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.
Resultant force makes mass accelerate by F = ma, and weight is just mass times g.
More to read
5.3 Newton's Second Law
The second law turns forces into motion with one equation: net force equals mass times acceleration. It tells you exactly how much push each kilogram needs.
Watch how mass resists. The same force on double the mass gives half the acceleration, which is why the law divides, not multiplies, by mass.
5.4 Mass and Weight
Mass and weight sound alike but live in different units. Mass counts matter in kilograms; weight is gravity pulling on it in newtons.
Weight changes with g while mass does not. A bag that weighs 50 N here would weigh less on the Moon but still hold 5 kg of shopping.
Key Terms
Force, mass, weight, newton and net force each get a sharp definition here. Mixing them up is the fastest way to lose marks.
Use this page as a vocab drill. Say each meaning from memory before you touch any calculation.
Conceptual Questions
These questions test whether the law makes sense to you, not whether you can crunch numbers. They ask what happens when forces or masses change.
Predict first, then reason with F = ma. The habit of predicting will catch many algebra slips later.
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Where it sits
Learn first
Nothing; this is a starting point.
Optional connections
14 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.