Newton's Second Law: Force, Mass and Acceleration
A resultant force gives an acceleration in its own direction, with F = ma. Weight is mass times g, which is why objects of different mass fall together.
What a learner can do afterwards
- Find the acceleration of a 2 kg object under a 6 N resultant force
- Keep mass and weight apart, with the right units for each
- Find the resultant force needed to slow a moving object at a stated rate
1 · Read
Resultant force equals mass times acceleration: F = ma. A 2 kg object under a 6 N resultant accelerates at 6 over 2 = 3 m/s squared. Mass resists: the same force on double the mass gives half the acceleration.
A 5 kg trolley accelerating at 2 m/s squared needs F = 5 times 2 = 10 N driving it. The law works for slowing too: a 1000 kg car braking at 2 m/s squared feels 1000 times 2 = 2000 N against its motion.
Mass counts matter in kilograms and stays the same everywhere, while weight is gravity pulling on it in newtons. On Earth with g = 10 N/kg, a 5 kg bag of food weighs 5 times 10 = 50 N. A 2 kg bag weighs 20 N, never 2 N.
Weight is mass times g, and F = ma divides it straight back out: a = mg over m = g. That is why a 2 kg and a 5 kg object fall with the same acceleration. Bigger weight, same extra mass to move, same g.
Resultant force makes mass accelerate by F = ma, and weight is just mass times g.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.