Newton's Laws and Free-Body Diagrams: Mechanism

≈ 30 min

Mechanism

Follow the mechanism step by step and distinguish what causes the change from what is merely observed.

This extension applies that lens specifically to Newton's Laws and Free-Body Diagrams.

Newton's three laws turn a description of motion into an explanation.

First law (inertia). A body remains at rest or in uniform straight-line motion unless a net external force acts on it. This is a statement about the existence of inertial frames as much as about objects.
Second law. $\sum \vec{F} = m\vec{a}$ — the net force and the acceleration are parallel vectors, and mass is the constant of proportionality.
Third law. If A exerts a force on B, then B exerts an equal and opposite force on A. The two forces of the pair act on different bodies.

The working method is the free-body diagram: isolate one body, draw every force that other objects exert on it (weight, normal, tension, friction, applied), pick axes, and write $\sum F_x = ma_x$ and $\sum F_y = ma_y$ .

Worked example. A $20\ \text{kg}$ crate of oranges is dragged along a level floor by a horizontal force of $80\ \text{N}$ . Kinetic friction opposes the motion with $49\ \text{N}$ .

Vertical: the crate does not accelerate upward, so $N = mg = (20)(9.8) = 196\ \text{N}$ — which is consistent with $\mu_k = 49/196 = 0.25$ .

Horizontal:

\sum F_x = 80 - 49 = 31\ \text{N}

a = \frac{\sum F_x}{m} = \frac{31}{20} = 1.55\ \text{m/s}^2

The crate accelerates at $1.55\ \text{m/s}^2$ in the direction of the pull.

Physics — Year 1 — Mechanism: Which property of a body measures its resistance to a change in its state of motion? Give the one-word name.

Physics — Year 1 — Mechanism: A textbook rests on a desk. According to Newton's third law, what is the reaction to the Earth's gravitational pull on the book?

The normal force the desk exerts upward on the book

The gravitational pull the book exerts on the Earth

The weight of the book acting downward

The force the book exerts downward on the desk

Physics — Year 1 — Mechanism: A $20\ \text{kg}$ crate is dragged across a level floor by an $80\ \text{N}$ horizontal force while kinetic friction of $49\ \text{N}$ opposes it. What is the crate's acceleration, in $\text{m/s}^2$ ?

Name the original topic being extended by this mechanism lesson.

Which statement is the best evidence-led starting point for Newton's Laws and Free-Body Diagrams: Mechanism?

Newton's three laws turn a description of motion into an explanation. 1. First law (inertia). A body remains at rest or in uniform straight-line motion unless a net external force acts on it. This is a statement about the existence of inertial frames as much as about objects. 2. Second law. $\sum \vec{F} = m\vec{a}$ — the net force and the accelerati

Kinematics in One Dimension

Work, Energy and Power

A conclusion needs no observations, calculations or stated assumptions.