Newton’s first law is about what happens when the forces on an object are balanced: it simply carries on doing what it was already doing. No resultant force means no change in motion — and that includes an object that’s moving, not just one that’s standing still.
📘 What you need to know
The law: a body stays at rest, or keeps moving at constant velocity, unless a resultant (net) force acts on it.
A resultant force is what’s needed to change motion — to speed up, slow down, or change direction.
If the resultant force is zero, the forces are balanced and the object is in translational equilibrium.
Equilibrium does not mean “stopped” — it can also mean moving at a constant velocity.
For balanced forces: the forces up = down and left = right (resolve into components first).
What the law says
Newton’s three laws connect the forces on an object to how it moves. The first law deals with the case where those forces add up to nothing:
Newton’s first law
An object stays at rest, or moves at constant velocity, unless acted on by a resultant force.
In other words:
An object at rest will stay at rest until a resultant force acts on it.
An object moving at constant velocity will keep moving at that same velocity until a resultant force acts on it.
What a resultant force does
A non-zero resultant force is the only thing that can change an object’s motion. It can do one of three things, depending on its direction relative to the motion:
A resultant force in the direction of motion speeds an object up; opposite to it, slows it down; at an angle, changes its direction.
Translational equilibrium
When the resultant force on an object is zero, we say it is in translational equilibrium. The key (and slightly surprising) point is that this covers two situations: an object at rest, and an object moving at a steady velocity. In both, the forces simply cancel out.
In both cases the resultant force is zero — equal up/down and equal left/right forces. Constant velocity is just as much “equilibrium” as standing still.
Because force is a vector, it’s easiest to split everything into horizontal and vertical directions. If the object is in equilibrium then, in each direction, the forces one way exactly match the forces the other way.
Worked examples
WE 1
A car at constant velocity
A car moves along a flat road at a constant velocity. The only horizontal forces are the driving force of 6 kN forwards and friction backwards. Find the size of the frictional force Ff.
Constant velocity → resultant force = 0so the horizontal forces must be balanceddriving force = frictionFf = 6 kN
WE 2
A crate pulled across the floor
A 50 N crate is pulled across a level floor at constant velocity by a horizontal force of 18 N. State the normal force from the floor and the frictional force on the crate.
Vertical: at equilibrium, up = downFN = weight = 50 Nnormal force = 50 N (up)Horizontal: constant velocity, so left = rightFf = pull = 18 Nfriction = 18 N (backwards)
Quick reference: resultant force = 0 ⇄ balanced forces ⇄ translational equilibrium ⇄ at rest or constant velocity. A resultant force is needed to speed up, slow down, or turn.
💡 Top tips
Constant velocity is the giveaway. If a question says “constant velocity”, read it as “resultant force = 0” and balance the forces.
Resolve into directions. Treat up/down separately from left/right — each pair must balance on its own.
Equilibrium ≠ stationary. An object cruising at steady speed in a straight line is in equilibrium just like one at rest.
A resultant force changes velocity, not position — it causes acceleration (that’s the second law).
⚠ Common mistakes
Thinking a moving object needs a forward force to keep going. At constant velocity the forces are balanced; the motion continues on its own.
Assuming “no resultant force” means “not moving”. It can equally mean moving at a steady velocity.
Forgetting the invisible forces. Drag and friction are easy to leave out, but they’re often what balances the driving force.
Adding forces from different directions together without resolving them into components first.
Why can something move with balanced forces? Because a resultant force causes acceleration, not motion itself. No acceleration means the forces must be balanced — even at high speed. Up next: Newton’s Second Law, which tells you exactly how a resultant force produces acceleration.
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