Forces & Newton's laws (intro)
The idea
Every push and pull you have ever felt is a force, and Newton's three laws describe how forces and motion are connected. The first law says an object keeps doing what it is doing — staying still or coasting at constant velocity — unless a net force acts on it. The second law makes it quantitative: net force = mass × acceleration, so the same push accelerates a skateboard far more than a loaded moving truck. The third law says forces come in pairs: when you push a wall, the wall pushes back on you just as hard.
The oldest misconception in physics is that keeping something moving requires a constant push. It feels true because friction is always secretly pushing back; stop pedaling and friction, not a lack of effort, slows the bike. Remove friction — think of an ice rink or deep space — and things coast forever. So train yourself to ask two questions about any situation: what are ALL the forces acting, and what is the mass? Force changes motion; it does not maintain it.
One more careful point about the third law: the action and reaction forces act on DIFFERENT objects, so they never cancel each other. The Earth pulls you down and you pull the Earth up; you accelerate noticeably and the Earth does not, because the same size of force meets wildly different masses.
Worked example
You pull a 25 kg wagon along a sidewalk with a force of 65 N while friction drags backward with 15 N. What is the wagon's acceleration?
- First find the net force, because only the leftover force changes motion: 65 N forward − 15 N backward = 50 N forward.
- Apply Newton's second law rearranged for acceleration: acceleration = net force ÷ mass = 50 N ÷ 25 kg = 2 m/s².
- Interpret the number: as long as you keep pulling this hard, the wagon gains 2 m/s of speed every second.
- Sanity-check with the mass: if a friend hopped in and doubled the mass to 50 kg, the same 50 N net force would only produce 1 m/s² — more mass means more inertia, so the same force changes the motion less.
Answer. The wagon accelerates at 2 m/s² in the direction of your pull.
Check your understanding
- Why does a rolling ball on grass stop on its own, and what would happen to it on a perfectly frictionless surface?
- If action and reaction forces are always equal, how does anything ever start moving — what resolves the apparent standoff?
- How would the same 100 N push affect a shopping cart versus a parked car, and which law predicts the difference?
- What changes about a problem when you are told the net force is zero, even though several forces are acting?
Build the foundations first
Forces & Newton's laws (intro) builds on these concepts. If any feel shaky, start there.