5.7.2 Newton's Second Law

Newton's Second Law of Motion

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Resultant force means that an object is accelerating or decelerating. Newton's Second Law helps us understand how we can measure the size of this acceleration or deceleration.

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The acceleration of an object is directly proportional to the resultant force acting on the object and inversely proportional to the mass of an object.

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Formula

\text{Force (F)} = \text{mass (m)} \times \text{acceleration (a)}

The size of the acceleration (positive or negative) depends on two factors:

Resultant Force – The larger the resultant force, the larger the acceleration. Remember, if the resultant force is zero, the object is at constant speed and does not accelerate.
Mass – the heavier an object is, the more force needed to accelerate it, and hence the lower the acceleration for a given resultant force.

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For example, both vehicles below have a resultant force of 2N. The car is much lighter than the truck, so will experience a larger acceleration. Newton's Second Law

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Key Points

If two objects of the same mass experience different resultant forces, the object experiencing the higher resultant force will have a greater acceleration.
If two objects of different masses experience the same resultant force, the object with the higher mass will have a lower acceleration.

Relationships of Proportionality

In Newton's Second Law, there are relationships of proportionality.

We know that the acceleration of an object is proportional to the force acting on it (as the force increases, the acceleration increases).
We also know that the acceleration is inversely proportional to the mass of the object (as the mass increases, the acceleration decreases).

Inertia

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We can use Newton's Second Law to help us find a value for inertial mass

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This is the measure of how difficult it is to change the velocity of an object.

\text{inertial mass} = \frac{\text{force}}{\text{acceleration}}

\text{inertial mass} = \frac{f}{a}

Newton's Second Law Simplified Revision Notes for GCSE Edexcel Physics

5.7.2 Newton's Second Law

Newton's Second Law of Motion

Relationships of Proportionality

Inertia

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