Section 2.3 in Matter and Interactions (4th edition)
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===== Applying the Momentum Principle =====
===== Applying the Momentum Principle =====
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**Your primary job in mechanics is to be able to predict or explain the motion of systems.** Previously, you read about the [[:183_notes:displacement_and_velocity#what_s_so_special_about_constant_velocity_motion|position update formula]], which allows you to predict the future location of a system given information about its current location and its velocity (or [[183_notes:momentum|momentum]]).
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Your primary job in mechanics is to be able to predict or explain the motion of systems. Previously, you read about the [[:183_notes:displacement_and_velocity#what_s_so_special_about_constant_velocity_motion|position update formula]], which allows you to predict the future location of a system given information about its current location and its velocity (or [[183_notes:momentum|momentum]]).
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But, a system doesn't need to move with [[183_notes:displacement_and_velocity|constant velocity]] (or momentum); [[183_notes:momentum_principle|it can change its momentum (or velocity) as a result of interacting with it surroundings]]. In these notes, you will learn how to predict the future motion of an system that interacts with its surroundings.
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But, a system doesn't need to move with [[183_notes:displacement_and_velocity|constant velocity]] (or momentum); [[183_notes:momentum_principle|it can change its momentum (or velocity) as a result of interacting with it surroundings]]. In these notes, you will read how to predict the future motion of an system that interacts with its surroundings.
==== Predicting the Future Momentum ====
==== Predicting the Future Momentum ====
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→pf=→pi+→FnetΔt
→pf=→pi+→FnetΔt
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It is critical that the time step over which we are doing the prediction be small enough such that the net force can be considered a constant vector.
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It is critical that the time step over which we are doing the prediction be small enough such that the [[183_notes:constantf|net force can be considered a constant vector]].
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In later notes, you will learn about the [[183_notes:constantf|special case of constant force motion]] -- in that case, the length of the time interval will not matter. But for all other cases you will work with (e.g., [[183_notes:springmotion|spring-like interactions]]), the length of the time interval absolutely matters.
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In later notes, you will learn about the [[183_notes:constantf|special case of constant force motion]] -- in that case, the length of the time interval will not matter. But for all other cases you will work with (e.g., [[183_notes:gravitation|gravitational interactions]], [[183_notes:springmotion|spring-like interactions]]), the length of the time interval absolutely matters.
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=== Separation of Components ===
=== Separation of Components ===
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This might seem trivial, but there is a critical implication. If the force in any direction is zero, then the momentum, and thus the velocity, does not change in that direction.
This might seem trivial, but there is a critical implication. If the force in any direction is zero, then the momentum, and thus the velocity, does not change in that direction.
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===== Examples =====
===== Examples =====
[[:183_notes:examples:finalP|Predicting the final momentum & velocity using the Momentum Principle]]
[[:183_notes:examples:finalP|Predicting the final momentum & velocity using the Momentum Principle]]