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Newton's Laws and Free-Body Diagrams: Worked Reasoning

≈ 30 min
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Worked Reasoning

Make each reasoning step visible: identify the knowns, choose the governing idea, apply it and check the result.

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.

  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. ∑F⃗=ma⃗\sum \vec{F} = m\vec{a} — the net force and the acceleration are parallel vectors, and mass is the constant of proportionality.
  3. 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 ∑Fx=max\sum F_x = ma_x and ∑Fy=may\sum F_y = ma_y.

Worked example. A 20 kg20\ \text{kg} crate of oranges is dragged along a level floor by a horizontal force of 80 N80\ \text{N}. Kinetic friction opposes the motion with 49 N49\ \text{N}.

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

Horizontal:

∑Fx=80−49=31 N\sum F_x = 80 - 49 = 31\ \text{N}
a=∑Fxm=3120=1.55 m/s2a = \frac{\sum F_x}{m} = \frac{31}{20} = 1.55\ \text{m/s}^2

The crate accelerates at 1.55 m/s21.55\ \text{m/s}^2 in the direction of the pull.

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

Physics — Year 1 — Worked Reasoning: 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?

Physics — Year 1 — Worked Reasoning: A 20 kg20\ \text{kg} crate is dragged across a level floor by an 80 N80\ \text{N} horizontal force while kinetic friction of 49 N49\ \text{N} opposes it. What is the crate's acceleration, in m/s2\text{m/s}^2?

Name the original topic being extended by this worked reasoning lesson.

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