Newton's First Law of Motion — Long Answer Questions (Class 9 Physics)
Medium Level (Application & Explanation)
Q1. Explain Newton’s First Law of Motion and illustrate it using at least three different real-life examples.
Answer:
Newton’s First Law (Law of Inertia) states that an object at rest stays at rest and an object in motion continues in motion with the same speed and direction unless acted upon by an unbalanced external force.
Example 1: A book on a table remains at rest because the forces (gravity and normal reaction) are balanced; it will not move unless you push it.
Example 2: A hockey puck slides on ice and keeps moving for a long time because friction (an external force) is very small; it stops only when friction or a wall acts on it.
Example 3: When you let go of an inflated balloon, the escaping air produces an unbalanced force that propels the balloon forward; without that unbalanced force it would remain at rest.
These examples show that motion or rest continues until an unbalanced force changes it, which is the core idea of the law.
Q2. What is inertia? Explain how the amount of inertia depends on mass with suitable examples.
Answer:
Inertia is the property of matter that makes an object resist any change in its state of motion—whether from rest to motion or from one motion to another.
The amount of inertia of an object depends directly on its mass. A larger mass means greater inertia, so more force is needed to change its motion.
Example 1: Pushing an empty trolley is easy, but pushing a loaded trolley (greater mass) requires much more force because the loaded trolley has greater inertia.
Example 2: A small stone and a heavy boulder at rest: it is effortless to move the stone but nearly impossible to push the boulder without a very large force.
Thus, mass is the measure of inertia; the heavier the object, the stronger its tendency to remain at rest or keep moving.
Q3. Differentiate between balanced and unbalanced forces. How do they relate to Newton’s First Law? Give examples.
Answer:
Balanced forces are equal in magnitude and opposite in direction, producing no change in motion. Under balanced forces, an object at rest stays at rest, and a moving object continues with the same velocity.
Unbalanced forces are not equal and cause a change in motion (acceleration or deceleration). According to Newton’s First Law, it is the unbalanced force that changes the state of motion.
Example of balanced forces: A box on the floor experiences gravity down and normal force up; these forces are balanced so the box remains at rest.
Example of unbalanced force: Kicking a soccer ball applies an unbalanced force and the ball accelerates forward.
In short, balanced → no change; unbalanced → change, which directly connects to the First Law.
Q4. Explain why moving objects on Earth eventually come to rest even when no one touches them. Discuss the role of friction and how surface conditions affect it.
Answer:
On Earth, a moving object usually encounters friction, an external force arising from contact between surfaces, and air resistance. These forces are unbalanced and act opposite to motion, causing the object to slow down and eventually stop.
The amount of friction depends on the roughness of surfaces and the normal force (which relates to weight). Rough surfaces produce large friction, and smooth or lubricated surfaces produce less friction.
For example, a hockey puck on ice travels far because the ice provides small friction, while the same puck on a rough floor stops quickly.
Thus, even without a push, surface conditions and air resistance are responsible for stopping moving objects on Earth, demonstrating how external forces change motion in real situations.
Q5. Describe a simple classroom experiment using a coin, card, and glass to demonstrate Newton’s First Law. Explain the observations and the conclusion.
Answer:
Place a glass on the table. Put a card on top of the glass and place a coin at the center of the card.
Quickly flick the card horizontally with your finger so that it is pulled out from under the coin. The coin will drop straight into the glass instead of moving forward with the card.
Observation: The coin appears to remain at rest while the card moves away. This happens because the coin’s inertia resists the sudden change in motion; no large horizontal force acts on the coin, so it stays roughly in the same place and then falls due to gravity into the glass.
Conclusion: The experiment demonstrates Newton’s First Law—the coin remains at rest until an unbalanced force acts; the quick removal of the card does not apply sufficient horizontal force to the coin.
High Complexity (Analytical & Scenario-Based)
Q6. A bus suddenly brakes and passengers without seat belts lurch forward. Using Newton’s First Law, explain why this happens and how seat belts and airbags protect passengers.
Answer:
When the bus suddenly brakes, the bus decelerates quickly because of an unbalanced force acting on it. Passengers’ bodies, due to inertia, tend to maintain their forward motion, so they lurch forward relative to the bus.
Seat belts provide a backward unbalanced force on the passenger’s body, stopping forward motion gently by distributing the force over the chest and pelvis; this reduces injury by reducing sudden deceleration of internal organs.
Airbags add another protective layer by providing a cushioned surface that increases the time over which the passenger slows down, thereby reducing the force felt.
In short, without restraints, the passenger’s inertia causes them to keep moving forward; seat belts and airbags supply the necessary safe external forces to change that motion and prevent harm.
Q7. An astronaut working on a spacewalk accidentally drops a wrench. Explain why the wrench does not fall to the “ground” and describe how the astronaut can retrieve it using the idea of forces and motion in space.
Answer:
In space, both the astronaut and the wrench are in microgravity and orbiting Earth; there is effectively no significant external horizontal force to change their motion, so the wrench continues moving along the same orbital path—this is an example of Newton’s First Law. It does not fall down as on Earth because both wrench and astronaut are in free fall around Earth.
To retrieve it, the astronaut must apply an external force to change the wrench’s velocity relative to them. The astronaut can: gently push off a structure to change their own velocity and meet the wrench, or use a tether, rope, or magnetic tool to apply force and pull the wrench closer.
Small thruster bursts (reaction control system) can also change the astronaut’s motion to rendezvous with the tool; any change requires applying a force to overcome the existing inertia and align velocities.
Q8. Compare a heavy truck and a small car moving at the same speed when brakes are applied. Use the concept of inertia to analyze which vehicle stops sooner and which requires more braking force. Explain consequences for road safety.
Answer:
Both vehicles have inertia, but the truck has much greater mass, so it possesses greater inertia and more momentum at the same speed. This means the truck resists change in motion more strongly than the small car.
To stop in the same distance and time as the car, the truck’s brakes must provide a much larger unbalanced force; otherwise, the truck will require longer distance to stop. In practice, trucks need stronger braking systems and longer stopping distances.
Consequences: Trucks that cannot stop quickly pose serious safety risks in traffic, especially in sudden braking situations. Drivers need to maintain greater following distance behind heavy vehicles and be aware that heavy vehicles take longer to slow down due to higher inertia.
Thus, greater mass → greater inertia → more force or longer distance needed to stop safely.
Q9. Explain how Newton’s First Law applies to an object moving with constant speed in a circular path, such as a car turning on a curved road. Why does the direction change even if speed stays constant?
Answer:
Newton’s First Law describes motion in a straight line when no unbalanced force acts. For circular motion, even if speed is constant, the direction of velocity changes continuously, so the velocity vector is changing. A changing velocity means there is an acceleration, which requires an unbalanced force (centripetal force) directed toward the center of the circle.
In the example of a car rounding a curve, friction between tires and road supplies the centripetal force to change the car’s direction. Without this sideways frictional force, the car would continue moving straight (as the First Law predicts) and would not follow the curved path.
Therefore, constant speed does not imply no net force; the constant change in direction requires an unbalanced force, consistent with Newton’s laws.
Q10. You are asked to design a plan to secure grocery items in a car so they do not move forward when the car brakes suddenly. Using Newton’s First Law, suggest practical measures and explain how each reduces the chance of items shifting.
Answer:
Newton’s First Law predicts that groceries in motion will continue moving forward during sudden braking. To prevent this:
Use a trunk or closed compartment to create physical barriers so items cannot slide forward; barriers provide the unbalanced forces needed to stop them.
Place heavy items low and close to the back seat to lower center of mass and reduce their tendency to move; increased contact with the floor increases friction.
Use non-slip mats or cargo nets/straps to increase friction and apply rest...
