Newton's First Law and Inertia
A body continues at rest or in uniform motion unless acted on by a net external force; this property is inertia, measured by mass.
What is Newton's First Law and Inertia?
A body continues at rest or in uniform motion unless acted on by a net external force; this property is inertia, measured by mass.
Key formula / rule: Inertia is measured by mass, not by weight.
Key points
- Define inertia and identify inertial frames.
- Explain everyday phenomena using the first law.
Common exam trap
Believing a moving body needs a continuous force to keep moving.
Definitions
- Term
Newton's First Law and Inertia
- Meaning
A body continues at rest or in uniform motion unless acted on by a net external force; this property is inertia, measured by mass.
- Term
Newton's First Law and Inertia — explanation
- Meaning
The first law defines the inertial frame in which the other two laws hold. Everyday experience misleads because friction is always present, which is why bodies appear to need a force to keep moving.
Learning objectives
Define inertia and identify inertial frames.
Explain everyday phenomena using the first law.
Formulae
- Key point
Inertia is measured by mass, not by weight.
- Key point
The law identifies inertial frames; it is not a special case of F = ma.
- Key point
Net force zero means constant velocity, not necessarily rest.
Prerequisites
PHY-U1-KIN-T1-S1-C3
Common mistakes
Believing a moving body needs a continuous force to keep moving.
Applying the law in an accelerating frame without a pseudo-force.
Keywords
Newton
First
Inertia
Practice preview
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