Motion and Forces

Motion describes changing position; net force explains changing velocity. Start by choosing a reference frame, identifying the body and drawing the forces acting on it.

✓ Sources checked · 10 September 2026 · View verification

Motion describes changing position; net force explains changing velocity. Start by choosing a reference frame, identifying the body and drawing the forces acting on it.

By Matan · Updated

A short route through mechanics

Position tells you where an object is. Velocity tells you how that position changes, including direction. Acceleration tells you how velocity changes. A constant speed around a corner still requires acceleration. In an inertial frame, a constant-mass body follows ΣF = ma. [1], [2]

Worked cart: moving with balanced forces

A 10 kg cart moves along a straight level track. A 12 N pull balances 12 N resistance, so net force and acceleration are zero while it keeps its existing velocity. Increase the pull to 17 N and net force becomes 5 N: acceleration is 0.50 m/s². Over 4 s of those constant forces, velocity increases by 2.0 m/s.

Walking and turning

When your foot pushes backward against the ground, forward friction can accelerate you. Friction is therefore not always a force that slows the whole body. During a turn, the inward net force changes velocity direction. A free-body diagram helps distinguish interactions from labels such as “centripetal.” [3]

Choose the next question

  • Describing a path: study kinematics and projectile motion.
  • Explaining acceleration: study Newton’s laws and friction.
  • Following energy transfers: study work and kinetic energy.
  • Comparing before and after an impact: study impulse and momentum.
  • Turning and balancing: study torque and moment of inertia.

Every model has a boundary

Neglecting drag, deformation or rotation is a choice, not part of every physical law. State the approximations and include the omitted effects when they change the answer. The cart figures are illustrative.

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How this article was checked

Sources checked; calculations checked where shown. The definitions, model assumptions and main explanatory claims were cross-checked against the sources below. Worked-example arithmetic and units were checked separately. These examples are illustrative calculations, not experimental measurements.

Source cross-check and editing assisted by AI. No independent human scientific review is recorded for this revision. The check applies to the version dated 10 September 2026; it is not a guarantee that every possible interpretation is correct.

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Sources and further reading

  1. OpenStax: Position and velocity — Position and velocity.
  2. OpenStax: Net force and acceleration — Net force and acceleration.
  3. OpenStax: Friction — Friction.

Cite this article

Matan. “Motion and Forces.” Your Physicist. Originally published 2026-06-11; updated 2026-09-10. https://your-physicist.com/motion-and-forces/

Revision history

: Condensed repeated introductions into a learning route; added a checked balanced-versus-unbalanced force example. Added linked sources and a transparent verification record.

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