CBSE Class 9 Science Chapter 06 How Forces Affect Motion MCQs Set 01

Science Objective Questions and Answers: Chapter 06 How Forces Affect Motion

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Question: When a canoeist pushes water backward with a paddle, what makes the canoe move forward?
A. The water pushes back on the paddle with an equal and opposite force
B. The canoe's own weight creates forward thrust
C. The buoyant force of water acts forward
D. The canoeist's body weight shifts forward
Show Answer & Explanation

Answer: (A) The water pushes back on the paddle with an equal and opposite force

Explanation:
This is a direct application of Newton's third law: the paddle pushes water backward, and the water simultaneously pushes the paddle (and hence the canoe) forward with an equal force, since the two forces act on different objects and don't cancel out.

Question: Why does a fresh coconut crack open when dropped forcefully onto hard ground?
A. The ground exerts a very large force on it because its velocity changes to zero in a very short time
B. The coconut's shell becomes lighter upon impact
C. Gravity increases suddenly on contact
D. The coconut loses its acceleration before hitting the ground
Show Answer & Explanation

Answer: (A) The ground exerts a very large force on it because its velocity changes to zero in a very short time

Explanation:
A rapid change in velocity over a very short time interval demands a large force, as force relates to how quickly momentum or velocity changes; this large force is what breaks the shell.

Question: A fielder pulls their hands backward while catching a fast cricket ball. What is achieved by this action?
A. It increases the time taken for the ball's velocity to reach zero, lowering the required force
B. It increases the mass of the ball temporarily
C. It reverses the direction of the ball's momentum instantly
D. It removes the effect of friction between the ball and hand
Show Answer & Explanation

Answer: (A) It increases the time taken for the ball's velocity to reach zero, lowering the required force

Explanation:
By stretching out the duration over which the ball slows down, the fielder reduces the acceleration needed to stop it, and since force equals mass times acceleration, a smaller acceleration means a smaller force acts on the hand — reducing injury.

Question: In the rubber-band-and-coins activity, why does the stack of coins eventually come to rest after leaving the rubber band?
A. The force of friction acts opposite to its motion and gradually reduces its velocity
B. The rubber band continues pulling it backward
C. The coins lose mass as they slide
D. The surface pushes the coins upward, stopping them
Show Answer & Explanation

Answer: (A) The force of friction acts opposite to its motion and gradually reduces its velocity

Explanation:
Once contact with the rubber band ends, only friction acts on the moving coins, opposing their motion and steadily decreasing their speed until they stop.

Question: Which pair of surfaces from the chapter's activity would most likely allow the coin stack to travel the greatest distance before stopping?
A. Polished marble or tiled floor
B. Wooden table top
C. Cemented floor
D. Laminated table top with rough patches
Show Answer & Explanation

Answer: (A) Polished marble or tiled floor

Explanation:
Smoother surfaces like polished marble offer the least resistance to sliding, so friction is weakest there, letting the coins travel farther before their velocity drops to zero.

Question: A block remains stationary even though a person is pushing it with 50 N of force. What can be concluded about the forces acting on it?
A. The applied force is exactly balanced by an equal force of friction
B. No force of friction is acting on the block at all
C. The block has zero mass
D. The normal force exceeds the gravitational force
Show Answer & Explanation

Answer: (A) The applied force is exactly balanced by an equal force of friction

Explanation:
If a push does not move an object, the net force must be zero — meaning friction is matching the applied force in magnitude but acting in the opposite direction, as explained through the block-pushing example in the text.

Question: According to the chapter, what historical shift in understanding did Galileo's thought experiments bring about?
A. He showed that no force is needed to keep an object moving at constant velocity once friction is removed
B. He proved that heavier objects always move faster than lighter ones
C. He discovered the exact numerical value of gravitational acceleration
D. He established that force is always proportional to an object's weight
Show Answer & Explanation

Answer: (A) He showed that no force is needed to keep an object moving at constant velocity once friction is removed

Explanation:
Before Galileo, people believed a continuous force was necessary to sustain motion. Galileo argued that if all resistance to motion were removed, an object would keep moving forever without needing any force — laying groundwork for Newton's first law.

Question: In Activity 6.3 with the cart and pulley system, doubling the pulling force on the same cart mass should ideally double the acceleration, but the actual measured increase is usually somewhat less. What explains this gap?
A. Friction between the cart's wheels and the surface affects the results
B. The pulley itself gains mass during the experiment
C. The string used stretches permanently
D. Gravity acts differently on heavier weights
Show Answer & Explanation

Answer: (A) Friction between the cart's wheels and the surface affects the results

Explanation:
Real-world friction between the wheels and the table introduces resistance not accounted for in the ideal calculation, so the measured acceleration falls a bit short of doubling as theory would suggest without friction.

Question: Why are grooves cut into the soles of shoes and treads placed on vehicle tyres?
A. To increase friction so the foot or tyre does not slip backward while pushing against the ground
B. To decrease the weight of the footwear or tyre
C. To make the shoes or tyres last longer regardless of grip
D. To reduce the normal force acting on the ground
Show Answer & Explanation

Answer: (A) To increase friction so the foot or tyre does not slip backward while pushing against the ground

Explanation:
Walking and driving both depend on friction pushing the foot or tyre forward as it presses backward on the ground; grooves and treads boost this grip, preventing slipping, which is especially important on wet or icy surfaces.

Question: Two identical spring balances are hooked together and pulled from opposite ends. What does the activity demonstrate about the readings on both balances?
A. The readings are always equal, showing that the forces the balances exert on each other are equal in magnitude
B. The balance held stationary always reads higher
C. The readings differ depending on which hand pulls harder
D. Only the moving balance shows a nonzero reading
Show Answer & Explanation

Answer: (A) The readings are always equal, showing that the forces the balances exert on each other are equal in magnitude

Explanation:
Since both scales consistently show identical readings no matter how much force is applied, the activity confirms Newton's third law — the two objects exert equal and opposite forces on each other.

Question: Consider two boxes connected by a string on a frictionless surface, with an external force F pulling one box. What role does the string tension play when the two boxes are treated as a single system?
A. It is an internal force and can be ignored while calculating the system's overall acceleration
B. It is the only force considered while finding the system's acceleration
C. It cancels out the external force entirely
D. It acts as the sole force responsible for the system's motion
Show Answer & Explanation

Answer: (A) It is an internal force and can be ignored while calculating the system's overall acceleration

Explanation:
When boxes and string are viewed as one combined system, forces exchanged between them internally (like tension) don't affect the system's net acceleration — only the outside applied force matters, simplifying the calculation to F divided by total mass.

Question: In the bullet-and-gun example, both experience a recoil force of equal magnitude, yet the bullet accelerates far more than the gun. What accounts for this difference?
A. The bullet has much smaller mass, so the same force produces a much larger acceleration in it
B. The gun does not actually experience any force
C. The bullet is affected by an extra unseen force
D. Acceleration depends only on force, not on mass
Show Answer & Explanation

Answer: (A) The bullet has much smaller mass, so the same force produces a much larger acceleration in it

Explanation:
Since a = F/m, a lighter object like the bullet accelerates much more than a heavier one like the gun even when they experience forces of identical magnitude — this is why the recoil felt by the gun is much gentler than the bullet's launch.

Question: What is the key idea conveyed by the airbag and coconut examples together in this chapter?
A. Changing the time over which velocity changes alters the magnitude of force involved, whether reducing injury or breaking a shell
B. Airbags and coconuts both rely on gravity to function
C. Both examples show that friction is the main force involved in stopping motion
D. Both examples demonstrate that mass has no effect on force
Show Answer & Explanation

Answer: (A) Changing the time over which velocity changes alters the magnitude of force involved, whether reducing injury or breaking a shell

Explanation:
• Airbags increase the time taken for a passenger's velocity to drop to zero, reducing the force experienced and lowering injury risk
• A dropped coconut hits the ground and stops almost instantly, so the very short time results in a very large force strong enough to break its shell
• Both cases illustrate how, for the same change in velocity, a shorter or longer duration changes the force involved

Question: A rocket's engine expels exhaust gas downward during launch. Based on the chapter's explanation, what ultimately allows the rocket to lift off the ground?
A. The upward reaction force from the expelled gas exceeds the rocket's weight, producing a net upward force
B. The exhaust gas pulls the rocket upward through suction
C. The rocket becomes weightless once the engine starts
D. Air pressure beneath the rocket pushes it upward
Show Answer & Explanation

Answer: (A) The upward reaction force from the expelled gas exceeds the rocket's weight, producing a net upward force

Explanation:
As the engine pushes gas downward, the gas pushes back on the rocket upward with equal force (Newton's third law); when this upward force is greater than the rocket's weight, the net force points upward, causing the rocket to accelerate off the ground.

Question: How is the SI unit of force, the newton, formally defined in terms of mass and acceleration?
A. The force needed to give a 1 kg object an acceleration of 1 m/s^2
B. The force needed to give a 10 kg object an acceleration of 1 m/s^2
C. The force needed to give a 1 kg object a velocity of 1 m/s
D. The weight of a 1 kg object at the Earth's surface
Show Answer & Explanation

Answer: (A) The force needed to give a 1 kg object an acceleration of 1 m/s^2

Explanation:
Using F = ma, when mass equals 1 kg and acceleration equals 1 m/s^2, the resulting force works out to exactly 1 kg m/s^2, which is defined as 1 newton. Confusing this with weight (option d) is a common error since weight depends on g, not just the definition of the unit.

Question: Why is force always described as having both magnitude and direction rather than just a size?
A. Because it behaves like a vector quantity, similar to velocity and acceleration
B. Because its unit changes depending on the surface it acts on
C. Because only gravitational and magnetic forces need direction
D. Because direction only matters when two forces act together
Show Answer & Explanation

Answer: (A) Because it behaves like a vector quantity, similar to velocity and acceleration

Explanation:
The chapter groups force with position, displacement, velocity and acceleration as quantities needing both magnitude and direction. Every example given, such as friction opposing motion or buoyant force acting upward, shows direction is essential to fully describe the force.

Question: A weightlifter holds a steady barbell whose total mass is 20 kg. Taking g as 9.8 m/s^2, how much upward force must she apply?
A. 98 N
B. 196 N
C. 20 N
D. 9.8 N
Show Answer & Explanation

Answer: (B) 196 N

Explanation:
The downward gravitational pull on the barbell is F = mg = 20 kg x 9.8 m/s^2 = 196 N. Since the barbell stays steady, the lifter must supply an equal force in the opposite (upward) direction to balance it.

Question: A book rests on a table without sinking into it or flying off. What does this tell us about the gravitational force and the normal force acting on it?
A. They are equal in magnitude and opposite in direction, so they balance
B. They act in the same direction and add up
C. The normal force is much larger than the gravitational force
D. There is no gravitational force acting on the book
Show Answer & Explanation

Answer: (A) They are equal in magnitude and opposite in direction, so they balance

Explanation:
The weight pulls the book downward while the surface pushes back upward with the normal force; since the book neither moves down nor up, these two forces must cancel each other out, leaving no net vertical force.

Question: Why does a person often need to push harder than expected to get a heavy stationary box moving, even though the box eventually slides once moving?
A. The floor's normal force increases as the box speeds up
B. The initial force of friction opposes the applied push until the applied force exceeds it
C. Gravity acts more strongly on stationary objects
D. The box's mass increases while it is at rest
Show Answer & Explanation

Answer: (B) The initial force of friction opposes the applied push until the applied force exceeds it

Explanation:
As long as the pushing force is smaller than or equal to the opposing friction, the net force is zero and the box stays put. Only once the applied force grows larger than this friction does a net forward force appear, allowing the box to start moving.

Question: In the spring-balance activity where a block is pulled across different surfaces, what does a smaller reading at the moment the block just starts moving suggest about that surface?
A. The surface offers a larger force of friction
B. The surface offers a smaller force of friction
C. The block's mass is smaller on that surface
D. The reading has no connection to friction
Show Answer & Explanation

Answer: (B) The surface offers a smaller force of friction

Explanation:
The spring balance reading at the instant of starting motion approximately equals the friction force being overcome; a lower reading means less friction had to be overcome, and such surfaces earlier also let the coin stack slide farther before stopping.

Question: Newton used a particular term to describe an object's natural tendency to resist any change in its state of rest or steady motion. What is this term?
A. Momentum
B. Inertia
C. Acceleration
D. Tension
Show Answer & Explanation

Answer: (B) Inertia

Explanation:
This tendency, named inertia by Newton, forms the basis of his first law of motion — that objects continue in their state of rest or uniform velocity unless a net force disturbs them.

Question: According to Newton's third law, a swimmer's hand pushes water backward and the water pushes the hand forward with equal force. Why do these two equal and opposite forces not cancel each other out?
A. Because one force is much smaller due to water's low density
B. Because the forces act on two different objects, not on the same object
C. Because friction between water and hand absorbs one of the forces
D. Because the forces act only for an instant and are ignored
Show Answer & Explanation

Answer: (B) Because the forces act on two different objects, not on the same object

Explanation:
Forces cancel only when they act simultaneously on the same object. Here, one force acts on the water and the other acts on the hand, so together they produce motion instead of balancing out.

Question: When someone climbs a coconut tree by pressing their legs against the trunk, which force actually lifts the climber upward?
A. The gravitational force from the trunk
B. The normal force from the ground below
C. The force of friction between the legs and the trunk, reacting to the climber's downward push
D. The buoyant force of surrounding air
Show Answer & Explanation

Answer: (C) The force of friction between the legs and the trunk, reacting to the climber's downward push

Explanation:
As the climber's legs push down against the trunk, the trunk exerts an equal and opposite frictional force upward on the legs; this is why smoother trunks with less friction are much harder to climb.

Question: Two boxes connected by a string are pulled across a frictionless surface by an external force F. Why can the string's tension be left out when finding the acceleration of the combined system?
A. Tension is an internal force that acts equally within the system and does not affect its overall acceleration
B. Tension is always zero in such systems
C. Tension only matters when the surface has friction
D. Tension cancels the effect of F completely
Show Answer & Explanation

Answer: (A) Tension is an internal force that acts equally within the system and does not affect its overall acceleration

Explanation:
When two connected objects are treated as one system, only forces from outside the system determine its acceleration. The tension pulls one box forward and the other backward internally, so it has no net effect on the system's overall motion, unlike the external force F.

Question: In the block example where a 6 N force pushes toward the right and a 10 N force pushes toward the left at the same time, what is the resulting net force on the block?
A. 16 N acting to the right
B. 4 N acting to the left
C. 4 N acting to the right
D. 16 N acting to the left
Show Answer & Explanation

Answer: (B) 4 N acting to the left

Explanation:
When two forces act in opposite directions, the net force equals their difference and points along the larger one. Here 10 N minus 6 N leaves 4 N directed toward the left, since the leftward force was stronger.

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