Class 9 · ScienceNCERT 2026 Revised Syllabus
Chapter 6: How Forces Affect Motion (0906CH06)
Exploration · Newton's laws, inertia, momentum and conservation.
Chapter code: 0906ch06 · Book: Exploration · Class 9 Science
Find complete NCERT solutions for Class 9 Science, Exploration Chapter 6. Every question has a clear, step-by-step answer for chapter code 0906CH06.
Pause and Ponder(10 questions)
1
A weightlifter lifts a barbell (Fig. 6.8). List two forces that are acting on the barbell. Are these forces balanced if the weightlifter keeps the barbell steady?
School Level Solution
Ans.
The barbell has its weight acting downward and the upward force exerted by the weightlifter. When the barbell is held steady, acceleration is zero, so the two forces are equal in magnitude and opposite in direction; the net force is zero.
2
Two players R and S are participating in an arm-wrestling match (Fig. 6.9). At the instant when the arms tilt to the front direction (out of the page towards you), are the forces exerted by the players balanced? If not, which player exerted the larger force?
School Level Solution
Ans.
The arms tilt toward the front, so the net force is not zero. The player whose force produces the observed direction of acceleration exerts the larger force; in the figure this is S.
3
An object is moving with a constant velocity. Is there a net force acting upon it?
School Level Solution
Ans.
By Newton's second law, Fnet = ma. Constant velocity means a = 0, therefore the net force is zero.
4
Suppose, no net force is acting on an object. Which of the following situations are possible? (i) Object remains at rest if at rest. (ii) Object keeps moving with a constant velocity if already moving. (iii) Object is moving with a constant acceleration.
School Level Solution
Ans.
Zero net force means zero acceleration. Therefore an object can remain at rest if initially at rest, or continue with constant velocity if already moving. A non-zero constant acceleration is not possible when the net force is zero.
5
In the real world, it is difficult to find a situation where no forces are acting on an object. But by applying additional forces, a condition can be achieved where the net force on the object is zero. Explain with the help of an example.
School Level Solution
Ans.
Example: a book resting on a horizontal table has gravitational force downward and an equal normal force upward. Forces are present, but their vector sum is zero, so the book has no acceleration.
6
A toy car of mass 100 g is moving with a constant velocity of 0.5 m s⁻¹. What is the net force acting on the toy car?
School Level Solution
Ans.
The velocity is constant, so acceleration is 0 m s−2. Therefore Fnet = ma = 0.1 × 0 = 0 N.
7
Two children of different masses are sitting on identical swings. To impart identical initial acceleration, for which child would you require to apply a larger force? Explain why.
School Level Solution
Ans.
From F = ma, for the same acceleration, force is directly proportional to mass. Therefore the child with the larger mass requires the larger applied force.
8
How are glass items packed for transportation using a bubble wrap or hay protected from damage?
School Level Solution
Ans.
When the package is dropped or struck, the cushioning material increases the time over which the item comes to rest. For the same change in momentum, increasing stopping time decreases the average impact force, reducing damage.
9
Why does a fireperson sometimes struggle when holding the pipe issuing water?
School Level Solution
Ans.
Water is accelerated forward out of the nozzle and gains momentum. The corresponding reaction force acts backward on the pipe and the fireperson, so a substantial opposing force must be resisted.
10
Suppose a spacecraft is moving in a region of space where the gravitational force acting upon it is negligible. Suggest how can it change its velocity.
School Level Solution
Ans.
The spacecraft can change its velocity by expelling propellant. The expelled gases gain momentum in one direction and the spacecraft receives an equal and opposite reaction, changing its velocity.
Chapter Exercises(10 questions)
1
A weightlifter lifts a barbell. List two forces acting on the barbell. Are these forces balanced if the weightlifter keeps the barbell steady?
School Level Solution
Ans.
Two forces: weight (gravity) pulling down and the lifter’s upward muscular force. Since the barbell is steady (no acceleration), the net force is zero — the two forces are balanced.
2
Two players R and S are arm-wrestling. At the instant the arms tilt out of the page towards you, are the forces exerted by the players balanced? If not, which player exerted the larger force?
School Level Solution
Ans.
Motion means a net force exists, so the forces are unbalanced. The player whose push dominates and moves the arms in that direction is exerting the larger force at that instant.
3
An object is moving with a constant velocity. Is there a net force acting upon it?
School Level Solution
Ans.
By Newton’s First Law, constant velocity (fixed speed and direction) means zero acceleration, so the net force is zero. Individual forces may act, but they cancel out.
4
Suppose no net force acts on an object. Which are possible: (i) stays at rest if at rest; (ii) keeps moving with constant velocity; (iii) moves with constant acceleration?
School Level Solution
Ans.
With zero net force, acceleration is zero: a resting object stays at rest (i) and a moving object keeps its velocity (ii). (iii) is impossible — constant acceleration requires a constant net force (F = ma).
5
In the real world no object is force-free, yet net force can be zero by applying additional forces. Explain with an example.
School Level Solution
Ans.
Example: a book on a table — gravity pulls it down while the table pushes up with an equal normal force; they cancel → net force zero, book stays put. Similarly a car at steady speed: engine force cancels friction and air drag.
6
A toy car of mass 100 g moves with a constant velocity of 0.5 m/s. What is the net force acting on it?
School Level Solution
Ans.
Mass and speed are distractors. Since velocity is constant, acceleration a = 0, so F = ma = 0 N. The net force on the toy car is zero.
7
Two children of different masses sit on identical swings. To give both the same initial acceleration, for which child must you apply a larger force? Why?
School Level Solution
Ans.
By F = ma, force and mass are directly proportional for fixed acceleration. So the heavier child needs the larger push to achieve the same initial acceleration as the lighter child.
8
How does packing glass items in bubble wrap or hay protect them during transportation?
School Level Solution
Ans.
A sudden stop delivers a fixed impulse (change in momentum). Bubble wrap/hay extends the stopping time, so the same impulse acts with a much smaller force (F = impulse ÷ time) — below the glass’s breaking limit.
9
Why does a firefighter sometimes struggle while holding a pipe issuing water?
School Level Solution
Ans.
The pipe pushes water forward at high speed; by Newton’s Third Law the water pushes the pipe backwards with equal force. This strong, jerky reaction is what the firefighter must wrestle to control.
10
A spacecraft moves where gravity is negligible. How can it change its velocity?
School Level Solution
Ans.
It must throw something away: firing thrusters expels gas backwards, and the reaction pushes the craft forwards (Newton’s Third Law — same principle as a balloon rocket). Steering uses small side thrusters; nothing to push against is needed.
Activities — How to Do(6 questions)
1
Activity 6.1 (rubber band and coins): launch a taped coin-stack with a stretched rubber band on wood, laminated top and polished marble; measure travel distance each time. What do you conclude?
School Level Solution
Ans.
Stretch the band to mark C and release identically each time. The stack travels farthest on polished marble, less on laminate, least on wood. Conclusion: smoother surfaces exert less friction, so velocity decreases slowest there; friction opposes motion.
2
Activity 6.2 (spring balance and block): pull a wooden block on the same three surfaces and note the balance reading just as motion starts. What does the reading mean, and how do surfaces compare?
School Level Solution
Ans.
Zero the horizontal balance, attach the block, pull gently: the reading just as it slips equals the limiting friction (net force zero at that instant). Readings differ by surface — smallest on marble, largest on wood — matching Activity 6.1’s distances in reverse.
3
Activity 6.3 (cart, pulley, varying force): pull a wheeled cart with a thread over a pulley loaded by a cup of coins; time it over a fixed distance, then double the cup mass and repeat. Using s = ½aT², what is the conclusion?
School Level Solution
Ans.
Same distance s both times: a₂/a₁ = (T₁/T₂)². The heavier cup gives a shorter time, so a₂ > a₁. Conclusion: for a fixed cart mass, acceleration rises as net force rises — verifying F = ma qualitatively.
4
Activity 6.4 (double the cart mass): repeat Activity 6.3 keeping the cup mass fixed but doubling the cart’s mass. What happens to acceleration?
School Level Solution
Ans.
Same pull, heavier cart → the cart takes longer to cover the distance, so acceleration decreases. Conclusion: for a given force, acceleration is inversely proportional to mass (a = F/m) — heavier objects resist motion change more.
5
Activity 6.6 (two spring balances): hook two identical balances together, fix one end, pull the other, and compare readings at rest. What do you predict and observe?
School Level Solution
Ans.
Prediction: the readings match. Observation (repeated with different pulls): both scales read identically every time. Each balance pulls the other with an equal and opposite force — a direct static demo of action–reaction pairs.
6
Activity 6.7 (balloon rocket): thread a straw-mounted balloon along a taut line, release its tied neck, and watch. Which way does it move, and how does Newton’s Third Law explain it?
School Level Solution
Ans.
Air rushes out of the neck backwards; the reaction drives the balloon-straw forwards along the thread. This action–reaction pair is exactly how rockets and jets work — expelled mass one way, craft the other.