Class 9 Science: How Forces Affect Motion — Chapter Question Set

(a)

(b)

(c)

(a)

(a)

(a)

(d)

(a)

(c)

(d)

(b)

(a)

(d)

(a)

(a)

(a)

(d)

(c)

(b)

(d)

(a)

(b)

(a)

(c)

(a)

(d)

(b)

(b)

(a)

(a)

(d)

(c)

(c)

(d)

(a)

(c)

(a)

(c)

(b)

(a)

(a)

(c)

(a)

(b)

(c)

(d)

(b)

(b)

(d)

(d)

(b)

(c)

(b)

(c)

(a) ---

newton; N

balanced

non-zero net

opposite

net

ma

1 m s⁻²; 1 kg

9.8

equal; opposite

normal

inertia

friction

third

m₁ + m₂

force of friction

True

True

False

False

False

True

False

True

False

True

False

True

False

False

True

(a)

(a)

(a)

(a)

(a)

(c)

(a)

(c) ---

Balanced forces are equal in magnitude and act in opposite directions, so their net force is zero — for example, the two equal pulls on a tug-of-war rope when the teams are evenly matched. Unbalanced forces leave a non-zero net force — for example, when one team pulls harder, the rope moves in that direction.

Newton's first law states that an object at rest stays at rest, and an object in motion continues to move with constant velocity in a straight line, unless acted upon by a net external force. Inertia is the natural tendency of an object to resist any change in its state of rest or of uniform motion.

Newton's second law states that when a net force acts on an object, the object accelerates in the direction of the net force; the acceleration is directly proportional to the net force and inversely proportional to the mass of the object. Mathematically, a = F/m, or equivalently F = ma.

Newton's third law states that whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first. For example, while walking we push the ground backwards, and in return the ground pushes us forward, enabling us to move.

By pulling his hands backward, the fielder increases the time in which the ball's velocity falls to zero. This reduces the ball's rate of change of velocity (a = Δv/Δt), so a much smaller force acts on his hands, preventing injury.

Grooves and treads increase the friction between the sole or tyre and the ground by providing a better interlocking grip. This increased friction prevents slipping and gives a firm hold on the road while walking or driving.

On a wet polished floor or on ice, the friction between our feet and the surface is extremely small. When we push the ground backwards, our foot slips instead of getting a firm grip, and the forward frictional push is too small to move us forward, making walking difficult.

No, this does not violate Newton's first law, because the first law applies only when the net force on an object is zero. Here friction is an unbalanced force acting opposite to the motion, which decelerates the box and brings it to rest.

Since the forces act in opposite directions, the net force is their difference: 8 N − 3 N = 5 N. It acts in the direction of the larger force, i.e. along the 8 N force.

By Newton's second law, F = ma = 1000 kg × 2 m s⁻² = 2000 N. The net force of 2000 N acts in the direction of the acceleration of the car.

Weight is the gravitational force on a body, W = mg. Therefore W = 5 kg × 10 m s⁻² = 50 N, acting vertically downward towards the centre of the Earth.

One newton is defined as the force that produces an acceleration of 1 m s⁻² in an object of mass 1 kg. Hence 1 N = 1 kg m s⁻².

The pipe pushes the water forward with a large force to eject it at high speed. By Newton's third law, the water pushes the pipe backward with an equal and opposite force, which the fireperson has to struggle to hold against.

A rocket changes its velocity by firing its engine and expelling gases. When the gases are expelled backward, they push the rocket forward, speeding it up; expelling gases forward exerts a backward reaction force that slows it down, in accordance with Newton's third law.

While climbing, a person grips the trunk and pushes his legs downward against it. The friction between the trunk and the legs then pushes the person upward with an equal force, allowing the climb — this is harder on smooth, low-friction trunks.

Action and reaction forces are equal in magnitude and opposite in direction, but they always act on two different objects. Only equal and opposite forces acting on the same object can balance or cancel each other, so an action-reaction pair never cancels out.

For the bullet, a = F/m = 2/0.1 = 20 m s⁻²; for the gun, a = 2/5 = 0.4 m s⁻². The same force gives very different accelerations because their masses are very different — the bullet accelerates 50 times more than the gun.

A thought experiment is an imagined experiment whose idealised conditions are difficult or impossible to create in reality. Galileo imagined a surface with all friction (all impediments) removed, and concluded that on such a frictionless horizontal plane a moving body would continue moving forever with constant velocity.

Since both forces act in the same direction, they add up: net force = 300 N + 200 N = 500 N. This net force acts on the car in the direction in which both people are pushing.

Internal forces act between the parts of the system themselves, such as the tension in the string joining the two boxes. External forces act on the system from outside it, such as the applied force F, gravity and the normal reaction from the floor.

Introduction: Forces decide whether an object moves or stays still, as seen in a tug of war between two teams.
Key Points:
1. Balanced forces — Both teams pull with equal magnitude in opposite directions, so the net force on the rope is zero.
2. Unbalanced forces — When one team pulls harder, forces become unequal and the rope moves toward the larger force.
3. Role of net force — Only a non-zero net force can change the state of rest or motion of the rope or any object.
Conclusion: Thus, the motion of an object depends only on the net force acting upon it, not on individual forces.

Introduction: Newton formulated three laws in 1687 that connect force and motion for all objects.
Key Points:
1. First law — An object stays at rest or in uniform motion unless a net force acts; e.g. passengers jerk forward when a bus suddenly brakes.
2. Second law — Acceleration is proportional to net force and inversely proportional to mass; e.g. a harder hit sends a ball farther.
3. Third law — Every action has an equal and opposite reaction; e.g. a rocket expels gas downward and moves upward.
Conclusion: Together, the three laws allow us to predict how any object moves under given forces.

Introduction: Activities with a rubber band and a coin stack demonstrate how friction changes with surface type.
Key Points:
1. Setup — A stretched rubber band launches a taped stack of four coins across different horizontal surfaces.
2. Observation — The coins travel the shortest distance on a rough wooden top and farthest on polished marble.
3. Conclusion — Smaller friction slows the coins more gradually, so they travel farther before coming to rest.
Conclusion: Hence, the force of friction depends on the nature of the surfaces in contact.

Introduction: Newton's second law links force, mass and acceleration through a = F/m for any object.
Key Points:
1. Law relation — For a fixed net force, acceleration is inversely proportional to the mass of the object.
2. Gun-bullet example — A 2 N recoil force acts on a 5 kg gun and a 0.1 kg bullet simultaneously.
3. Calculation — Gun: a = 2/5 = 0.4 m s⁻²; bullet: a = 2/0.1 = 20 m s⁻² — very different accelerations.
Conclusion: Equal action-reaction forces need not give equal accelerations because masses of interacting objects differ.

Introduction: Safety devices work by stretching the time of an impact, which reduces the force experienced.
Key Points:
1. Second law insight — For a given velocity change, smaller acceleration needs a smaller force on the body.
2. Airbags — The inflated soft cushion increases the stopping time of the passenger's head during a collision.
3. Fielder's technique — Pulling hands back with the moving ball increases stopping time, reducing the required force.
Conclusion: By increasing collision time, both devices lower the force on the human body and prevent serious injuries.

Introduction: Newton's second law and kinematics together describe the block's motion after the net force is found.
Key Points:
1. Net force — F(net) = 55 N − 50 N = 5 N, acting in the forward direction of the push.
2. Acceleration — a = F/m = 5 N / 25 kg = 0.2 m s⁻² in the forward direction.
3. Displacement — s = ut + ½at² = 0 + ½ × 0.2 × (2)² = 0.4 m forward.
Conclusion: The block accelerates at 0.2 m s⁻² and covers 0.4 m in the first two seconds.

Introduction: A rocket moves by expelling gas, in accordance with the action-reaction principle.
Key Points:
1. Action — The rocket engine expels hot gas downwards with a large force.
2. Reaction — The gas exerts an equal and opposite upward force on the rocket.
3. Lift-off — When this upward force exceeds the rocket's weight, the net force is upward and it lifts off.
Conclusion: The same principle was used by the Vikram lander of Chandrayaan-3 to slow down for a soft Moon landing.

Introduction: Walking forward relies on friction pushing the foot, not on friction opposing it.
Key Points:
1. Push on ground — While walking, your foot pushes the ground backwards along the surface.
2. Friction's direction — The ground exerts an equal forward frictional force on your foot.
3. Consequence — This forward friction moves you ahead; without it the foot would slip and you would fall.
Conclusion: Thus friction, usually seen as a hindrance, actually enables walking, running and driving.

Introduction: The velocity-time graph reveals the acceleration, from which Newton's second law gives the force.
Key Points:
1. 0–5 s — Constant acceleration a = (10−0)/5 = 2 m s⁻²; F = ma = 1500 × 2 = 3000 N forward.
2. 5–10 s — Flat line means constant velocity; acceleration and hence net force are zero.
3. 10–15 s — a = (0−10)/5 = −2 m s⁻²; F = −3000 N, i.e. 3000 N opposing the motion.
Conclusion: The graph's slope gives acceleration, and F = ma converts it into the force for every phase.

Introduction: Newton's laws can be applied to several connected objects treated as one system.
Key Points:
1. System idea — Two boxes joined by a string on a frictionless surface are pulled by external force F.
2. Internal forces — The tension in the string acts within the system and is not considered.
3. Result — The system accelerates as a single object: a = F/(m₁ + m₂).
Conclusion: Treating connected objects as one system greatly simplifies the analysis of their motion.

Introduction: Newton's second law links the force, the velocity change and the contact time.
Key Points:
1. Convert speed — 108 km h⁻¹ = 108 × 1000/3600 = 30 m s⁻¹; the ball starts from rest.
2. Acceleration — a = F/m = 800 N / 0.4 kg = 2000 m s⁻² during the kick.
3. Time — t = v/a = 30/2000 = 0.015 s of contact between foot and ball.
Conclusion: The foot remains in contact with the ball for about 0.015 s during the penalty kick.

Introduction: Mass measures the quantity of matter, while inertia describes resistance to change of motion.
Key Points:
1. Mass — Mass is the amount of matter in an object and its SI unit is the kilogram.
2. Inertia — Inertia is the tendency of an object to resist any change in its state of rest or motion.
3. Link — A larger mass means larger inertia — a loaded truck is harder to start or stop than a bicycle.
Conclusion: Therefore, mass is the measure of inertia: heavier objects resist changes to motion more strongly.

Introduction: The motion of a canoe follows directly from Newton's third law and second law.
Key Points:
1. Third law — The paddle pushes water backwards; the water pushes the paddle and canoe forward equally.
2. Second law — When the canoeist pushes harder, the larger forward force gives the canoe a larger acceleration.
3. Mass effect — With a passenger aboard, the same paddle force produces a smaller acceleration of the canoe.
Conclusion: The canoe's forward motion and its response to harder paddling follow Newton's second and third laws.

Introduction: The combined opposing forces decelerate the object until it comes to rest.
Key Points:
1. Total opposing force — F = 7 N + 3 N = 10 N opposite to the motion of the object.
2. Deceleration — a = F/m = 10 N / 2 kg = 5 m s⁻² against the direction of motion.
3. Stopping distance — v² = u² − 2as → 0 = 10² − 2×5×s, giving s = 10 m.
Conclusion: The object travels 10 m along the rough patch before it finally comes to rest.

Introduction: The jump demonstrates Newton's third law between the sailor and the boat.
Key Points:
1. Action — The sailor's legs exert a backward force on the boat while jumping toward the shore.
2. Reaction — The boat exerts an equal and opposite forward force on the sailor, launching them ahead.
3. Boat's motion — The backward force on the light boat gives it a noticeable backward acceleration.
Conclusion: The boat moves backward because the action force acts on it while the reaction propels the sailor forward.

(a) The coins travelled the largest distance on the polished marble floor, because it offers the least friction among the three surfaces.
(b) It indicates that after release only the frictional force acts on the coins, opposing their motion and gradually decelerating them to rest.
(c) On a perfectly frictionless floor the coins would continue moving with constant velocity forever, as stated by Newton's first law.
(d) They could use a spring balance, pulled horizontally with a wooden block on each surface, to measure the force of friction.

(a) The applied force exactly balances the maximum friction, so the net force is zero and the block remains stationary.
(b) Net force = 55 − 50 = 5 N; acceleration a = F/m = 5/25 = 0.2 m s⁻².
(c) Displacement s = ½at² = ½ × 0.2 × (2)² = 0.4 m.
(d) Newton's second law of motion (F = ma) was used, along with a kinematic equation for displacement.

(a) He pulls his hands backwards to increase the time in which the ball's velocity becomes zero, which reduces the hurt or injury.
(b) Newton's second law of motion is involved, since the force depends on the rate of change of velocity.
(c) Force is proportional to the rate of change of velocity; by stretching the stopping time, the acceleration of the ball is reduced and hence the force on the hands becomes smaller.
(d) Airbags in vehicles work on the same principle — they inflate to cushion the passenger and increase the stopping time during a collision.

(a) Acceleration during 0–5 s = 10/5 = +2 m s⁻²; during 5–10 s = 0; during 10–15 s = (0 − 10)/5 = −2 m s⁻².
(b) Net force = ma: during 0–5 s it is 1500 × 2 = 3000 N east; during 5–10 s it is 0 N; during 10–15 s it is 1500 × (−2) = −3000 N, i.e. 3000 N west.
(c) The net force is zero during 5–10 s, because the car moves with constant velocity, so its acceleration and hence F = ma are zero.
(d) During 10–15 s the force acts opposite to the direction of motion, i.e. towards the west.

(a) Newton's third law of motion explains how rockets change their velocity in space.
(b) The engine throws the exhaust gases forward, and the gases exert an equal and opposite reaction force backward on the spacecraft, reducing its velocity.
(c) The expelled gas pushes the rocket upward with an equal force; when this upward force exceeds the rocket's weight, the net force lifts the rocket off the ground.
(d) The equal and opposite forces act on two different objects — the gas and the rocket — so they cannot cancel each other.

(a) The two spring balances always show equal readings, whatever force is applied.
(b) The experiment verifies Newton's third law of motion — the mutual forces of action and reaction are equal and opposite.
(c) The two forces would then balance each other, giving zero net force and producing no change in the object's state of motion.
(d) Rowing a canoe is an example — the paddle pushes the water backwards, and the water pushes the canoe forward with an equal force.

(a) By Newton's third law, the gun experiences an equal and opposite recoil force of 2 N backward.
(b) Bullet: a = F/m = 2/0.1 = 20 m s⁻²; gun: a = 2/5 = 0.4 m s⁻².
(c) The accelerations differ because acceleration a = F/m is inversely proportional to mass, and the masses of the bullet and the gun differ greatly.
(d) The heavier gun's larger mass gives it a much smaller recoil acceleration and hence a smaller recoil velocity, so it hurts the shoulder less.

(a) Total mass = 10 kg (bar) + 10 kg + 10 kg (plates) = 30 kg.
(b) Gravitational force F = mg = 30 × 9.8 = 294 N, directed vertically downward.
(c) To hold the barbell steady she must apply an equal force of 294 N in the upward direction, so that the net force is zero.
(d) No, she does no work on the barbell because there is no displacement of the barbell while she holds it steady, although her muscles tire internally.

(a) A larger force is required for Kabir, the heavier child.
(b) By Newton's second law, a = F/m; for the same acceleration, the required force F = ma is directly proportional to the mass.
(c) a = F/m = 60/40 = 1.5 m s⁻².
(d) F = ma = 20 × 1.5 = 30 N would give Riya's swing the same acceleration.

(a) The 95 oarsmen together apply 95 × 200 N on the water backwards, and by reaction the water pushes the boat forward with 19000 N.
(b) The 5 mistaken oarsmen apply 5 × 200 N = 1000 N in the reverse direction, i.e. backward on the boat.
(c) Net horizontal force = 19000 − 1000 = 18000 N in the forward direction.
(d) Newton's third law (the water pushes the boat forward as the oars push the water back) and Newton's second law (the net force accelerates the boat) are used.

Introduction: Newton's three laws, presented in 1687, form the foundation of classical mechanics.
Key Points:
1. First law — An object stays at rest or in uniform motion unless acted upon by a net external force.
2. Second law — The acceleration produced is proportional to the net force and inversely proportional to mass (F = ma).
3. Third law — For every action, there is a simultaneous equal and opposite reaction on the other object.
4. Example — first law — A book lying on a table remains at rest until someone pushes it with a net force.
5. Examples — second and third laws — A harder kick sends a ball farther; the recoil of a gun when a bullet is fired.
Conclusion: Together, these laws explain and predict the motion of objects from everyday life to planets.

Introduction: Real objects usually experience several forces at once, whose combined effect decides the motion.
Key Points:
1. Balanced forces — Equal and opposite forces on the same object give zero net force and no change of motion.
2. Example — balanced — A tug-of-war rope pulled equally by both teams does not move at all.
3. Unbalanced forces — Unequal or non-cancelling forces leave a net force that accelerates the object.
4. Example — unbalanced — A box pushed with 55 N against 50 N friction accelerates due to a 5 N net force.
5. Key idea — Only the net force matters for motion; individual forces may act but cancel out.
Conclusion: Hence, whether an object starts moving, stops, or maintains its motion depends solely on the net force.

Introduction: Two classroom activities together show how friction varies and how it can be measured.
Key Points:
1. Activity 6.1 — A stretched rubber band launches a coin stack on different surfaces — wood, laminate, marble.
2. Observation — The stack travels farthest on marble and shortest on wood, i.e. least friction on marble.
3. Activity 6.2 — A spring balance dragged with a wooden block measures the friction at the moment of sliding.
4. Result — The spring balance reading is smallest for marble and largest for the wooden table top.
5. Conclusion — Smaller friction means slower deceleration and larger travel distance before the coins stop.
Conclusion: These activities establish that friction depends on the nature of the surfaces in contact.

Introduction: The need for continuous pedalling or pushing in real life seems to contradict the first law but does not.
Key Points:
1. Real situation — Friction always acts on moving objects, opposing their motion on any surface.
2. Effect of friction — Friction is an unbalanced force that decelerates the object until it stops.
3. Why push continuously — A continuous applied force merely cancels friction, making the net force zero.
4. First law reconciliation — The first law speaks of zero NET force; with friction balanced, velocity stays constant.
5. Ideal case — On a frictionless surface, an object would need no force to keep moving uniformly forever.
Conclusion: Thus Newton's first law holds perfectly once all forces, including friction, are properly accounted for.

Introduction: A velocity-time graph encodes acceleration through its slope, which Newton's second law converts to force.
Key Points:
1. Phase 0–5 s — Straight inclined line: a = 2 m s⁻²; F = 1500 × 2 = 3000 N towards the east.
2. Phase 5–10 s — Line parallel to time axis: a = 0; net force is zero though the car keeps moving.
3. Phase 10–15 s — Inclined falling line: a = −2 m s⁻²; F = −3000 N, i.e. 3000 N towards the west.
4. Slope meaning — Positive slope means accelerating force; zero slope means zero net force.
5. Negative slope — A negative slope indicates a force opposite to the direction of motion.
Conclusion: The slope of the v-t graph, combined with F = ma, gives the magnitude and direction of the force.

Introduction: Inertia is the resistance of matter to changes in its state of motion, an idea rooted in Galileo's work.
Key Points:
1. Meaning of inertia — Inertia is the tendency of an object to remain at rest or continue in uniform motion.
2. Everyday examples — Passengers lurch backward when a bus starts and forward when it brakes suddenly.
3. Galileo's thought experiment — Galileo imagined a body moving on a horizontal plane with all impediments removed.
4. Conclusion drawn — Such an ideal body would continue moving indefinitely without any force.
5. Newton's contribution — Newton coined the word 'inertia' and used Galileo's idea to frame his first law.
Conclusion: Galileo's reasoning and Newton's formulation together established that uniform motion needs no cause.

Introduction: Newton's second law quantitatively relates the net force on an object to the acceleration it produces.
Key Points:
1. Statement — The object accelerates along the net force; a ∝ F and a ∝ 1/m, giving a = F/m.
2. Definition of newton — F = ma with m = 1 kg and a = 1 m s⁻² defines 1 N = 1 kg m s⁻².
3. First calculation — F = ma = 5 kg × 2 m s⁻² = 10 N is the applied force.
4. Second calculation — Same 10 N on 10 kg gives a = F/m = 10/10 = 1 m s⁻².
5. Insight — For a fixed force, doubling the mass halves the acceleration produced.
Conclusion: The relation F = ma lets us predict accelerations, forces or masses whenever two are known.

Introduction: Newton's third law appears in many everyday phenomena wherever two objects interact.
Key Points:
1. Statement — When object A pushes B, B simultaneously pushes A with equal force in the opposite direction.
2. Walking — Feet push the ground backward; the ground's forward friction pushes the walker ahead.
3. Rowing a canoe — The paddle pushes water backward; the water pushes the paddle and canoe forward.
4. Rocket propulsion — The engine expels gas downward; the gas pushes the rocket upward with equal force.
5. No cancellation — Action and reaction act on two different objects, so they never balance each other.
Conclusion: Every interaction involves a pair of equal, opposite forces acting on the two bodies involved.

Introduction: Connected objects can be analysed simply by treating them as one system.
Key Points:
1. System view — The two boxes plus the string form one system of total mass m₁ + m₂ = 5 kg.
2. Internal force — The tension in the string acts between the boxes and is internal to the system.
3. External force — The applied 10 N pull is the only external horizontal force on the system.
4. Acceleration — a = F/(m₁ + m₂) = 10/5 = 2 m s⁻² for both boxes together.
5. Glued case — Gluing changes nothing — the system still accelerates at 2 m s⁻² as one object.
Conclusion: This shows the power of Newton's laws: complicated systems behave like single objects of total mass.

Introduction: Newton's laws explain why impact time determines the damage in collisions and strikes.
Key Points:
1. Airbags — The soft cushion lengthens the stopping time of the passenger, reducing the force on the body.
2. High-jump mat — The sand bed/mat deforms, increasing stopping time so the athlete lands gently without injury.
3. Bubble wrap — The wrapping increases the impact time for glass items, so a much smaller force reaches them.
4. Coconut cracking — The coconut stopped in a very short time by hard ground experiences a huge breaking force.
5. Common principle — Force = mass × acceleration; longer stopping times mean smaller accelerations and forces.
Conclusion: All four situations show that stretching or shortening the collision time controls the force experienced.