Newton's second law Revision Notes for AQA GCSE Physics

Newton's second law

What Newton's second law tells us

Newton's second law explains what happens when forces act on objects. When there is a resultant force acting on an object, it will change the object's velocity. This means the object will speed up, slow down, or change direction.

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The key idea is: the bigger the force, the bigger the acceleration.

Understanding resultant force

When multiple forces act on the same object, we can combine them to find the resultant force. This is the overall effect of all the forces working together.

Forces in the same direction are added together
Forces in opposite directions are subtracted from each other
If forces are balanced (cancel each other out), the resultant force is zero

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Example: Finding Resultant Force

If a 400N force pushes right and a 200N force pushes left:

Resultant force = 400N - 200N = 200N to the right

The force, mass and acceleration relationship

Newton's second law can be written as an equation:

$F = ma$

Where:

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

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This equation is not given in exams, so you must memorise it!

What this relationship tells us

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Key Relationships:

Larger force = larger acceleration (if mass stays the same)
Larger mass = smaller acceleration (if force stays the same)
Acceleration is in the same direction as the resultant force

Key points about forces and acceleration

Understanding how forces and acceleration work together is essential for solving physics problems.

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Important Principles:

When the resultant force is zero, acceleration is also zero
A negative force means the object is slowing down or moving backwards
The acceleration always happens in the same direction as the resultant force

Worked example: calculating acceleration

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Worked Example: Finding 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 $300N = 400kg × a$ $a = 300N ÷ 400kg = 0.75 m/s²$

Answer: The boat accelerates at 0.75 m/s² forwards

Worked example: calculating mass

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Worked Example: Finding Mass

A basketball player catches a ball. The force on the ball is -1.44N and its acceleration is -2.4 m/s².

Using $F = ma$ : $-1.44N = m × (-2.4 m/s²)$ $m = -1.44N ÷ (-2.4 m/s²) = 0.6kg$

Answer: The mass of the ball is 0.6kg

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Key Points to Remember:

Newton's second law: A resultant force causes acceleration proportional to the force and inversely proportional to the mass
$F = ma$ is the key equation - memorise it!
Bigger force = bigger acceleration (for the same mass)
Bigger mass = smaller acceleration (for the same force)
Force and acceleration are in the same direction
Zero resultant force = zero acceleration

Newton's second law (AQA GCSE Physics): Revision Notes