Newton’s second law Revision Notes for AQA GCSE Physics Combined Science

Newton's second law

Newton's second law tells us what happens when forces act on objects. When a force pushes or pulls an object, it makes the object speed up, slow down, or change direction. This change in motion is called acceleration.

What Newton's second law means

When forces act on an object, they change its velocity (speed and direction). The bigger the force, the bigger the change. The heavier the object, the smaller the change for the same force.

infoNote

Key point: The resultant force controls both how much acceleration happens and which direction it happens in.

Resultant forces

When several forces act on the same object, we need to find the resultant force. This is the overall effect of all the forces combined.

Forces in the same direction add together
Forces in opposite directions subtract
The resultant force shows the overall pushing or pulling effect

lightbulbExample

Simple Force Calculation

If a 400N force pushes right and a 200N force pushes left, the resultant force is 200N to the right.

Calculation: 400N - 200N = 200N (to the right)

The F = ma equation

Newton's second law gives us a very important equation that connects force, mass, and acceleration.

chatImportant

Newton's Second Law Equation:

$F = ma$

Where:

F = resultant force (measured in Newtons, N)
m = mass (measured in kilogrammes, kg)
a = acceleration (measured in metres per second squared, m/s²)

This equation tells us several important relationships:

Bigger force = bigger acceleration
Heavier object = less acceleration for the same force
Zero resultant force = zero acceleration (object stays at constant speed)

Direction matters

Understanding the direction of forces and acceleration is crucial for applying Newton's second law correctly.

chatImportant

Direction Rules:

Acceleration always happens in the same direction as the resultant force
If the resultant force is zero, there's no acceleration
A negative force means the object slows down or accelerates backwards

Worked example

lightbulbExample

Worked Example: Boat Acceleration

A boat has two forces acting on it:

Driving force = 900N (forwards)
Resistive force = 600N (backwards)
Mass of boat = 400kg

Step 1: Find the resultant force Resultant force = 900N - 600N = 300N forwards

Step 2: Use $F = ma$ to find acceleration $300N = 400kg \times a$ $a = \frac{300N}{400kg} = 0.75 \text{ m/s}^2$

Answer: The boat accelerates forwards at 0.75 m/s².

Using the equation in different ways

The $F = ma$ equation is flexible and can be rearranged to solve for different variables depending on what information you have and what you need to find.

The three arrangements are:

To find force: $F = ma$
To find mass: $m = \frac{F}{a}$
To find acceleration: $a = \frac{F}{m}$

bookmarkSummary

Key Points to Remember:

Newton's second law connects force, mass and acceleration
$F = ma$ is the key equation you must know
Bigger forces create bigger accelerations
Heavier objects accelerate less with the same force
Acceleration always happens in the same direction as the resultant force

Newton’s second law (AQA GCSE Physics Combined Science): Revision Notes