A spacecraft coasting between stars keeps the same speed and direction forever, with no engine running. A car cruising at a steady 100 km/h has its engine working flat out, yet both have zero net force. Newton's first law explains how "no net force" can mean either standing still or sailing on unchanged. This is the law of inertia.
A puck slides across perfectly smooth (frictionless) ice with no one touching it.
Before reading: what happens to the puck, does it slow down, speed up, or keep gliding at the same velocity? Write your answer and one sentence on why.
Objects keep doing what they are doing unless a net force interferes
Left alone, an object will not change its motion at all, a still object stays still and a moving object keeps the same speed and direction, and it takes a net force to alter that.
Newton's first law: an object remains at rest, or continues to move with constant velocity, unless acted upon by a non-zero net force. This tendency to resist a change in motion is called inertia, and it grows with mass, a loaded truck is far harder to start or stop than a bicycle. The frictionless puck keeps gliding forever precisely because nothing pushes against it.
$$\vec{F}_{net} = 0 \;\Rightarrow\; \text{velocity is constant (or zero)}$$
Newton's first law (inertia): an object stays at rest or moves at constant velocity unless a non-zero net force acts on it. Inertia is the resistance to a change in motion, and it increases with mass.
Pause, copy the highlighted statement into your book before moving on.
Did you get this? A puck slides on frictionless ice with no forces pushing it horizontally. What does it do?
Rest and steady motion are both zero-net-force states
We just saw that an object needs a net force to change its motion. That raises a question: does "no change" only mean standing still? This card answers it → no, equilibrium covers both rest and constant velocity, because both have zero net force.
An object is in equilibrium whenever its net force is zero, and that can describe either staying still or moving along at a steady velocity, physics treats the two the same way.
In equilibrium, all the forces balance: the horizontal components sum to zero and the vertical components sum to zero. For the cruising car, the engine's driving force forward exactly balances air resistance and friction backward, while the normal force balances the weight. The net force is zero, so the velocity stays constant.
A car drives at a constant $100\ \text{km h}^{-1}$ on a flat road. The total resistive force (air drag + friction) on it is $600\ \text{N}$ backward. What driving force must the engine provide?
Equilibrium means $\vec{F}_{net} = 0$: both an object at rest and one at constant velocity are in equilibrium. All horizontal forces balance and all vertical forces balance. A cruising car's driving force just equals the resistive force.
Add the highlighted definition to your notes before the check below.
Quick check: True or false: an object moving at constant velocity in a straight line is in equilibrium.
Pattern A, Analyse equilibrium
A $2.0\ \text{kg}$ lamp hangs motionless from a single vertical cord (take $g = 9.8$). In your book: (1) draw the free-body diagram; (2) state why it is in equilibrium; (3) calculate the tension in the cord.
Fill the gaps. Newton’s first law says an object keeps its velocity unless acted on by a non-zero . The tendency to resist a change in motion is called , and it depends only on the object’s .
Dot point: Apply Newton's first law of motion to stationary objects and objects moving with a constant velocity
Reading off the balanced forces when nothing accelerates
We just learned that constant velocity is equilibrium. That raises a question: how do we use that to work out an unknown force on a steadily moving object? This card answers it → set each direction's forces to balance and solve.
Because the net force is zero, the forces in each direction must cancel exactly, so an unknown force can be found by setting it equal to whatever opposes it.
For an object at constant velocity, write that $\sum F_x = 0$ and $\sum F_y = 0$ from the free-body diagram, then solve for the unknown. A skydiver at terminal velocity is a vivid example: their weight downward exactly equals the air resistance upward, so the net force is zero and they fall at a steady speed. This balanced-force reasoning is the core of every constant-velocity problem.
For an object at constant velocity, the net force is zero, so $\sum F_x = 0$ and $\sum F_y = 0$. Set the opposing forces equal to find an unknown, e.g. a skydiver at terminal velocity has air resistance equal to weight.
Pause, copy the highlighted method into your book before the check below.
Last check: Decide if the statement is true or false.
A car moving at a steady speed in a straight line must have a non-zero net force pushing it forward.
Match each constant-velocity situation with the balancing pair of forces:
Dot point: Analyse the forces acting on an object moving with constant velocity
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
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UnderstandBand 5(4 marks) 1. A skydiver falls at a constant terminal velocity. Using Newton's first law, explain the forces acting on the skydiver and why their velocity stays constant.
1 mark: two forces, weight down and air resistance up · 1 mark: these forces are equal in size · 1 mark: so the net force is zero · 1 mark: by Newton's first law, zero net force means constant velocity
ApplyBand 5(4 marks) 2. A skydiver of weight $700\ \text{N}$ falls at a constant terminal velocity. State the air resistance acting on them, justify your answer using Newton’s first law, and explain what changes when the parachute opens.
1 mark: air resistance $= 700\ \text{N}$ · 1 mark: constant velocity means zero net force · 1 mark: opening the parachute increases air resistance above the weight · 1 mark: the net force is now upward, so the skydiver decelerates to a new, lower terminal velocity
Dot point: Analyse the forces acting on an object moving with constant velocity
UnderstandBand 5(3 marks) 3. Explain why Newton’s first law applies equally to a stationary object and to one moving at constant velocity, and why a common intuition says otherwise.
1 mark: both have zero acceleration, so both require zero net force · 1 mark: rest is simply the special case of constant velocity equal to zero · 1 mark: the intuition comes from friction disguising the law in everyday experience
Dot point: Apply Newton's first law of motion to stationary objects and objects moving with a constant velocity
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): Two forces act on the skydiver: their weight ($W = mg$) pulling downward, and air resistance pushing upward. At terminal velocity these two forces are equal in magnitude and opposite in direction, so they balance and the net force is zero. According to Newton's first law, an object with zero net force continues to move with constant velocity. Therefore the skydiver falls at a steady terminal velocity, neither speeding up nor slowing down, because there is no net force to change their motion.
Q2 (4 marks): At terminal velocity the skydiver is not accelerating, so by Newton’s first law the net force on them is zero. The only two vertical forces are the weight down and the air resistance up, so the air resistance must be $700\ \text{N}$ upward, exactly balancing the weight. When the parachute opens the surface area increases sharply, so the air resistance at that same speed jumps well above $700\ \text{N}$. The forces are now unbalanced with a net upward force, and the skydiver decelerates. As the speed falls the air resistance falls with it, until it once again equals the $700\ \text{N}$ weight and a new, much lower terminal velocity is reached.
Q3 (3 marks): Newton’s first law is a statement about acceleration, not about speed: an object continues with whatever velocity it has unless a non-zero net force acts. A stationary object has a constant velocity of zero, and an object cruising at $20\ \text{m s}^{-1}$ has a constant velocity of $20\ \text{m s}^{-1}$. Both are unaccelerated, so both require a net force of exactly zero, and rest is simply the special case. The intuition that motion needs a continuing force comes from everyday experience, where friction and drag are always present: a pushed object stops when you stop pushing, so it looks as though force sustains motion. In fact the push was only balancing friction, and it is the unbalanced friction that produces the deceleration.
You can now apply Newton's first law to stationary and constant-velocity objects and analyse their balanced forces. Next lesson asks what happens when the forces do not balance, Newton's second law, where a net force produces acceleration.
↩ Back to focus-area hubAt the start a puck slid on frictionless ice. It keeps gliding at constant velocity forever: with no friction and no push, the net force is zero, and by the first law its motion never changes. The everyday intuition that "moving things slow down" comes from friction, not from any need for a force to keep things moving. Revisit Card 1 if this still feels counter-intuitive.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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