Looking for reliable SEBA Class 9 Science Chapter 9 (Force and Laws of Motion) extra questions and answers for the 2026–27 academic year? Access important short questions, conceptual explanations on inertia and momentum, worked numericals on $F = ma$, and exam-oriented practice notes to score top marks.
Extra question and answer
- Explain balanced and unbalanced forces with examples. How do they affect the motion of an object?
Answer: Forces act on objects in pairs. When two or more forces act on a body and cancel each other out, they are called balanced forces. In this case, the object does not change its state of rest or uniform motion.
- Example: If two people pull a rope from opposite sides with the same force, the rope does not move. A book lying on a table also remains at rest because the downward gravitational weight and upward normal force are balanced.
Unbalanced forces do not cancel each other, and the net force acting on the object is non-zero. They cause a change in the state of motion, speed, or direction of an object.
- Example: When one team in a tug-of-war pulls harder than the other, the rope moves toward the stronger team. A stationary football on the ground moves when a player kicks it because an unbalanced force acts on it.
Thus, balanced forces do not cause motion, while unbalanced forces produce motion, change speed, or alter direction.
- What is friction? Explain with examples how friction affects motion in daily life.
Answer: Friction is a force that opposes the relative motion between two surfaces in contact. It always acts in the direction opposite to the direction of motion.
Effects of Friction:
- Friction prevents objects from moving easily: When we push a heavy box, it does not move until the applied force overcomes the maximum static friction.
- Friction slows down moving objects: A bicycle slows down when we stop pedaling due to friction between the tires and the road, as well as among moving mechanical parts.
- Friction can stop motion completely: A rolling ball on the ground eventually comes to rest due to ground friction.
- Friction is useful: We can walk without slipping because friction provides grip, vehicles move safely on roads due to tire grip, and writing with a pen or pencil is possible due to friction between the tip and paper.
Thus, friction resists motion, but it is essential for daily activities.
SEBA Class 9 Science Chapter 9 Extra Questions
- State Newton’s First Law of Motion. Why is it called the law of inertia? Explain with examples.
Answer: Newton’s First Law states: “An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.”
This means:
- A body at rest will stay at rest.
- A moving body will continue moving with the same speed and direction.
- Motion changes only when an external unbalanced force acts.
It is called the Law of Inertia because it defines inertia—the inherent property of an object to resist any change in its state of rest or uniform motion.
Examples:
- When a bus stops suddenly, passengers fall forward because their upper bodies were in motion and tend to stay in motion.
- When a bus starts moving suddenly, passengers fall backward because their bodies were at rest and tend to stay at rest.
- A coin placed on a cardboard card drops vertically into a glass when the card is flicked sharply because the coin tends to remain at rest due to inertia.
- What is inertia? Explain the different types of inertia with examples.
Answer: Inertia is the natural property of an object to resist any change in its state of rest or uniform motion. It depends directly on mass—heavier objects have greater mass and therefore greater inertia.
Types of Inertia:
- Inertia of Rest: The tendency of a body to remain at rest.
- Example: Dust particles on a carpet remain at rest until the carpet is beaten with a stick, causing the carpet to move and the dust to fall off.
- Inertia of Motion: The tendency of a moving body to continue moving in a straight line at a constant speed.
- Example: A moving bicycle continues moving forward for some distance even after we stop pedaling.
- Inertia of Direction: The tendency of a body to maintain its direction of motion.
- Example: When a car takes a sharp turn, passengers lean outward because their bodies tend to continue moving along the original straight path.
Mass is the physical measure of inertia. For instance, a stone has more inertia than a rubber ball of the same size.
Class 9 Science Chapter 9 Important Questions SEBA 2026-27
- Define momentum. Derive the relation between force, mass, and acceleration using the concept of momentum.
Answer: Momentum (p) is defined as the product of the mass (m) and velocity (v) of an object. It is a vector quantity.
p = m * v
Derivation of F = m * a (Newton’s Second Law):
Let:
- Mass of object = m
- Initial velocity = u
- Final velocity = v
- Time taken = t
Initial momentum (p_i) = m * u
Final momentum (p_f) = m * v
Change in momentum = m * v – m * u = m * (v – u)
Rate of change of momentum = [m * (v – u)] / t
According to Newton’s Second Law, the applied force (F) is directly proportional to the rate of change of momentum:
F ∝ [m * (v – u)] / t
F = k * [m * (v – u)] / t
In SI units, the constant of proportionality k = 1. Since acceleration a = (v – u) / t, substituting this gives:
F = m * a
This is the mathematical formula for Newton’s Second Law of Motion.
- Explain Newton’s Second Law of Motion with daily life examples.
Answer: Newton’s Second Law states that the rate of change of momentum of an object is directly proportional to the applied unbalanced force and takes place in the direction in which the force acts.
Mathematically: F = m * a
Examples:
- A cricket fielder pulls their hands backward while catching a fast-moving ball: This increases the time taken to bring the ball’s velocity to zero, thereby decreasing the rate of change of momentum and reducing the impact force on the hands to prevent injury.
- Pushing a heavy car versus a light scooter: A car has more mass and greater inertia, requiring a much larger force to achieve the same acceleration as a lighter scooter.
- High impact of a bullet: A bullet has a small mass but an extremely high velocity, resulting in a large momentum that exerts a massive force upon impact.
- Kicking a football versus a stone: A football moves easily when kicked, whereas a heavy stone barely moves and hurts the foot because its larger mass requires much more force to accelerate.
Force and Laws of Motion Class 9 SEBA Extra Question Answer
- State Newton’s Third Law of Motion. Give three examples to explain it.
Answer: Newton’s Third Law states: “To every action, there is an equal and opposite reaction, and they act on two different bodies.”
Examples:
- Walking on the ground: When walking, our foot pushes the ground backward (action). In response, the ground exerts an equal and opposite force pushing us forward (reaction).
- Rocket launch: Hot gases produced by burning fuel are expelled downward at high velocity through the nozzle (action). These gases exert an equal and opposite upward force on the rocket (reaction), lifting it.
- Recoil of a gun: When a bullet is fired, the gun exerts a forward force on the bullet (action). The bullet exerts an equal and opposite backward force on the gun (reaction), causing it to recoil.
- Jumping from a boat: As a person leaps forward onto the bank (action), they push the boat backward into the water (reaction).
- Why does a horse find it difficult to pull a cart at rest but easier once it starts moving?
Answer: When a cart is at rest, the horse must overcome static friction between the cart wheels and the ground. Static friction is the maximum resistive force that opposes the initiation of motion.
Once the cart starts moving, the static friction transitions into kinetic (sliding/rolling) friction. Because kinetic friction is noticeably smaller than static friction, less force is required to keep the cart moving than was needed to start it moving initially. Therefore, the horse finds it much easier to maintain motion once the cart is already moving.
SEBA Class 9 Science Chapter 9 Practice Numericals
- Why does a person sitting in a bus fall in the forward direction when the bus stops suddenly? Explain using Newton’s laws.
Answer: When a bus is in motion, a person sitting inside moves along with the bus at the same speed.
When the driver applies the brakes and the bus stops suddenly, the lower part of the passenger’s body, which is in direct contact with the bus seat and floor, comes to rest immediately. However, the upper part of the body tends to continue moving forward at the original speed due to the inertia of motion.
According to Newton’s First Law of Motion, an object continues in its state of motion unless acted upon by an external unbalanced force. Because no stopping force acts directly on the upper body, it keeps moving forward, causing the passenger to lunge forward.
- What is the importance of friction in our daily life? Should friction be increased or reduced? Explain.
Answer: Friction plays a critical role in daily life—it can be both beneficial and disadvantageous depending on the context.
Importance of Friction:
- Walking: Friction between shoe soles and the ground prevents slipping.
- Transportation: Tire treads utilize friction against the road to provide traction and enable braking.
- Writing: Friction between the pen tip and paper allows ink to transfer and deposit properly.
- Handling objects: Friction permits holding glass, tools, and objects securely without dropping them.
Increasing or Reducing Friction:
- Increasing friction: Required where extra grip or stability is needed. Examples include grooving tire treads, spiking athletic shoes, and using brake pads in vehicles.
- Reducing friction: Required where smooth motion is needed and energy loss or wear needs to be minimized. Examples include using lubricants (oil/grease) in machinery, applying powder on a carrom board, and using ball bearings in wheel hubs.
Class 9 Science Chapter 9 Important Short Questions
🔬 Updated Extra Questions Guide Notice: This page features an extensive collection of SEBA Class 9 Science Chapter 9 (Force and Laws of Motion) Extra Questions and Detailed Answers updated for the 2026–27 academic session. All conceptual notes, worked numerical problems, and application-based questions strictly follow the latest revised Assam Board (SEBA) curriculum.
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