NCERT Solutions Class 9 Science Exploration Chapter 06 How Forces Affect Motion

Question 1. Using a horizontal force F, a table is moved across the floor at a constant velocity. How much is the frictional force exerted by the floor on the table?
Answer: Since the table moves at constant velocity, its acceleration equals zero. Per Newton's first law of motion, when acceleration is zero, the net force acting on the object must also be zero. Therefore, the frictional force exerted by the floor on the table matches the applied force F in magnitude but acts in the opposite direction.
In simple words: The friction force must be the same size as the push force, but pointing the opposite way.

Exam Tip: Remember that constant velocity means zero acceleration, which means zero net force - applied force and friction must balance exactly.

Question 2. For a ball moving on a smooth frictionless surface, choose the appropriate option that will make the following statements physically correct.
(i) If no net force is applied on the ball, the velocity of the ball will remain the same/increase/decrease.
Answer: If no net force is applied on the ball, the velocity of the ball will remain the same. Per Newton's first law of motion, an object continues to move with the same velocity unless acted upon by an unbalanced force.
In simple words: Without any force pushing on it, the ball keeps moving at the same speed.

Exam Tip: This tests understanding of Newton's first law - state it clearly in your answer.

Question 2(ii). If a net force is applied on the ball in the direction of its motion, the magnitude of the velocity of the ball will remain the same/increase/decrease.
Answer: If a net force is applied on the ball in the direction of its motion, the magnitude of the velocity of the ball will increase. A force in the direction of motion produces acceleration in the same direction, so the speed increases.
In simple words: When the push goes the same way as the ball is moving, the ball speeds up.

Exam Tip: Force in the direction of motion causes acceleration in that direction - the object speeds up.

Question 2(iii). If a net force is applied on the ball in a direction opposite to the direction of its motion, the magnitude of the velocity of the ball will remain the same/increase/decrease.
Answer: If a net force is applied on the ball in a direction opposite to the direction of its motion, the magnitude of the velocity of the ball will decrease. A force opposite to the direction of motion produces retardation, so the speed decreases.
In simple words: When the push goes against the ball's movement, the ball slows down.

Exam Tip: Force opposite to motion causes deceleration - the object slows down.

Question 3. Two blocks P and Q on a smooth horizontal surface are shown in Fig. 6.36(a) and Fig. 6.36(b). Two forces of magnitudes 4 N and 5 N are acting in opposite directions on block P, while block Q is moving with a constant velocity. Which of the following statement is correct? (i) P experiences a net force and Q does not experience a net force. (ii) P does not experience a net force and Q experiences a net force. (iii) Both P and Q experience a net force. (iv) Neither P nor Q experiences a net force.
Answer: (i) P experiences a net force and Q does not experience a net force.
In simple words: Block P has two opposing pushes (5 N and 4 N), so there is a net force of 1 N on it. Block Q moves at steady speed, meaning no net force acts on it.

Exam Tip: Find net force by subtracting opposite forces: 5 N - 4 N = 1 N on P. Constant velocity means zero net force on Q.

Question 4. While practising for the snake boat race (Vallum kali in Kerala), 100 oarsmen are rowing a boat together. Out of these, 95 row backwards to propel the boat forward. But by mistake, 5 oarsmen row in the opposite direction. If each oarsman applies a horizontal force of 200 N, what is the net force on the snake boat?
Answer: Number of oarsmen rowing in the forward-driving direction equals 95. Force applied by each oarsman equals 200 N. Total forward force equals 95 × 200 = 19000 N. Number of oarsmen rowing in the opposite direction equals 5. Total backward force equals 5 × 200 = 1000 N. Net force equals Forward force - Backward force, which is 19000 N - 1000 N = 18000 N. The net force on the snake boat is 18000 N in the forward direction.
In simple words: Add up all the pushes in one direction (19000 N), then subtract the pushes in the other direction (1000 N), and you get the total push: 18000 N forward.

Exam Tip: Always find net force by subtracting forces in opposite directions; direction matters.

Question 5. When a net force acts on an object, we observe that the object accelerates: (i) opposite to the direction of force, with acceleration proportional to the force acting on the object. (ii) opposite to the direction of force, with acceleration proportional to the mass of the object. (iii) in the direction of force, with acceleration inversely proportional to the force acting on the object. (iv) in the direction of force, with acceleration proportional to the force acting on the object.
Answer: (iv) in the direction of force, with acceleration proportional to the force acting on the object.
In simple words: The object speeds up in the same direction as the push, and the harder you push, the more it speeds up.

Exam Tip: Newton's second law states acceleration is in the direction of net force and directly proportional to force but inversely proportional to mass.

Question 6. The position-time graph for four objects A, B, C and D moving along a straight line are given in Fig. 6.37. A net force acts on: (i) Object A (ii) Object B (iii) Object C (iv) Object D
Answer: (iii) Object C
In a position-time graph, a straight line means constant velocity, so acceleration is zero and net force is zero. A horizontal line means the object is at rest, so acceleration is zero and net force is zero. A curved line means velocity is changing, so acceleration is present and hence a net force acts.

Observing the graphs:
Object A: straight line upward - constant velocity - no net force
Object B: horizontal line - at rest - no net force
Object C: curved line upward - changing velocity - net force acts
Object D: straight line downward - constant negative velocity - no net force
In simple words: Only object C has a curved line in the graph, which means its speed is changing, so a force must be acting on it.

Exam Tip: In position-time graphs, straight lines mean no acceleration (no net force), while curved lines mean acceleration is present (net force acts).

Question 7. A sailor jumps out from a small boat to the shore (Fig. 6.38). As the sailor jumps forward, will the boat move? If yes, in which direction and why?
Answer: Yes, the boat will move. The boat will move in the direction opposite to the direction in which the sailor jumps. When the sailor jumps forward, they push the boat backward. Per Newton's third law of motion, every action has an equal and opposite reaction. The forward force applied by the sailor on the ground (or shore) results in an equal backward force on the boat. Hence, the boat moves backward.
In simple words: When the sailor pushes off, the boat gets pushed the opposite way, like a springboard bouncing back.

Exam Tip: Apply Newton's third law clearly - every action and reaction pair must be identified and explained.

Question 8. During a high jump event, a landing mat or sand bed is placed for the athlete to fall upon (Fig. 6.39). Explain the reason behind it.
Answer: A landing mat or sand bed serves to reduce the impact force on the athlete. When the athlete lands, their momentum becomes zero. Per Newton's second law, force depends on the rate of change of momentum. A soft surface (mat or sand) increases the time taken to stop. Increasing the time of impact reduces the force experienced by the body. The mat or sand bed reduces injury by decreasing the force of impact.
In simple words: The soft mat lets the athlete slow down over a longer time, which means less force hits the body and causes less injury.

Exam Tip: Explain using the concept that force depends on time - longer stopping time means smaller force.

Question 9. A hand cart loaded with vegetables collides with an identical but empty hand cart. During the collision: (i) the loaded cart exerts a force of larger magnitude on the empty cart. (ii) the empty cart exerts a force of larger magnitude on the loaded cart. (iii) neither cart exerts a force on the other. (iv) the loaded cart and the empty cart both exert an equal magnitude of force on each other.
Answer: (iv) the loaded cart and the empty cart both exert an equal magnitude of force on each other.
Per Newton's third law of motion, every action has an equal and opposite reaction. Hence, both carts exert equal and opposite forces on each other.
In simple words: When two things bump into each other, they push on each other with equal force, no matter how heavy they are.

Exam Tip: Newton's third law applies to all interactions - forces always come in equal and opposite pairs.

Question 10. The acceleration-mass graph for the acceleration produced by a force on objects of different masses is plotted in Fig. 6.40. Plot the force-mass graph for this case.
Answer: From Newton's second law, F equals ma. The given graph shows acceleration decreases as mass increases, which means force is constant. Therefore, force does not change with mass. The force-mass graph will be a straight horizontal line parallel to the mass axis.
In simple words: If you keep pushing with the same force on heavier and heavier objects, they speed up less. So the force stays the same while the mass changes.

Exam Tip: From the acceleration-mass relationship, identify whether force is constant, then plot the force-mass graph as a horizontal line.

Question 11. The velocity-time graph of an object of mass 10 kg moving along a straight line is shown in Fig. 6.41. Calculate the force acting on the object by using the graph.
Answer: From the graph: Initial velocity (u) equals 10 m/s. Final velocity (v) equals 30 m/s. Time (t) equals 8 s.

Acceleration: a = (v - u) / t = (30 - 10) / 8 = 20 / 8 = 2.5 m/s²

Force: F = ma = 10 × 2.5 = 25 N. Force acting on the object equals 25 N.
In simple words: Find how fast the speed is changing from the graph (that's acceleration), then multiply by the mass to get the force.

Exam Tip: Always extract initial velocity, final velocity, and time from the graph first, then calculate acceleration, then use F = ma.

Question 12. A bullet of mass 50 g moving with a speed of 100 m/s enters a heavy stationary wooden block and stops after penetrating a distance of 50 cm. Estimate the stopping force acting on the bullet. (assume that the bullet undergoes constant acceleration within the block)
Answer: Mass equals 50 g = 0.05 kg. Initial velocity (u) equals 100 m/s. Final velocity (v) equals 0 m/s. Distance (s) equals 50 cm = 0.5 m.

Using: v² = u² + 2as

⇒ 0 = (100)² + 2a(0.5)

⇒ 0 = 10000 + a

⇒ a = -10000 m/s²

Force: F = ma = 0.05 × (-10000) = -500 N. Stopping force equals 500 N (opposite to direction of motion).
In simple words: The bullet slows down very fast over a short distance, so the stopping force is huge.

Exam Tip: Use the kinematic equation v² = u² + 2as to find acceleration when time is not given, then apply F = ma.

Question 13. An ace footballer converted a penalty shot by kicking the football with a speed of 108 km h⁻¹. The estimated force they imparted was 800 N. The mass of the football was 0.4 kg. Calculate the time of contact between their foot and the ball.
Answer: Speed equals 108 km/h = 30 m/s. Initial velocity (u) equals 0. Final velocity (v) equals 30 m/s. Mass equals 0.4 kg. Force equals 800 N.

Using: F = ma ⇒ a = F/m = 800/0.4 = 2000 m/s²

Using: v = u + at ⇒ 30 = 0 + (2000)t ⇒ t = 30/2000 = 0.015 s. Time of contact equals 0.015 s.
In simple words: The foot pushes the ball very hard, so even though the contact time is tiny (0.015 seconds), the ball shoots forward at high speed.

Exam Tip: Convert speed to m/s first, find acceleration from F = ma, then find time from v = u + at.

Question 14. An object of mass 2 kg moving with a constant velocity of 10 m s⁻¹ encounters a rough patch where the force of friction on the object is 7 N. At the same time, an additional constant force of 3 N opposing the motion is applied on the object. After entering the rough patch, how much distance does the object travel before coming to rest?
Answer: Given: Mass of object, m = 2 kg. Initial velocity, u = 10 m s⁻¹. Force of friction = 7 N. Additional opposing force = 3 N.

Total opposing force: F = 7 N + 3 N = 10 N

Using Newton's second law: F = ma ⇒ a = F/m = 10 / 2 = 5 m s⁻²

Since the force opposes the motion, acceleration is negative: a = -5 m s⁻². Final velocity, v = 0 m s⁻¹. Using the equation: v² = u² + 2as ⇒ 0 = (10)² + 2(-5)s ⇒ 0 = 100 - 10s ⇒ 10s = 100 ⇒ s = 10 m. The object travels 10 m before coming to rest.
In simple words: Two forces (friction and the extra push) slow the object down, making it stop after traveling 10 meters.

Exam Tip: Always add opposing forces to get total retarding force, then use v² = u² + 2as to find stopping distance.

Question 15. A tractor pulls a harrow (a ploughing tool) of mass m₁ with a net force F resulting in an acceleration of a₁. The same tractor pulls a trolley of mass m₂ with a force F producing an acceleration of a₂. If the tractor now pulls the trolley with the harrow placed on it (with the same force F), then obtain an expression for the resulting acceleration in terms of a₁ and a₂. Ignore friction.
Answer: For the harrow: F = m₁a₁. So, m₁ = F/a₁.

For the trolley: F = m₂a₂. So, m₂ = F/a₂.

When both are together: Total mass = m₁ + m₂. Let resulting acceleration be a.

Then, F = (m₁ + m₂)a. Substituting values of m₁ and m₂:

F = (F/a₁ + F/a₂)a

⇒ F = F(1/a₁ + 1/a₂)a

⇒ 1 = (1/a₁ + 1/a₂)a

⇒ a = 1/(1/a₁ + 1/a₂)

⇒ a = 1 / ((a₂ + a₁)/(a₁a₂))

⇒ a = (a₁a₂)/(a₁ + a₂)

Therefore, the resulting acceleration is a = (a₁a₂)/(a₁ + a₂).
In simple words: When you combine two things and push with the same force, the new acceleration depends on both their individual accelerations in a special way.

Exam Tip: Express masses in terms of force and acceleration for each object separately, then combine them using F = ma for the system.

Question 16. When the pole of a bar magnet is brought close to a magnetic compass, the bar magnet and the compass needle (which is also a magnet) exert a magnetic force on each other. As per Newton's third law of motion, both the forces are equal in magnitude and opposite in direction. However, the compass needle moves, whereas the bar magnet does not move (Fig. 6.42). Explain why.
Answer: Per Newton's third law, the bar magnet and the compass needle exert equal and opposite forces on each other. However, motion depends not only on force but also on mass and support. The compass needle has very small mass, so even a small force produces noticeable acceleration and motion. The bar magnet has much larger mass, so the same force produces very little acceleration. Also, the bar magnet is usually held firmly in the hand or kept fixed, so it does not move. Thus, although the forces are equal and opposite, the compass needle moves while the bar magnet appears not to move because the needle has much smaller mass and is free to rotate.
In simple words: Both feel equal forces, but the tiny needle moves easily while the heavy magnet barely moves.

Exam Tip: Remember that equal forces produce different accelerations in objects with different masses - use a = F/m to explain the difference.

Very Short Answer Type Questions

Question 1. What is the SI unit of force?
Answer: The SI unit of force is newton (written with lowercase 'n'). Its symbol is N (uppercase). One newton is the force that produces an acceleration of 1 m/s² in a 1 kg object.
In simple words: Force is measured in newtons (N), and one newton is the push needed to speed up a 1 kg weight by 1 meter per second every second.

Exam Tip: Always use "newton" and symbol "N" - note the lowercase 'n' and uppercase 'N' distinction.

Question 2. State Newton's First Law of Motion in one sentence.
Answer: An object at rest remains at rest and an object in motion continues to move with constant velocity, unless a net (unbalanced) force acts upon it.
In simple words: Things stay as they are unless something pushes them to change.

Exam Tip: Include both parts - objects at rest AND objects in motion - to state the law fully.

Question 3. Define balanced forces.
Answer: Two forces are called balanced when they are equal in magnitude and opposite in direction, resulting in zero net force. Balanced forces produce no change in the state of motion of an object.
In simple words: Balanced forces are equal pushes from opposite sides that cancel each other out.

Exam Tip: State that balanced forces result in zero net force, which means no acceleration or change in motion.

Question 4. What is inertia?
Answer: Inertia is the natural tendency of an object to resist any change in its state of rest or of uniform motion in a straight line. The term was used by Isaac Newton.
In simple words: Inertia means things like to stay as they are - resting things want to stay still, and moving things want to keep going.

Exam Tip: Mention that inertia resists changes in motion - both starting and stopping.

Question 5. Write the mathematical form of Newton's Second Law.
Answer: Newton's Second Law is expressed as F = ma, where F is net force in newtons, m is mass in kilograms, and a is acceleration in m/s². Acceleration acts in the direction of force.
In simple words: Force equals mass times acceleration - a bigger push or heavier thing causes more speed-up.

Exam Tip: Write the formula clearly and note that acceleration direction matches the force direction.

Question 6. What does a negative sign in force indicate?
Answer: A negative sign indicates the force acts in the direction opposite to the chosen positive direction. For example, -3000 N for a westward-moving car means the force is acting westward, opposing eastward motion.
In simple words: A minus sign means the push goes the opposite way from what we called positive.

Exam Tip: Always clarify what "positive direction" means when interpreting negative forces.

Question 7. What is the value of acceleration due to gravity (g) near Earth's surface?
Answer: The acceleration due to gravity near Earth's surface is g = 9.8 m/s². For quick estimations, g = 10 m/s² is used. Importantly, g does not depend on the mass of the falling object.
In simple words: Gravity makes all things speed up at about 10 meters per second every second when falling, no matter how heavy they are.

Exam Tip: Remember both values (9.8 and 10 m/s²) and note that g is the same for all objects.

Question 8. State Newton's Third Law of Motion.
Answer: Whenever object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A. These action-reaction forces always act on two different objects.
In simple words: If A pushes B, then B pushes A back just as hard, but in the opposite way.

Exam Tip: Always mention that action and reaction act on different objects - this is key to understanding why they don't cancel.

Question 9. Why does a fielder pull their hand back while catching a fast cricket ball?
Answer: Pulling the hand back increases the time for the ball to stop. Per Newton's Second Law, longer stopping time means smaller acceleration and therefore smaller force on the hand, reducing injury.
In simple words: By moving the hand back while catching, the fielder makes the ball slow down over a longer time, which makes the force smaller and safer.

Exam Tip: Explain using the relationship F = m(Δv/Δt) - larger Δt means smaller force for the same change in velocity.

Question 10. Give one example of Newton's Third Law involving non-contact forces.
Answer: When two like poles of bar magnets face each other, magnet A repels magnet B with a magnetic force; simultaneously, magnet B repels magnet A with an equal and opposite magnetic force.
In simple words: Two magnets push each other apart with equal force, even without touching.

Exam Tip: Non-contact forces (magnetic, gravitational) also follow Newton's third law - forces always come in equal and opposite pairs.

Question 11. What happens to the net force on a box moving at constant velocity?
Answer: If a box moves at constant velocity, its acceleration is zero. Per Newton's Second Law (F = ma = m × 0 = 0), the net force on the box is zero. Applied force exactly balances friction.
In simple words: When something moves at the same steady speed, all forces pushing on it must add up to zero.

Exam Tip: Constant velocity means zero acceleration, which means zero net force - this is key.

Question 12. How does friction help a person walk?
Answer: When a person pushes the ground backward with their foot, friction acts in the forward direction on the foot, propelling the person forward. Without friction, the foot would slip backward.
In simple words: Your foot pushes the ground back, and friction pushes your foot (and you) forward.

Exam Tip: Use Newton's third law here - the person pushes ground back, ground pushes person forward via friction.

Question 13. What is the formula for gravitational force on an object of mass m?
Answer: The gravitational force on an object of mass m near Earth's surface is F = mg, where g = 9.8 m/s². This force is also called the weight of the object, measured in newtons.
In simple words: The pull of gravity on any object is its mass times 10 (or 9.8 exactly).

Exam Tip: Weight is different from mass - weight is the force of gravity, measured in newtons.

Question 14. For two connected objects of masses m₁ and m₂ pulled by force F, what is the acceleration?
Answer: Using Newton's Second Law for the combined system: a = F/(m₁ + m₂). Only external force F matters; internal forces (tension) cancel within the system and are not separately considered.
In simple words: When you pull two things tied together, they speed up together - divide the push by the total weight.

Exam Tip: Treat connected objects as a single system - use total mass and only external forces.

Question 15. Why is it harder to climb a smooth tree trunk than a rough one?
Answer: When climbing, legs push the trunk down. Friction pushes the person upward (Newton's Third Law). A smooth trunk has less friction, so the upward force is smaller, making climbing much harder.
In simple words: Rough bark grips your hands and feet, giving you a bigger upward push. Smooth trunks have less grip, so you can't hold on as well.

Exam Tip: Explain using action-reaction - you push trunk down, it pushes you up via friction.

Short Answer Type Questions

Question 1. Explain with an example why action-reaction pairs do not cancel each other.
Answer: Action and reaction forces are equal and opposite but act on two different objects. When a paddle pushes water backward (action on water), water pushes the paddle forward (reaction on paddle). Since they act on different objects, they cannot cancel. The canoe moves forward as a result.
In simple words: The paddle pushes water back and water pushes the paddle forward - they're on different things, so they both do something instead of canceling out.

Exam Tip: Always identify which object each force acts on - action and reaction are on different objects, so they don't cancel.

Question 2. A boy pushes a wall with 50 N force. What force does the wall exert on the boy? Does the boy move?
Answer: Per Newton's Third Law, the wall exerts an equal and opposite force of 50 N back on the boy. The boy typically does not move because the wall's reaction force is transmitted through his body; friction from the floor on his feet provides another opposing force, keeping net force on him near zero.
In simple words: The wall pushes back on the boy with 50 N, but the floor grips his feet, so the boy stays still.

Exam Tip: Note that the wall always pushes back (Newton's third law), but the boy may not move if other forces (like friction) balance the reaction force.

Question 3. Distinguish between balanced and unbalanced forces with one example each.
Answer: Balanced Forces: Equal and opposite forces producing zero net force. Example - a book on a table (gravity balanced by normal force). The book remains stationary. Unbalanced Forces: Forces of unequal magnitude producing a nonzero net force. Example - one team pulling harder in tug of war. The rope accelerates toward the stronger team because the net force is nonzero.
In simple words: Balanced forces cancel out and nothing changes. Unbalanced forces don't cancel, so something moves or speeds up.

Exam Tip: Provide both examples - show that balanced forces produce no change, while unbalanced forces cause acceleration.

Question 4. Why are airbags fitted in cars? Explain using Newton's Second Law.
Answer: In a collision, a car stops very abruptly - the passenger's head decelerates very rapidly (large a), requiring a very large force (F = ma), which can cause fatal injuries. The airbag inflates quickly, increasing the stopping time. Per F = m(Δv/Δt), larger Δt means smaller deceleration and therefore smaller force on the passenger's head and chest, preventing serious injury.
In simple words: When you crash and stop suddenly, your head needs a huge force to stop quickly. The airbag makes you stop slowly instead, so the force is smaller and you don't get hurt as badly.

Exam Tip: Use F = m(Δv/Δt) to show that increasing time decreases force for the same change in velocity.

Question 5. How does a rocket lift off? Which Newton's law explains this?
Answer: A rocket engine burns fuel and expels hot exhaust gases downward at high speed (action). Per Newton's Third Law, these gases exert an equal and opposite force upward on the rocket (reaction). When this upward thrust is greater than the rocket's weight (gravitational force downward), the net force is upward and the rocket accelerates skyward.
In simple words: The rocket pushes hot gases down, and those gases push the rocket up with equal force.

Exam Tip: Clearly state that Newton's Third Law explains rockets - action (gases pushed down) and reaction (rocket pushed up).

Question 6. Two forces of 8 N and 5 N act on an object. Find the net force when they act (a) in the same direction and (b) in opposite directions.
Answer: (a) Same direction: Net force equals 8 + 5 = 13 N, in the direction of both forces. (b) Opposite directions: Net force equals 8 - 5 = 3 N, in the direction of the larger (8 N) force. These results follow directly from the vector addition and subtraction of parallel forces.
In simple words: When forces push the same way, add them. When they push opposite ways, subtract them.

Exam Tip: For parallel forces, use simple addition when same direction, subtraction when opposite.

Question 7. What would happen to a moving object if friction suddenly disappeared? Relate this to Newton's First Law.
Answer: If friction disappears, no net force opposes the object's motion. Per Newton's First Law, an object in motion continues with constant velocity indefinitely in the absence of a net force. The object would never slow down or stop on its own. This is exactly what Galileo's thought experiment predicted - remove all impediments, and a body moves forever on a horizontal plane.
In simple words: Without friction to stop it, something moving would keep going forever at the same speed.

Exam Tip: Connect Newton's first law to the thought experiment about motion on frictionless surfaces.

Question 8. Explain why a gun recoils when a bullet is fired.
Answer: When the gun fires, the explosion exerts a large force on the bullet, propelling it forward at very high speed (action on bullet). Per Newton's Third Law, the bullet exerts an equal and opposite force backward on the gun (reaction on gun). This backward force causes the gun to recoil. Because the gun's mass is much greater than the bullet's mass, its recoil acceleration is much smaller than the bullet's forward acceleration.
In simple words: The gun pushes the bullet forward hard, so the bullet pushes the gun backward equally hard. The gun barely moves because it's much heavier.

Exam Tip: Explain the recoil using Newton's third law and show why acceleration is smaller for the heavier gun.

Question 9. From Newton's Second Law, what happens to acceleration if (a) force is doubled at constant mass, and (b) mass is doubled at constant force?
Answer: From a = F/m: (a) If force is doubled (2F) at constant mass m: New acceleration equals 2F/m = 2a, so acceleration doubles. Force and acceleration are directly proportional. (b) If mass is doubled (2m) at constant force F: New acceleration equals F/2m = a/2, so acceleration is halved. Mass and acceleration are inversely proportional.
In simple words: Double the push and you double the speed-up. Double the weight and the speed-up is cut in half.

Exam Tip: Show the math clearly and note the direct/inverse proportionality relationships.

Question 10. A sailor jumps from a small boat to the shore. What happens to the boat and why?
Answer: When the sailor jumps forward toward the shore, they push the boat backward with their feet (action on boat). Per Newton's Third Law, the boat exerts an equal and opposite force on the sailor (reaction on sailor, which helps propel them forward). Since no anchoring force holds the boat, it moves backward - away from the shore - in the direction opposite to the sailor's jump.
In simple words: The sailor pushes off the boat to jump toward the shore, so the boat gets pushed the opposite way (away from the shore).

Exam Tip: Use Newton's third law clearly - identify the action and reaction pair on different objects.

Long Answer Type Questions

Question 1. State and explain Newton's three laws of motion with one real-life example for each.
Answer:
Newton's First Law (Law of Inertia): An object at rest remains at rest and an object in motion continues moving with constant velocity unless a net force acts on it. Example: Passengers lurch forward when a bus brakes suddenly - their bodies resist the change in motion due to inertia.

Newton's Second Law: When a net force acts on an object, it accelerates in the direction of that force. F = ma. Example: A loaded cart needs more force than an empty cart to achieve the same acceleration.

Newton's Third Law: Every action has an equal and opposite reaction on a different object. Example: Jumping is possible because feet push the ground down (action) and the ground pushes the body up (reaction).
In simple words: Things like to stay as they are (first law), harder pushes cause more speed-up (second law), and when you push something, it pushes you back (third law).

Exam Tip: State each law clearly, explain it in simple terms, and provide a real-life example that students can relate to.

Question 2. Describe the force of friction. How does it depend on surfaces? Explain its useful and harmful effects.
Answer: Friction is a contact force that opposes relative motion between surfaces. It always acts opposite to the direction of motion. Its magnitude depends on the nature of surfaces in contact - rougher surfaces produce more friction, smoother surfaces produce less.

Useful effects:
1. Enables walking - friction from ground propels us forward
2. Allows vehicles to brake safely
3. Grooves on tyres and shoe soles increase grip

Harmful effects:
1. Causes wear and tear in machine parts
2. Generates unwanted heat, reducing engine efficiency
3. Wastes energy - vehicles must continuously overcome friction

Friction is reduced using lubricants, ball bearings, polished surfaces, streamlined shapes and magnetic levitation.
In simple words: Friction is the rubbing force between surfaces - rough ones have more friction, smooth ones have less. It helps us walk and stop, but it wears things out and wastes energy.

Exam Tip: Explain both benefits and drawbacks - friction is not "bad," but its effects vary by situation. Provide specific examples and ways to reduce it.

Question 3. A sports car of mass 1500 kg moves east. Its velocity increases from 0 to 10 m/s in 5 s, stays constant for the next 5 s, then decreases to 0 in 5 s. Calculate force in each phase.
Answer:
Phase 1 (0-5 s): Using v = u + at, we get a = (10-0)/5 = 2 m/s². Applying F = ma: F = 1500 × 2 = 3000 N directed eastward. The car speeds up as a net forward force is applied.
Phase 2 (5-10 s): The velocity remains the same, so acceleration becomes 0. Therefore F = ma = 1500 × 0 = 0 N (no net force). Newton's First Law explains this: an object moving at constant velocity needs zero net force acting on it.
Phase 3 (10-15 s): Acceleration is now a = (0-10)/5 = -2 m/s². The force becomes F = 1500 × (-2) = -3000 N. The minus sign shows that the force points westward (opposite to the direction of motion). This braking force slows the car down until it stops.
In simple words: When the car speeds up, a forward force acts (Phase 1). When it moves at the same speed, no force acts (Phase 2). When it slows down, a backward force acts (Phase 3).

Exam Tip: Always show the direction of force (eastward or westward) and explain what the negative sign means - this is what separates full marks from partial credit.

Question 4. Explain Newton's Third Law using the rocket launch. How was it applied in Chandrayaan-3? Support with the balloon rocket activity.
Answer: A rocket engine forces hot gases out downward at very high speed (action). By Newton's Third Law, those gases push back on the rocket with an equal force directed upward (reaction). When this upward push is stronger than the rocket's weight, the net force points upward and the rocket takes off.
In Chandrayaan-3, the Vikram Lander used a technique called retro-firing. The engine fired in the direction the lander was already moving, creating a backward reaction force. This force reduced the lander's speed to exactly the right level for a safe, gentle landing near the Moon's south pole.
The balloon rocket activity demonstrates the same idea. When you let air escape from the balloon, it rushes out backward (action). That escaping air pushes the balloon forward (reaction). This small-scale model shows exactly how rocket propulsion works.
In simple words: Every action has an equal reaction. A rocket pushes gases down, and gases push the rocket up. The Chandrayaan-3 used this to land safely on the Moon.

Exam Tip: Clearly name the action and the reaction, and remember they act on different objects - the gas and the rocket are two separate things.

Question 5. Describe Activities 6.3 and 6.4 that led to Newton's Second Law. Derive F = ma. Explain why a cricket fielder and a car airbag work on the same principle.
Answer:
Activity 6.3 (constant mass, changing force): When the hanging cup's mass doubled, the cart's acceleration also doubled. This proved that acceleration grows in proportion to force when mass stays the same (a ∝ F).
Activity 6.4 (constant force, changing mass): When the cart's mass doubled, its acceleration became half. This showed that acceleration shrinks in proportion to the increase in mass (a ∝ 1/m at constant force).
Derivation: Combining both findings gives us a ∝ F/m, which rearranges to
\( F = ma \)
Cricket Fielder and Airbag - Same Principle: Both depend on the formula F = m(Δv/Δt). The change in velocity (Δv) is already set - the person or object must slow down by a certain amount. By making the stopping time (Δt) longer, we reduce the acceleration and therefore reduce the force.
1. A cricket fielder pulls the hand backward and downward while catching - this stretches out the stopping time. Longer stopping time means less force strikes the hand, so no injury happens.
2. An airbag inflates slowly when a car crashes - it increases the stopping time for the passenger. More stopping time leads to less force on the body, which prevents serious or fatal injury.
In simple words: Force depends on mass and how fast things speed up or slow down. If you spread out the slowing down over more time, the force becomes smaller and causes less harm.

Exam Tip: For applications like fielding and airbags, always mention "stopping time increases" and "force decreases" - examiners specifically mark for understanding this inverse relationship.

Quick Revision for Exam Day

• State all three Newton's laws word-for-word exactly as in the textbook
• Know F = ma, F = mg and a = F/(m1 + m2) - with units
• Can draw position-time and velocity-time graphs for zero net force (at rest and constant velocity)
• Can identify action-reaction pairs correctly in any scenario
• Remember: action and reaction act on different objects - they do NOT cancel each other
• Know that friction depends on the nature of surfaces in contact
• Can solve numericals using v = u + at and s = ut + ½at²
• g = 9.8 m/s² (use 10 m/s² for quick estimations)
• SI unit of force = newton (N); mass = kg; acceleration = m/s²
• Know real-life examples for each of Newton's three laws - at least two per law

Important Formulae