Team Physics -
Created by: Team Physics -, Last Updated: July 5, 2024


Inertia is a fundamental concept in physics that describes the resistance of an object to any change in its state of motion. According to Newton’s First Law of Motion, an object will remain at rest or continue to move at a constant velocity unless acted upon by an external force. This property of matter means that a stationary object will not start moving on its own, and a moving object will not stop or change direction without an external influence. Inertia depends solely on the mass of the object: the greater the mass, the greater the inertia. This principle explains why heavier objects are more difficult to accelerate or decelerate compared to lighter ones.

What is Inertia?

Inertia is the tendency of an object to resist changes in its motion. According to Newton’s First Law of Motion, an object at rest stays at rest, and an object in motion continues moving at a constant speed in a straight line unless acted upon by an external force. The greater the object’s mass, the greater its inertia.

Inertia Formula

The formula for inertia is often associated with the moment of inertia (I), which measures an object’s resistance to rotational motion around an axis. The moment of inertia depends on the object’s mass distribution relative to the axis of rotation. The general formula is:



  • I is the moment of inertia.
  • mᵢ is the mass of each particle in the object.
  • rᵢ​ is the distance of each particle from the axis of rotation.

Inertia Units

The unit of inertia, particularly the moment of inertia, is expressed in terms of mass and distance squared. In the International System of Units (SI), the unit for moment of inertia is:

SI Unit = kilogram metre squared (kg⋅m²)

In the imperial or US customary units, the unit for moment of inertia is:

pound-foot-second squared (lbf⋅ft⋅s²)

These units reflect the dependence of the moment of inertia on both the mass of the object and the square of the distance from the axis of rotation.

Examples of Inertia

Examples of Inertia
  1. Car Braking: When a car suddenly stops, passengers lurch forward due to their bodies’ inertia, which resists the change from moving to stopping.
  2. Tablecloth Trick: Quickly pulling a tablecloth from under dishes without moving them demonstrates inertia. The dishes remain in place because they resist the sudden change in motion.
  3. Space Travel: In the vacuum of space, a spacecraft will continue moving in a straight line at a constant speed until acted upon by an external force, such as thrusters or gravitational pull.
  4. Books on a Table: A stack of books remains at rest on a table until a force, like a push, causes them to move. The books resist this change in motion due to their inertia.
  5. Sports: A soccer ball will remain stationary on the field until kicked. Once in motion, it will continue moving until friction with the ground and air resistance slows it down.
  6. Train Movement: When a train starts moving, passengers feel pushed back into their seats due to their bodies’ inertia resisting the acceleration.
  7. Riding a Bicycle: When you suddenly stop pedaling, the bicycle continues to move forward due to inertia until friction and other forces slow it down.
  8. Swinging a Hammer: When you swing a hammer to hit a nail, the hammerhead continues its motion due to inertia, driving the nail into the wood even after you stop applying force.
  9. Bumper Cars: In an amusement park, when bumper cars collide, the passengers continue moving in the direction they were going due to inertia, causing them to jolt forward or sideways upon impact.
  10. Earth’s Rotation: The Earth keeps rotating on its axis due to inertia. Without external forces like gravitational pulls from other celestial bodies, it would continue spinning indefinitely at a constant speed.

Examples of inertia class 9

  1. Turning a Car: When a car makes a sharp turn, passengers feel pushed outward. This happens because their bodies resist the change in direction due to inertia.
  2. Pendulum Swing: A pendulum continues to swing back and forth due to inertia, maintaining its motion until friction gradually slows it down.
  3. Tightrope Walker: A tightrope walker uses a long pole to balance. The inertia of the pole helps stabilize their movements, making it easier to maintain balance.
  4. Bouncing Ball: When you drop a ball, it bounces back up due to inertia. Each bounce gets smaller as energy is lost, but the initial motion is due to the ball’s resistance to changing its state.
  5. Earth’s Orbit: The Earth continues to orbit the Sun due to inertia, maintaining a nearly constant speed and path unless acted upon by another force, like gravity from other celestial bodies.
  6. Roller Coaster: As a roller coaster goes over a hill, passengers feel lifted from their seats due to inertia. Their bodies continue to move upwards while the coaster starts descending.
  7. Sliding Hockey Puck: A hockey puck glides across the ice due to inertia. It continues moving in a straight line until friction or an obstacle stops it.
  8. Hanging Objects: An object hanging from a string remains stationary due to inertia until an external force, like wind or a push, sets it in motion.
  9. Kickstarting a Scooter: When you kickstart a scooter, the scooter resists the initial push due to inertia until the force is strong enough to overcome this resistance and start the engine.
  10. Basketball Free Throw: When shooting a basketball, the ball travels in a straight line towards the hoop due to inertia. It only changes direction if acted upon by external forces like gravity or air resistance.

Types of Inertia with Examples

  1. Inertia of Rest
    • Definition: The tendency of an object to remain at rest unless acted upon by an external force.
    • Examples:
      • Book on a Table: A book remains stationary on a table until someone applies a force to move it.
      • Ball on the Ground: A soccer ball will stay in place on the ground until kicked.
      • Sleeping Person: A person sleeping will remain in that position until woken up by an external force, like an alarm clock or someone shaking them.
  2. Inertia of Motion
    • Definition: The tendency of an object to continue moving in a straight line at a constant speed unless acted upon by an external force.
    • Examples:
      • Car Coasting: A car moving on a highway at a constant speed continues to move forward even when the driver takes their foot off the accelerator until friction and air resistance gradually slow it down.
      • Rolling Ball: A ball rolling on the ground keeps moving in the same direction until it is stopped by friction or an obstacle.
      • Runner: A sprinter continues moving forward after crossing the finish line due to inertia until they slow down gradually by applying force with their muscles.
  3. Inertia of Direction
    • Definition: The tendency of an object to resist a change in its direction of motion.
    • Examples:
      • Turning a Car: When a car makes a sharp turn, passengers feel pushed outward because their bodies resist the change in direction due to inertia.
      • Swinging a Hammer: When you swing a hammer to hit a nail, the hammerhead continues its forward motion due to inertia, making it easier to drive the nail into the wood.
      • Cyclist: When a cyclist makes a sudden turn, they have to lean into the turn to counteract the inertia that wants to keep them moving in a straight line.

Advantages and Disadvantages of Inertia

Advantages of Inertia

  1. Stability in Structures:
    • Example: Buildings and bridges are designed with inertia in mind to ensure they remain stable and resist sudden movements caused by wind or earthquakes.
  2. Predictable Motion:
    • Example: In sports, understanding inertia helps athletes predict the motion of balls and other equipment, improving accuracy and performance.
  3. Safety Mechanisms:
    • Example: Seat belts in cars take advantage of inertia by restraining passengers and preventing them from continuing to move forward in the event of a sudden stop.
  4. Efficient Motion:
    • Example: In outer space, spacecrafts use inertia to travel long distances without continuous propulsion, saving fuel and resources.
  5. Mechanical Systems:
    • Example: Flywheels in engines and machinery store rotational energy due to inertia, providing a smooth and continuous energy supply.

Disadvantages of Inertia

  1. Difficulty in Stopping:
    • Example: Large vehicles like trucks have significant inertia, making them harder to stop quickly and increasing the risk of accidents.
  2. Resistance to Change:
    • Example: In industrial settings, machines with high inertia take longer to start and stop, reducing efficiency and responsiveness.
  3. Safety Hazards:
    • Example: In sports like football, players can suffer injuries due to the inertia of their own bodies or equipment, leading to high-impact collisions.
  4. Control Challenges:
    • Example: Pilots and drivers must account for inertia when maneuvering vehicles, requiring skill and training to manage effectively.
  5. Energy Loss:
    • Example: In roller coasters and amusement park rides, overcoming inertia requires significant energy input, resulting in higher operational costs.


How does mass affect inertia?

Mass is directly proportional to inertia; objects with more mass have greater inertia and are harder to accelerate or decelerate.

What is an example of inertia in everyday life?

When a car suddenly stops, passengers lurch forward due to their body’s inertia, which resists the change in motion.

What is the difference between inertia and momentum?

Inertia is an object’s resistance to changes in motion, while momentum is the product of an object’s mass and velocity, describing its motion.

How does Newton’s First Law relate to inertia?

Newton’s First Law, or the law of inertia, states that an object will remain at rest or in uniform motion unless acted upon by an external force.

Why do seat belts help with inertia in cars?

Seat belts restrain passengers, preventing them from continuing forward motion due to inertia when a car abruptly stops.

What happens to an object with no net external force acting on it?

An object with no net external force will maintain its current state of rest or constant velocity due to inertia.

Does inertia depend on the shape of an object?

Inertia depends on mass distribution and the object’s shape in rotational motion, but for linear motion, only mass matters.

How is inertia important in sports?

Inertia affects how athletes start, stop, and change direction, influencing their performance and the design of sports equipment.

What role does inertia play in space?

In space, where there’s minimal external force, objects continue moving indefinitely in a straight line due to inertia.

Why does a heavy object take longer to stop?

A heavy object has more inertia, requiring more force and time to change its motion compared to a lighter object.

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