ScienceCBSE Class 9 · How Forces Affect Motion

Net Force, Friction, Inertia and Newton's First Law (Class 9)

Updated 17 October 2026 · Fact-checked

A force is a push or pull with magnitude and direction, measured in newtons (N). The net force on an object is the single force with the same effect as all the forces together: add forces in the same direction and subtract those in opposite directions. Newton's first law says that an object stays at rest or keeps moving with constant velocity unless a net force acts on it; inertia is this resistance to change in its state, and a larger mass means more inertia.

Subtract opposite forces: equal push and friction balance to zero, but a bigger push leaves a net force that changes the motion.

Illustration drawn for QuestionBuddy.

Understand Net Force, Friction and Newton's First Law

Force and net force. A force can start or stop motion, change speed or direction, or change shape. It has magnitude and direction, and its SI unit is the newton (N); a spring balance measures it, and holding a 100 g mass needs about 1 N. For forces along one line, the net force is the sum when they point the same way and the difference when they point opposite ways, in the direction of the larger one. If equal and opposite forces act on one object the net force is zero and the forces are balanced. A tug of war with equal pulls does not move the rope; if one team pulls harder the rope moves towards it. Motion depends only on the net force.

Friction. Friction acts between surfaces in contact and opposes motion or the tendency to move. A box at rest begins to move only when your push exceeds friction. Once you stop pushing, friction alone acts backwards and the box slows and stops. To keep a box at constant velocity you must push with a force equal to friction. Weight (down) and the normal force from the floor (up) balance on a horizontal floor. Friction depends on the nature of the surfaces: smoother surfaces give less friction. Friction is also useful: we walk because friction pushes us forward, and grooves on shoes and treads on tyres increase it. To reduce friction use lubricants, polishing, ball bearings and wheels, streamlined shapes.

Newton's first law. For centuries people believed a force is needed even to keep something moving. Galileo argued, using thought experiments, that if all obstacles were removed a moving body on a level surface would move forever; in practice things stop because of friction. Newton's first law: an object stays at rest, or keeps moving with constant velocity (same speed in a straight line), unless a net force acts on it. If net force = 0 then acceleration = 0. Constant velocity needs no net force; speeding up, slowing down or turning does. Inertia is the tendency of an object to resist any change in its state of rest or uniform motion, so the first law is also called the law of inertia, and more mass means more inertia.

Graphs with no net force. At rest: position-time graph horizontal, velocity-time graph on the time axis. Constant velocity: position-time graph is a straight sloping line, velocity-time graph horizontal.

Examples of inertia. Passengers lurch forward when a bus brakes suddenly because their bodies keep moving, and fall back when it starts suddenly. Dust leaves a carpet when it is beaten; a coin on a card drops into the glass when the card is flicked away; a seat belt stops us continuing forward in a crash.

Using the second law with friction. A net force F gives a = F/m (Newton's second law), so for a box of mass m pushed with force P against friction f, the acceleration is (P − f)/m, and after the push stops the deceleration is f/m.

Net Force, Friction and Newton's First Law: formulas and key facts

Net force along a line: Same direction: F₁ + F₂ | opposite directions: larger − smaller, along the larger
Balanced forces: $N e t f or ce = 0 \Leftrightarrow a cce l er a t i o n = 0 (r es t or co n s t an t v e l oc i t y)$
Acceleration under a net force: $a = F \div m, so a = (P - f) \div m f or p u s h P a g ain s t f r i c t i o n f$
Weight: $W = m g, w i t h g = 9.8 m s^{- 2}$
Unit of force: $1 N = 1 k g m s^{- 2}$

How to solve Net Force, Friction and Newton's First Law questions

Use this method for net force, friction and inertia questions.

1Draw the object and mark every force on it with arrows (weight, normal force, push or pull, friction).
2Choose a positive direction and add the forces along it with signs to get the net force.
3If the net force is zero, state 'balanced: no acceleration: rest or constant velocity'.
4If it is not zero, use a = F ÷ m for the acceleration in the direction of the net force.
5Use v = u + at, s = ut + ½at² or v² = u² + 2as for time, speed or distance.
6For inertia reasoning, say what the body 'tends to keep doing' and which part is acted on by a force.

Net Force, Friction and Newton's First Law in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Pushing a carton across a floor

Suppose a delivery boy pushes a 10 kg carton on a rough floor where friction is 40 N, and the carton moves at a steady speed.

(a) How much force is he applying? (b) He pushes with 55 N. Find the acceleration. (c) He stops pushing. Find the deceleration.

Try it first, then show the solutionSolution

(a) Steady speed means zero net force, so the push equals friction: 40 N.
(b) Net force = 55 − 40 = 15 N, so a = 15 ÷ 10 = 1.5 m s⁻².
(c) Only friction acts backwards: a = 40 ÷ 10 = 4 m s⁻² (a deceleration).

Answer: (a) 40 N; (b) 1.5 m s⁻²; (c) 4 m s⁻² backwards.

Memory hook: Steady speed means balanced forces.

Inertia and a sudden brake

Suppose a bus travelling at 36 km h⁻¹ brakes to a stop, and a standing passenger without a handhold lurches forward.

Explain why the passenger lurches forward.

Try it first, then show the solutionSolution

Before braking, the passenger and the bus are moving forward together at the same velocity.
Braking applies a backward force to the bus and to the passenger's feet, but the upper body does not feel any such force immediately.
Because of inertia, the upper body tends to keep its forward motion, so the passenger lurches forward.

Answer: Inertia: the body tends to continue in its state of motion when the bus stops.

Memory hook: The body wants to keep going.

Tricks to remember Net Force, Friction and Newton's First Law

Short memory aids. Say each one aloud once, then use it on a question the same day.

Zero net force, zero acceleration
No net force does not mean no motion. It means no change of velocity: rest or constant velocity.
Use it when: First-law reasoning.
Mass is a measure of inertia
The heavier it is, the harder it is to start and to stop, so the more inertia it has.
Use it when: Comparing inertia of two objects.
Push minus friction
Net force on a pushed box = push − friction. Steady speed when they are equal.
Use it when: Friction numericals.
Handle the same object
Balanced forces act on one object. Count only the forces on the object you are studying.
Use it when: Net force problems.

Net Force, Friction and Newton's First Law: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Net force from several forces · 3 marks · Short answerBoard level

(a) Two forces of 30 N and 18 N act on a cart in the same direction. Find the net force. (1)
(b) The same forces act in opposite directions. Find the net force and its direction. (1)
(c) In a tug of war, each team pulls the rope with 400 N. What is the net force on the rope and what happens to it? (1)

Try it first, then show the solutionSolution

(a) 30 + 18 = 48 N in the direction of the forces. (1)
(b) 30 − 18 = 12 N in the direction of the 30 N force. (1)
(c) 400 − 400 = 0 N. The forces are balanced, so the rope does not accelerate; if at rest it stays at rest. (1)

Answer: (a) 48 N; (b) 12 N along the larger force; (c) zero, the rope stays at rest.

Example 2: Pushing a box against friction · 3 marks · Short answerBoard level

A 10 kg box is on a rough floor and the friction is 40 N.
(a) What push keeps it moving at constant velocity? (1)
(b) Find the acceleration if the push is 55 N. (1)
(c) The push is removed when the speed is 6 m s⁻¹. Find the deceleration and the distance in which it stops. (1)

Try it first, then show the solutionSolution

(a) 40 N, equal to friction, so the net force is zero. (1)
(b) a = (55 − 40) ÷ 10 = 1.5 m s⁻². (1)
(c) a = −40 ÷ 10 = −4 m s⁻². From v² = u² + 2as: 0 = 36 + 2(−4)s, so s = 36 ÷ 8 = 4.5 m. (1)

Answer: (a) 40 N; (b) 1.5 m s⁻²; (c) 4 m s⁻² and 4.5 m.

Example 3: Inertia on a braking bus · 5 marks · Long answerHarder board level

A bus moving at 36 km h⁻¹ is brought to rest uniformly in 5 s.
(a) Find its deceleration. (1)
(b) Find the braking distance. (1)
(c) A passenger of mass 60 kg wearing a seat belt stops with the bus. Find the force on the passenger. (1)
(d) Another passenger without a belt sits 0.81 m behind the dashboard, and the friction on the seat is negligible. How long before he reaches the dashboard, and how fast is he moving relative to the dashboard then? (2)

Try it first, then show the solutionSolution

(a) 36 km h⁻¹ = 10 m s⁻¹. a = (0 − 10) ÷ 5 = −2 m s⁻², a deceleration of 2 m s⁻². (1)
(b) s = ½(10 + 0) × 5 = 25 m. (1)
(c) F = ma = 60 × 2 = 120 N, backwards, supplied by the seat belt. (1)
(d) With no net force on him he keeps moving at 10 m s⁻¹ (first law), while the dashboard slows with the bus. The gap closes by ½ × 2 × t² = t² metres, so t² = 0.81 and t = 0.9 s. At that moment the bus has slowed to 10 − 2 × 0.9 = 8.2 m s⁻¹, so he hits the dashboard at 10 − 8.2 = 1.8 m s⁻¹ relative to it. (2)

Answer: (a) 2 m s⁻²; (b) 25 m; (c) 120 N; (d) 0.9 s; he moves at 1.8 m s⁻¹ relative to the dashboard.

Example 4: Net force from a velocity-time graph · 5 marks · Long answerHarder board level

A 5 kg trolley's velocity rises uniformly from 0 to 8 m s⁻¹ in 4 s, stays at 8 m s⁻¹ from 4 s to 10 s, and falls uniformly to 0 in the next 4 s. Friction on the trolley is 4 N throughout.
(a) Find the net force in each stage. (2)
(b) Find the driving force (push) in the first two stages. (2)
(c) In the third stage, which extra force acts and how large is it? (1)

Try it first, then show the solutionSolution

(a) Stage 1: a = 8 ÷ 4 = 2 m s⁻², F = 5 × 2 = 10 N forward. Stage 2: a = 0, so net force 0. Stage 3: a = −8 ÷ 4 = −2 m s⁻², F = 5 × (−2) = −10 N (10 N backwards). (2)
(b) Stage 1: push − 4 = 10, so push = 14 N. Stage 2: net force is zero, so push = friction = 4 N. (2)
(c) In stage 3 the net force is 10 N backwards; friction supplies 4 N, so an extra brake force of 10 − 4 = 6 N acts backwards (with the push removed). (1)

Answer: (a) +10 N, 0, −10 N; (b) 14 N and 4 N; (c) 6 N extra backward force.

Example 5: A spacecraft with no engine thrust · 4 marks · Case-basedHOTS / competency

Suppose a spacecraft of mass 1000 kg is far from every planet and moves at 200 m s⁻¹ with its engine off.
(a) What force is needed to keep it moving at 200 m s⁻¹? (1)
(b) The engine fires forward with a 500 N thrust for 40 s. Find the new speed. (1)
(c) The engine now fires in the opposite direction with the same thrust for 100 s. Find the final velocity. (1)
(d) Why does a ball rolling on the floor stop but this craft does not? (1)

Try it first, then show the solutionSolution

(a) None. With zero net force the velocity stays constant (first law). (1)
(b) a = 500 ÷ 1000 = 0.5 m s⁻²; Δv = 0.5 × 40 = 20 m s⁻¹; new speed = 220 m s⁻¹. (1)
(c) a = −0.5 m s⁻²; Δv = −0.5 × 100 = −50 m s⁻¹; final velocity = 220 − 50 = 170 m s⁻¹, still forward. (1)
(d) The ball is slowed by friction (and air resistance), which is a net backward force; in space there is almost none, so the craft keeps its velocity. (1)

Answer: (a) zero force; (b) 220 m s⁻¹; (c) 170 m s⁻¹ forward; (d) no friction in space.

Where students lose marks in Net Force, Friction and Newton's First Law

The slips examiners see most often, and what to write instead.

Mistake: Saying a force is needed to keep an object moving at constant velocity.
Everyday objects stop because of friction, so students believe motion needs a push.
Wrong working
A moving car needs a net force to keep moving at constant speed.
Right working
At constant velocity the net force is zero; the engine force just balances friction and drag.
Do this: Only a change in velocity needs a net force.
Usually costs: 1 to 2 marks.
Mistake: Adding all forces without regard to direction.
The sign is forgotten.
Wrong working
30 N and 18 N in opposite directions: net force 48 N.
Right working
Net force = 30 − 18 = 12 N in the direction of the 30 N force.
Do this: Mark directions and subtract opposite forces.
Usually costs: 1 mark.
Mistake: Stating the first law without 'net force' or 'constant velocity'.
The law is written from memory as 'a body continues to move'.
Wrong working
A body keeps moving until a force acts on it.
Right working
A body remains at rest or in uniform motion in a straight line unless a net force acts on it.
Do this: Include 'net force' and 'constant velocity'.
Usually costs: ½ to 1 mark.
Mistake: Treating weight and normal force as an action-reaction pair.
They are equal and opposite on the book.
Wrong working
The normal force is the reaction to the weight.
Right working
They act on the same object; the reaction to the weight is the book's pull on the Earth.
Do this: Pairs act on two different objects.
Usually costs: 1 mark.
Mistake: Using the force of the push as the net force when friction acts.
Friction is not included.
Wrong working
a = 55 ÷ 10 = 5.5 m s⁻²
Right working
a = (55 − 40) ÷ 10 = 1.5 m s⁻²
Do this: Always subtract the opposing friction first.
Usually costs: 2 marks.

Exam tips

Draw a free-body sketch with arrows for every force; marks are given for the correct directions.
Write 'net force = ...' as a line in your answer before using a = F ÷ m.
In inertia answers, name the part of the body that is acted on and the part that tends to keep moving.
Convert km h⁻¹ to m s⁻¹ before substituting into equations of motion.
For the first law, quote 'net force', 'constant velocity' and 'rest'.

Practice questions on Net Force, Friction and Newton's First Law

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
State Newton's first law and define inertia. Which has more inertia, a 2 kg stone or a 20 kg stone?
Hint
Mass measures inertia.
Answer
An object stays at rest or keeps moving with constant velocity unless a net force acts on it; inertia is the tendency of an object to resist change in its state of rest or uniform motion (1 mark). The 20 kg stone has more inertia, because inertia increases with mass (1 mark).
Q2 · 3 marks · Short answerHarder board level
An 8 kg block moves at constant velocity on a rough floor when pulled with a horizontal 20 N force. (a) State the friction on the block. (b) Find the acceleration when the pull is increased to 36 N. (c) The pull is removed when the speed is 10 m s⁻¹; find the distance covered before it stops.
Hint
Friction stays the same.
Answer
(a) Zero net force, so friction = 20 N (1 mark). (b) a = (36 − 20) ÷ 8 = 2 m s⁻² (1 mark). (c) Deceleration = 20 ÷ 8 = 2.5 m s⁻²; s = u² ÷ 2a = 100 ÷ 5 = 20 m (1 mark).
Q3 · 2 marks · Assertion-reasonBoard level
Assertion (A): A passenger standing in a bus lurches forward when the bus brakes suddenly. Reason (R): The passenger's body tends to continue in its state of motion because of inertia. Choose: (a) A and R both true, R explains A; (b) A and R both true, R does not explain A; (c) A true, R false; (d) A false, R true.
Hint
Which law is about continuing in the same state of motion?
Answer
(a). The upper body keeps its forward velocity (inertia) while the bus slows, so the passenger moves forward relative to the bus.
Q4 · 3 marks · Short answerHarder board level
A book rests on a table. (a) Name the two forces on the book and state the net force. (b) A student says the weight and the normal force are an action-reaction pair. Is she right? Explain. (c) Give the real reaction to the table's push on the book.
Hint
An action-reaction pair acts on two different objects.
Answer
(a) Weight (downwards) and the normal force from the table (upwards); they are equal and opposite on the same book, so the net force is zero (1 mark). (b) No: both forces act on the same object (the book), but the two forces of an action-reaction pair act on different objects (1 mark). (c) The book pushes down on the table with an equal force (1 mark).
Q5 · 4 marks · Case-basedHOTS / competency
A cyclist and bicycle of total mass 80 kg are moving at 6 m s⁻¹ when she stops pedalling. The combined friction and air resistance is a constant 16 N. (a) Find the deceleration. (b) Find the time and distance to stop. (c) What pedalling force would keep her at 6 m s⁻¹? (d) Explain both parts using the first law.
Hint
Constant velocity needs balanced forces.
Answer
(a) a = 16 ÷ 80 = 0.2 m s⁻² backwards (1 mark). (b) t = 6 ÷ 0.2 = 30 s; s = 6² ÷ (2 × 0.2) = 90 m (1 mark). (c) 16 N forward, equal to the resistance so the net force is zero (1 mark). (d) With no net force she would keep a constant velocity; once she stops pedalling a net backward force acts, so her velocity changes (1 mark).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is the SI unit of force?Show answer
The newton (N).
2.A body moves at constant velocity. What is the net force on it?Show answer
Zero.
3.What is another name for Newton's first law?Show answer
The law of inertia.
4.Name one way to reduce friction.Show answer
Use a lubricant (also polishing, ball bearings or wheels, streamlining).
5.Who argued that a moving body on a level, frictionless surface would move forever?Show answer
Galileo.
6.What does inertia depend on?Show answer
Mass; more mass means more inertia.

Net Force, Friction and Newton's First Law: frequently asked questions

Can an object move when the net force on it is zero?

Yes. It then moves at constant velocity, in a straight line at constant speed.

Is friction always a nuisance?

No. Without friction we could not walk, hold things or stop vehicles; shoes have grooves and tyres have treads to increase it.

Why do we fall back when a bus starts suddenly?

Our feet are carried forward with the bus, but the upper body tends to stay at rest because of inertia.

Does a heavier object have a greater inertia?

Yes. More mass means a greater resistance to change in its state of motion.

Why does a ball stop rolling on grass?

Friction acts as a net backward force, so the ball's velocity decreases until it stops.