Classical mechanicsLesson 3 of 7 in the course Newton's Laws of Motion

Newton’s Second Law: Net Force, Components, and Momentum

5 flashcards · 5 quiz questions · 5 match pairs · 5 fill-in sentences · English

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Summary

Newton’s second law explains how forces change an object’s motion. In this lesson, we connect changes in velocity to acceleration and learn why identifying the system and its external forces is essential.

A change in motion means a change in velocity. Since acceleration is the rate at which velocity changes, a changing velocity means the object has nonzero acceleration. Newton’s first law tells us that a net external force is needed to change an object’s motion.

All flashcards

What is a system in mechanics?
The object or collection of objects selected for study; its boundary determines which forces are internal or external.
What is an external force?
A force exerted on a system by something outside its boundary.
What is an internal force?
A force acting between parts within the chosen system.
What is Newton’s second law for a constant-mass object?
The net force equals mass times acceleration: F⃗net=ma⃗\vec{F}_{\text{net}}=m\vec{a}.
How is momentum defined?
Momentum is the vector product of mass and velocity: p⃗=mv⃗\vec{p}=m\vec{v}.

Quiz questions

  1. In SI units, which expression gives the equivalent of one newton?Answer: 1 kg⋅m/s21\,\mathrm{kg}\cdot\mathrm{m}/\mathrm{s}^2
  2. For a positive-mass object, how is the acceleration direction related to the net-force direction?Answer: Acceleration points in the same direction as the net force.
  3. A free-body diagram is drawn for a selected object. What should it represent?Answer: The external forces acting on the selected object.
  4. A force FF acts at angle θ\theta above the positive xx-axis. Which pair gives its components?Answer: Fx=Fcos⁡θF_x=F\cos\theta and Fy=Fsin⁡θF_y=F\sin\theta
  5. What is Newton’s second law in its general momentum form?Answer: F⃗net=dp⃗/dt\vec{F}_{\text{net}}=d\vec{p}/dt

Based on University Physics Volume 1 by William Moebs, Samuel J. Ling, Jeff Sanny (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.

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