Forces
9th Grade Science · 12 lessons
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Lessons in this unit
- 1Newton's First Law and InertiaAn object stays at rest, or keeps moving in a straight line at constant velocity, unless a resultant force acts on it. Inertia is the name for that reluctance to change motion, and it scales with mass.Experiment
- 2Newton's Second Law: F = maThe resultant force on an object equals its mass times its acceleration. Doubling the force doubles the acceleration; doubling the mass halves it. Force is measured in newtons, where one newton accelerates one kilogram at one metre per second squared.Experiment
- 3Newton's Third Law: Forces Come in PairsIf A pushes B, then B pushes A with an equal force in the opposite direction. The pair always acts on two different objects, which is why they never cancel out — a rocket pushes gas down and the gas pushes the rocket up.Experiment
- 4Weight Is Not MassMass is the amount of matter, in kilograms, and does not change with location. Weight is the gravitational force on that mass, in newtons, and equals mg. A 60 kg astronaut still has 60 kg of mass on the Moon but weighs about a sixth as much.Experiment
- 5Free-Body DiagramsA free-body diagram shows one object as a dot with every force on it drawn as an arrow from that dot. Arrow length shows size and direction shows line of action. Getting the diagram right usually solves the problem before any arithmetic starts.Lesson
- 6Friction: Static, Kinetic, and UsefulFriction opposes relative sliding between surfaces. Static friction holds things in place up to a maximum; once sliding starts, kinetic friction is usually smaller. Friction is not simply a nuisance — without it you could not walk, grip or brake.Experiment
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