Newton's First Law — Revision Notes

⚡ 30-Second Revision

Newton's First Law (Law of Inertia): — An object at rest stays at rest, and an object in motion stays in motion with constant velocity ($vec{v} = \text{constant}$) unless acted upon by an unbalanced external force ($Sigma vec{F}

eq 0$).

Inertia: — Property of matter resisting changes in its state of motion. Measured by mass ($m$).
Net External Force ($Sigma vec{F}$): — Vector sum of all external forces. If $Sigma vec{F} = 0$, then $vec{a} = 0$ and $vec{v} = \text{constant}$.
Inertial Frame: — Reference frame where Newton's First Law holds true (non-accelerating frame).
Equilibrium: — State where $Sigma vec{F} = 0$, implying zero acceleration (rest or constant velocity).

2-Minute Revision

Newton's First Law, or the Law of Inertia, is fundamental. It states that objects are 'stubborn' about changing their motion. If an object is still, it wants to stay still. If it's moving at a steady speed in a straight line, it wants to keep doing that.

The only way to change this state – to make it start moving, stop moving, speed up, slow down, or change direction – is to apply an *unbalanced external force*. An 'unbalanced' force means the total push or pull on the object isn't zero.

If all forces cancel out (net force is zero), the object's velocity remains constant, which includes being at rest. Inertia is this inherent resistance to change in motion, and mass is its measure. This law also implicitly defines an 'inertial frame of reference,' a non-accelerating viewpoint from which these rules apply without needing 'fictitious forces.

' Key applications include explaining why you lurch in a braking car or why a book stays on a table.

5-Minute Revision

Let's consolidate Newton's First Law. At its core, it's about inertia – the inherent property of any object with mass to resist changes in its state of motion. This means if an object is currently at rest, it will remain at rest.

If it's moving, it will continue moving at the same speed and in the same straight-line direction. This persistence in its current state of motion is only broken if an *unbalanced external force* acts upon it.

An 'unbalanced' force means the vector sum of all forces acting on the object is not zero ($Sigma vec{F} eq 0$). If the net force *is* zero ($Sigma vec{F} = 0$), then the object's acceleration ($vec{a}$) is zero, and its velocity ($vec{v}$) remains constant.

This constant velocity can be zero (object at rest, known as static equilibrium) or a non-zero constant (object moving uniformly in a straight line, known as dynamic equilibrium).

Consider a simple example: a block on a frictionless table. If it's at rest, it stays at rest. If you give it a push, it starts moving. Once your hand leaves it, and assuming no friction, it will continue moving at the speed you imparted, in a straight line, forever – because the net external force on it is now zero.

Gravity is pulling it down, and the table is pushing it up, but these forces are balanced. If you then apply a constant force to it, it will accelerate, changing its velocity, because there's now an unbalanced force.

This law also introduces the concept of an 'inertial frame of reference,' which is a non-accelerating frame where these laws hold true. Understanding this law is crucial for all equilibrium problems in NEET, where the condition $Sigma vec{F} = 0$ is frequently applied.

Prelims Revision Notes

Newton's First Law (Law of Inertia) - NEET Revision

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Core Statement: — An object at rest remains at rest, and an object in motion continues in motion with constant velocity (constant speed and direction) unless acted upon by an unbalanced external force.
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Inertia:

* Definition: The inherent property of an object to resist any change in its state of motion. * Measure: Mass ($m$) is the quantitative measure of inertia. Greater mass means greater inertia. * Types: Inertia of rest (resistance to start moving), Inertia of motion (resistance to stop or slow down), Inertia of direction (resistance to change direction).

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**Unbalanced External Force ($Sigma vec{F}$):**

* Required to change an object's state of motion (i.e., cause acceleration). * If $Sigma vec{F} = 0$, then $vec{a} = 0$, which implies $vec{v} = \text{constant}$. * Constant velocity includes both $vec{v} = 0$ (at rest) and $vec{v} eq 0$ (uniform motion).

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Equilibrium:

* Definition: A state where the net external force acting on an object is zero ($Sigma vec{F} = 0$). * Consequence: Object is either at rest (static equilibrium) or moving with constant velocity (dynamic equilibrium). * Application: Crucial for solving problems where objects are not accelerating (e.g., block on a table, suspended masses, objects on inclined planes at rest).

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Inertial Frame of Reference:

* Definition: A reference frame in which Newton's First Law (and thus all of Newton's laws) is valid. * Characteristics: A non-accelerating frame. A frame moving with constant velocity relative to another inertial frame is also inertial. * Example: Earth's surface is approximately inertial for most NEET problems.

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Common Misconceptions to Avoid:

* Aristotelian View: Do not assume force is needed to *maintain* motion. Force is only needed to *change* motion. * Inertia is not a force: It's a property. * Objects don't naturally stop: They stop due to resistive forces like friction and air resistance.

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Real-World Examples: — Seatbelts, dusting carpets, passengers lurching in a bus, objects sliding in a turning car – all demonstrate inertia.

Vyyuha Quick Recall

Inertia Needs External Touch: An object's state of motion (rest or constant velocity) stays the same UNLESS an external force gives it a 'touch' (changes its motion).