NCERT Solutions for Class 9 Science Chapter 6 How Forces Affect Motion (PDF) – 2027

The canoe moves forward because of Newton’s third law of motion: the paddle pushes the water backwards, so the water pushes the paddle — and with it the canoe — forwards with an equal force. When the canoeist pushes harder, the water pushes forward harder too, and a larger force on the same canoe gives a larger acceleration (a = F/m), so the canoe moves faster.

The empty canoe moves faster.

Only the net force matters — the object behaves exactly as if that single net force were the only force acting on it.

• If the forces are balanced (equal magnitudes, opposite directions), the net force is zero. An object at rest stays at rest; an object already moving keeps moving with constant velocity. A ball floating on water is like this — the downward gravitational force and the upward buoyant force balance.
• If the forces are unbalanced, a non-zero net force acts. Forces in the same direction add; forces in opposite directions subtract, and the net force points along the larger one. The object then accelerates in the direction of the net force.

The two forces on the barbell are:

1. The gravitational force (weight) of the barbell, exerted by the Earth, acting vertically downwards.
2. The upward force applied by the weightlifter’s hands on the bar.

Yes — while she holds it steady, the two forces are balanced.

No, the forces are not balanced. The locked hands move in the direction of the larger force. In Fig. 6.9 both players use their right hands: R pushes the hands into the page (towards R’s left) and S pushes them out of the page (towards S’s left). Since the arms tilt out of the page, player S exerted the larger force.

Not necessarily. The object moves only if your push is larger than the force of friction.

Yes — but only because friction is present. The force is needed to cancel friction, not to keep the motion going.

Nothing that is moving would ever slow down on its own — and almost nothing could be started, steered or stopped by us.

• Moving objects would never stop. A ball rolled once, a cycle after you stop pedalling, a stack of coins released by a rubber band — each would keep moving with constant velocity for ever, exactly as Newton’s first law predicts when the net force is zero.
• Walking and running would be impossible. You move forward because you push the ground backwards and friction pushes your foot forwards. With no friction your foot would just slide back and you would fall.
• Vehicles could neither accelerate nor brake. Tyres would spin without gripping; brakes work purely by friction.

Yes. The stack slows down steadily and stops after covering some distance from C.

Yes to both. On the laminated top the stack goes further, and its velocity falls more gradually. The laminated top is smoother than wood, so the force of friction on the coins is smaller.

Yes. On polished marble or tiles the distance is the largest of the three and the slowing is the gentlest.

The force of friction depends on the nature of the surfaces in contact — and the smaller the friction, the slower the velocity falls and the further the object travels before stopping.

Measure it directly with a spring balance, as Activity 6.2 does.

1. Lay the spring balance horizontally on the surface and check that it reads zero.
2. Hook a wooden block to it and pull with a slowly increasing force.
3. Note the reading at the instant the block just begins to move. That reading is an approximate measure of the force of friction between the block and that surface.
4. Repeat on each of the four surfaces and compare the readings.

It indicates the magnitude of the force of friction acting on the block from that surface (an approximate value of the largest friction the surface can supply before sliding begins).

The net force on the block is zero.

Yes — approximately. Because the net force is zero, the spring’s pull and friction must be equal in magnitude and opposite in direction, so the scale reading gives the frictional force.

Yes to all three. The readings differ from surface to surface, and they line up exactly with Activity 6.1.

The object would be set moving by the rubber band, and then it would keep moving for ever with constant velocity. Its velocity would not decrease, and it would never come to rest.

No. The net force on it is zero.

(i) and (ii) are possible. (iii) is not possible.

You cannot remove forces, but you can balance them — add forces so that they cancel in pairs and the net force becomes zero.

Example — a book lying on a table.

Use weights of different magnitudes as the source of the force — the Earth pulls a heavier hanging load with a larger gravitational force.

The design of the test (Activity 6.3):

1. Make a cart with ball-bearing wheels and tie a thread to it.
2. Pass the thread over a pipe acting as a pulley at the edge of the table, and hang a paper cup from the free end.
3. Put objects in the cup. As the cup falls, the thread pulls the cart with a constant force equal to the weight of the cup and its contents.
4. Time the cart over a fixed distance s using a slow-motion video — call it T₁.
5. Double the mass in the cup (so the force doubles) and time it again — T₂.

Analysis: both runs start from rest and cover the same distance s.

What the numbers show: with double the load in the cup the cart reaches the pipe sooner, so T₂ < T₁, which makes T₁²/T₂² greater than 1 and therefore a₂ > a₁. So, for a fixed mass, the acceleration increases as the net force increases.

Run the same experiment again, but this time hold the force constant and change the mass of the cart.

1. Repeat Activity 6.3 with one change: keep the mass of the cup and the objects in it constant, so the pulling force stays the same.
2. Double the mass of the cart by adding objects inside it, and measure that mass on a weighing scale.
3. Record the time over the same distance s, exactly as before, from the slow-motion video.
4. Use a₂ ÷ a₁ = T₁² ÷ T₂² to compare the two accelerations.

Yes. The heavier cart takes longer over the same distance, which means its acceleration is smaller.

About the weight of a 100 g mass resting in your palm — a small apple, or a standard 100 g packet.

The mass presses down on your palm with about 1 N, and your palm pushes it back up with about 1 N. The two forces balance, which is why the mass stays at rest.

The net force is zero.

You need the larger force for the heavier child — the one with the greater mass.

The bubble wrap or hay stretches out the stopping time of every jolt, and a longer stopping time means a smaller force on the glass.

You (with the chair) move backwards — yes, the chair moves in the direction opposite to your push.

The chair moves forwards, towards the table.

Whenever you exert a force on the table, the table exerts an equal force on you in the opposite direction — at the same instant. This is Newton’s third law of motion.

You push the ground backwards with your feet. By Newton’s third law, the ground pushes your feet forwards with an equal and opposite force, and this force moves you forward along with the bicycle.

Prediction: both scales will read exactly the same number of newtons.

Yes. Whatever force you pull with — small, medium or large — the two scales read the same value every single time.

The balloon and straw shoot along the thread in the direction opposite to the direction in which the air rushes out of the neck.

The rocket slows down.

Because the water pushes the hose backwards with a force equal to the force the hose uses to push the water forwards — and that backward force can be very large.

By firing its engine and expelling gas — the only way to get a force when there is nothing outside to push against.

• To speed up: fire the exhaust backwards, opposite to the direction of motion.
• To slow down: fire the exhaust forwards, along the direction of motion.
• To turn: fire small side thrusters, so the reaction force acts sideways and changes the direction of the velocity.

No. Newton’s third law applies to all types of forces, contact or non-contact, that we come across in everyday mechanical situations. For example:

• Magnetic force: two bar magnets exert equal and opposite forces on each other (Fig. 6.31).
• Electrostatic force: two similarly charged balloons push each other apart with equal and opposite forces (Fig. 6.32).
• Gravitational force: the Earth pulls a fruit down, and the fruit pulls the Earth up with an equal force (Fig. 6.33).

Yes. Two or more connected objects can be treated as a single system and Newton’s laws applied to that system as a whole.

Treat the two boxes and the string as one system. Both boxes are tied together, so both have the same acceleration:

The frictional force is F — equal in magnitude to the applied force, and opposite in direction.

The correct statement is (i) — P experiences a net force and Q does not experience a net force.

The net force is 18 000 N in the forward direction.

The correct option is (iv) — in the direction of force, with acceleration proportional to the force acting on the object.

The correct option is (iii) — Object C.

Yes — the boat moves backwards, away from the shore.

The mat increases the time over which the athlete’s velocity falls to zero, which reduces the acceleration and therefore the force on the athlete.

The correct option is (iv) — both carts exert forces of equal magnitude on each other.

The force–mass graph is a horizontal straight line at F = 10 N, because the same force was applied to every object.

The force acting on the object is 25 N, in the direction of motion.

The stopping force is 500 N, acting opposite to the bullet’s motion.

The time of contact is 0.015 s, that is 15 milliseconds.

The object travels 10 m in the rough patch before stopping.

The resulting acceleration is a = a₁a₂ ÷ (a₁ + a₂).

Because equal forces do not produce equal accelerations — the compass needle has a far smaller mass and almost no friction opposing it, so the same force moves it easily.

The answers are: yes; yes; and yes. Here is how to establish each one experimentally, and what you should find.

What a good infographic must contain:

• A labelled diagram of the spring balance test, with the reading marked in N.
• A bar chart comparing your readings: one block, two blocks; just-about-to-move, sliding; sliding, rolling.
• One clear sentence of conclusion under each comparison.
• A short “so what” panel: ball bearings in cycle hubs, wheels on carts and suitcases, why brakes and tyre treads need more friction.

Yes — the two cars travel very nearly equal distances in opposite directions, and both distances shrink as you add mass.

What the graph should look like. Plot distance travelled (y-axis) against total mass of a car (x-axis). You get a falling curve — greater mass, shorter run — not a straight line.

What you will observe: with no turn you must hold nearly the full weight of the bucket. With one turn it becomes noticeably easier; with two turns you can hold the same bucket with a small fraction of the effort; with three or four turns you can hold it with two fingers.

Method for this project: read a short excerpt in translation, compare Newton’s own words with the modern statement in this chapter, and write about what changed and what did not.

Sample answer (an outline you can build on):