When an object is moving, it has kinetic energy – energy associated with motion. Without an external force acting on it, the object will eventually slow down and come to rest due to inertia. Inertia is the tendency of an object to resist changes in its motion. So even when no net force is applied, an object’s innate inertia causes it to gradually lose kinetic energy over time and come to a stop.
What is Inertia?
Inertia is defined as the resistance of any physical object to any change in its velocity. It is a property of matter that causes it to resist changes in motion. Inertia is related to an object’s mass – the more mass an object has, the more inertia it has and the more it resists changes to its motion.
Some key points about inertia:
- Inertia depends on the object’s mass – higher mass means higher inertia
- Inertia is a passive property – it doesn’t require any energy expenditure by the object
- Inertia always acts to keep the object at its current state of motion – it resists starting, stopping, or changing direction of motion
- Heavier objects (more mass) have more inertia than lighter objects
Inertia is what causes objects to resist changes in their velocity – their speed and direction of motion. So even when the net force on an object is zero, its inertia keeps it moving. Eventually other forces like friction will slow it down and inertia will bring it to rest.
Newton’s First Law of Motion and Inertia
Newton’s First Law of Motion is also known as the Law of Inertia. It states:
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
This law captures the concept of inertia – an object will not change its motion unless a force external to the object acts on it. So inertia opposes both starting motion from rest, and stopping motion once an object is moving.
Imagine pushing a heavy box across the floor. It takes a force to start the box moving from rest. Once it’s moving, its inertia keeps it moving – so you have to keep applying a force to sustain its motion. If you stop pushing, the box will gradually slow down due to friction and stop.
Or think about being in a car that suddenly stops. Your body’s inertia makes you lurch forward temporarily. That’s because your body’s inertia tries to keep you moving even after the car has stopped.
Newton’s First Law highlights that objects will keep moving at constant velocity if the net force on them is zero. An object’s inertia maintains its motion unless an unbalanced force interferes.
How Inertia Causes a Moving Object to Eventually Stop
We’ve seen that inertia keeps objects moving with the same speed and direction, unless a force causes a change. This tendency can make it seem as if objects with no net force should keep moving forever. But in reality, even with zero net force, friction and other factors cause objects to lose their kinetic energy and come to rest.
There are several effects which cause moving objects to slow down over time:
Friction
The rubbing of the object against surfaces introduces frictional forces that convert kinetic energy to heat. This causes the object to gradually lose speed. Without a driving force, friction will eventually drain an object’s kinetic energy and cause it to stop.
Air Resistance
Air pushback on the object creates drag that saps kinetic energy. Streamlined objects experience less drag. In a vacuum with no air, objects move at constant velocity unless another force interferes.
Deformation
If the object is not rigid or elastic, internal deformation as it moves converts some kinetic energy into heat that dissipates away. This leads to slowing down over time.
Emission
Photons, electrons, molecules, or other particles emitted by a moving object carry away some kinetic energy, reducing the object’s speed.
So in practice, even with no external net force, the loss of kinetic energy through internal dissipative forces causes the inertia of an object to bring it to rest eventually. The dissipative forces act at the microscopic level to convert macroscopic ordered motion into heat.
Examples of Inertia Bringing Motion to Rest
1. A hockey puck sliding across ice will gradually slow down and stop due to friction dissipating its kinetic energy as heat. This happens even if no external force disturbs it after being struck.
2. A ball thrown through the air initially maintains its motion. But air resistance creates drag forces that continuously sap the ball’s kinetic energy. The ball decelerates and eventually drops vertically down.
3. A spinning fidget spinner’s rotational speed decreases over time as friction converts kinetic energy to heat. Eventually the spinner comes to rest.
4. Space vehicles in orbits experience minute atmospheric drag that causes loss of kinetic energy and decay of the orbit over time. Re-boosts with rockets are needed to counter orbital decay.
5. Conserving angular momentum, an ice skater spinning with arms outstretched slows down when she pulls in her arms. Kinetic energy is lost to internal friction as she changes shape.
Why Inertia Results in Rest Without External Forces
The physics behind why inertia causes motion to stop without external forces lies in the First Law of Thermodynamics:
In a closed system, the total energy is conserved.
For an object with only inertia and no external forces, it is a closed system. The object has kinetic energy initially due to its motion. Over time, internal dissipative forces like friction convert macroscopic ordered motion (kinetic energy) into microscopic disordered motion – heat.
The First Law says total energy must stay constant. So as kinetic energy decreases, heat energy increases to keep the total energy unchanged. Eventually all kinetic energy becomes heat and the object comes to rest.
The dissipative forces and conversion of kinetic energy to heat all take place at the microscopic molecular level. But the effect is observed at the macroscale as the object gradually loses speed and stops moving. This is why an object’s inertia causes it to lose its motion and come to rest, even if no external forces act on it.
Factors Affecting How Quickly an Object Stops Due to Inertia
While inertia always causes a moving object to eventually come to rest absent external forces, some factors affect how quickly it will stop:
Mass
Heavier objects have more inertia, making them harder to slow down and stop. A bowling ball’s inertia keeps it rolling much longer than a tennis ball.
Velocity
Faster moving objects have more kinetic energy, so it takes longer to dissipate the energy and stop. A car coasting at 60 mph maintains its motion longer than at 20 mph.
Energy Conservation
Objects that conserve energy better retain kinetic energy over time. A well-lubricated car loses kinetic energy more slowly than one with sticky brake calipers dragging.
Aerodynamics
Sleek, streamlined objects experience less air drag and lose velocity more gradually than blunt objects. Bullets are designed to be aerodynamic.
Friction
Surfaces and medium that produce less friction allow objects to glide longer before stopping. Air hockey pucks on an air cushion maintain motion better than sliding on rough concrete.
Shape
Compact, symmetrical objects conserve angular momentum better, taking longer to slow their spin. Well-balanced fidget spinners resist changes to spin more than unevenly weighted ones.
Using Inertia in Practical Applications
Inertia can be harnessed in many useful applications:
Flywheels
Flywheels oppose changes in rotational speed. This provides stability in power output for engines and regulates forces in machines.
Gyroscopes
Spinning gyroscopes leverage their inertia to maintain orientation in navigation systems, stabilizers, compasses and more.
Momentum Transfer
In collisions, inertia results in momentum transfer between objects. This allows recoilless rifles and rocket thrusters to work.
Spinning Objects
Tops, yo-yos, and spinning dancers use inertia to maintain stability and resist toppling while rotating quickly.
Speed Modulation
Objects in cyclic motion use inertia to glide through the low-force portion. This happens in walking, pistons, swinging pendulums and more.
Understanding inertia allows us to smooth out and modulate motions by taking advantage of the tendency of objects to maintain their velocity. Inertia is a fundamental physical principle underlying many systems and technologies.
Frequently Asked Questions
Why does inertia cause objects to resist changes in motion?
Inertia originates from an object’s fundamental resistance to changes in its state of rest or motion. This is linked to its mass and connections between particles at the molecular level. More mass means more inertia.
How fast will an object stop moving due to inertia alone?
There is no set speed. The rate of slowing depends on factors like the object’s mass, shape, velocity and interaction with the environment through friction, air resistance etc. Heavier, sleeker objects maintain motion longer.
Does inertia mean no force is required to keep an object moving at constant speed?
Yes, due to Newton’s First Law. No net external force is needed to maintain constant linear velocity. But internal forces inside the object can cause it to slow down and stop over time.
What eventually brings a drifting rocket in space to rest?
Tiny atmospheric drag forces in space slowly drain a drifting rocket’s kinetic energy. Fuel leaks and emissions also contribute to slowing down. Given enough time, the loss of kinetic energy will bring it to rest.
Why don’t objects with inertia move forever in ideal conditions?
There are no perfectly ideal frictionless conditions in real systems. Some energy loss always occurs, converting ordered motion to heat due to microscopic dissipative effects. This causes the object to eventually stop.
Conclusion
In summary, inertia explains why objects that aren’t subject to external forces nonetheless slow down and come to rest over time. Inertia tries to maintain their state of uniform motion, but internal friction and other dissipative effects convert kinetic energy to heat. This causes the object to gradually lose speed. Factors like mass, velocity, lubrication and aerodynamics determine how quickly it decelerates. Harnessing inertia has allowed the development of numerous technologies that improve engineering and design. Understanding this fundamental property of matter helps explain motions we observe in everyday life.