Newton's second law
Resultant force = mass x acceleration (F = ma). A bigger force gives a bigger acceleration. A bigger mass gives a smaller one.
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Analogy
Pushing an empty trolley is easy. Pushing a full one with the same force barely moves it.
Example
mass = 1,000 kgforce = 3,000 Na = F / m = 3,000 / 1,000a = 3 m/s squared
In practice
Car makers use it to work out how much engine force a car needs.
Practice
Flashcards
- State Newton's second law as a formula.Resultant force = mass x acceleration (F = ma).
Interview questions
- A car of mass 800 kg has an engine force of 2,400 N and 400 N of drag. Find its acceleration.
- Resultant force = 2,400 - 400 = 2,000 N
- Use F = ma, so a = F / m
- a = 2,000 / 800 = 2.5 m/s squared
Questions people ask
How do I use F = ma?
Write down the values you know, with SI units. Rearrange the formula for the one you want: a = F / m, or m = F / a. A 3,000 N resultant force on a 1,000 kg car gives an acceleration of 3 m/s squared. Always use the resultant force, not just one of the forces.
Why does a heavier object accelerate more slowly?
Mass measures how hard an object is to speed up. With the same force, a larger mass gives a smaller acceleration, because a = F / m. A 2 kg object pushed by 10 N accelerates at 5 m/s squared. A 10 kg object with the same push only manages 1 m/s squared.