 6 = 5 m/s².
A 0.5 kg ball hit with 10 N: a = 10 ÷ 0.5 = 20 m/s².
Practice
- A 12 kg box accelerates at 2 m/s². Find the resultant force.
- A 500 N force acts on a 50 kg mass. Find the acceleration.
- Explain, using the third law, how a person walks forward.
Recap
Newton's laws describe inertia, F = ma and action-reaction pairs. They are the foundation of mechanics.
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Forces and Motion
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