Description and Assumptions
This model is applicable to a point particle (or to a system of objects treated as a point particle located at the system's center of mass) when the external forces are known or needed. It is a subclass of the model Momentum and External Force defined by the constraint dm/dt = 0.
Problem Cues
This model is typically applied to find the acceleration in cases where the forces will remain constant, such as an object moving along a flat surface like a ramp or a wall. It is also useful in combination with other models, such as when finding the normal force exerted on a passenger in a roller coaster at the top of a loop-the-loop (in which case, it would be combined with [Mechanical Energy and Non-Conservative Work]).
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Prerequisite Knowledge
Prior Models
Vocabulary
Compatible Systems
A single point particle, or a system of constant mass that is treated as a point particle located at the system's center of mass.
Relevant Interactions
External forces must be understood sufficiently to draw a free body diagram for the system. Internal forces will always cancel from the equations of Newton's 2nd Law for the system and can be neglected.
Model
Law of Change
\begin
[ \sum \vec
^
= m\vec
] \end
As with all vector equations, this Law of Interaction should really be understood as three simultaneous equations:
\begin
[ \sum F^
_
= ma_
]
[ \sum F^
_
= ma_
]
[\sum F^
_
= ma_
]\end
Diagrammatical Representations
Relevant Examples