What is Newton's second law and how do you use F = ma?
Short answer
Newton's second law says the resultant force on an object equals its mass multiplied by its acceleration: F = ma, with F in newtons (N), m in kilograms (kg) and a in metres per second squared (m/s²). A bigger force gives a bigger acceleration, and a heavier object accelerates less for the same force.
Step by step
- Find the resultant force: add forces acting in the same direction and subtract opposing forces such as friction.
- Convert the mass to kilograms if it is given in grams.
- Write F = ma and rearrange if needed: a = F ÷ m or m = F ÷ a.
- Substitute the values and give the unit.
Worked example
A 1200 kg car has a driving force of 3000 N and friction of 600 N. Find its acceleration.
- Resultant force: 3000 − 600 = 2400 N
- a = F ÷ m = 2400 ÷ 1200
- a = 2 m/s²
Common mistakes
- Using one force instead of the resultant force.
- Leaving the mass in grams.
- Confusing mass (kg) with weight (N). Weight is itself a force: W = mg.
A one-to-one lesson with a tutor is the quickest way to fill the gaps before a test or exam. Your first lesson is free.
Find a Physics tutor