Forces and net force
A force is a push or a pull, measured in newtons (N). Several forces usually act on an object at once. What matters for its motion is the net force — the overall result when all forces are added as vectors. Forces in opposite directions partly or completely cancel. When the net force is zero, the forces arebalanced.
The first law: inertia
An object at rest stays at rest, and an object in motion keeps moving at the same speed in the same direction, unless a net force acts on it. This tendency to resist changes in motion is called inertia, and more massive objects have more of it. On Earth, moving objects usually slow down because friction and air resistance act on them — not because motion naturally fades away.
The second law: F = ma
The acceleration of an object is proportional to the net force on it and inversely proportional to its mass.Double the net force and the acceleration doubles; double the mass and it halves. Acceleration is in the same direction as the net force, so a net force opposite to the motion slows the object down.
The third law: action and reaction
When one object exerts a force on another, the second object exerts an equal and opposite force on the first.When you push the cart, the cart pushes back on you just as hard. These two forces act on differentobjects, so they do not cancel each other out. The cart accelerates because of the forces acting on the cart.
Friction
Friction acts along surfaces in contact and opposes sliding. Static friction stops an object from starting to move and can grow up to a maximum value, μs × N. Once the object slides, the smallerkinetic friction, μk × N, acts. That is why it is harder to start pushing a heavy box than to keep it moving. The coefficients μ depend on the two surfaces: ice is very slippery, carpet is not.
Formulas
F_net = m · a a = F_net / mWeight W = m · g (g ≈ 9.8 m/s² on Earth) Normal force on level ground N = WStatic friction ≤ μs · N Kinetic friction = μk · NKey terms
- Newton (N)
- The unit of force: 1 N accelerates 1 kg at 1 m/s².
- Inertia
- The resistance of an object to changes in its motion; it depends on mass.
- Free-body diagram
- A sketch showing all the forces acting on one object as arrows.
- Normal force
- The push of a surface perpendicular to it, supporting an object.
- Equilibrium
- The state of zero net force: at rest or moving at constant velocity.
Real-world applications
- Seat belts and airbags exist because of inertia: in a crash, your body keeps moving forward when the car stops.
- Rockets push exhaust gas backward; by the third law, the gas pushes the rocket forward.
- Vehicle design: lighter cars accelerate faster with the same engine force (second law).
- Road safety: braking distances are longer on ice because the maximum friction is much smaller.
Common misconceptions
- “A moving object needs a force to keep it moving.” A force is needed only to overcome friction or to change velocity.
- “Action and reaction cancel, so nothing can move.” They act on different objects.
- “Heavier objects always need a force to stay still.” At rest with balanced forces, no net force is needed at all.
Key takeaways
- First law: without a net force, velocity stays constant.
- Second law: a = F_net / m.
- Third law: forces come in equal and opposite pairs acting on different objects.
- Friction opposes sliding; static friction can hold objects still up to a limit.
Model note: air resistance is ignored, and friction coefficients are typical values for each surface.