medium difficulty4 min

Newton's Second Law

Newton's second law states that force is equal to the rate of change of momentum; for constant mass, it is commonly used as F = ma.

In one line: Second law: force decides acceleration after mass is considered.

Practice This Concept

Understand it

The law connects force, mass, and acceleration. For the same mass, a greater force gives greater acceleration. For the same force, a greater mass gives smaller acceleration.

Example

A 5 kg object given an acceleration of 4 m/s² needs a force of 20 N.

Rule to remember

F = ma, where F is force in N, m is mass in kg, and a is acceleration in m/s².

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

If m = 10 kg and F = 50 N, then a = F/m = 50/10 = 5 m/s².

Method you can reuse

List given values, choose F = ma, rearrange if needed, substitute correctly, calculate, and attach SI unit.

Visual tip

A block with a force arrow and acceleration arrow can show that acceleration is in the direction of net force.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What happens to acceleration if the same force acts on a larger mass?

Think first, then show answer

The acceleration becomes smaller because acceleration is inversely related to mass when the applied force remains the same.

Common wrong answer

For F = 50 N and m = 10 kg, writing a = 500 m/s².

Why it loses marks: Force must be divided by mass.

Don't mix this up

Students may use F = m/a or forget to convert units before substitution.

Exam tip

Write the formula first, substitute with units, then give the answer in newton.