Inertia is an object’s resistance to a change in motion, related to Isaac Newton’s first law of motion. Inertia depends on mass and is used to measure mass independently of gravity. Examples of inertia include a car’s resistance to a change in motion and rotational inertia. The origin of inertia is still unknown.
In physics, inertia is the resistance of an object to a change in its motion. This could involve a change in speed or direction, an attempt to move a stationary object, or an attempt to stop an object that is already moving. The idea is related to Isaac Newton’s first law of motion, which states that the motion of an object will not change unless a force is acting on it. Inertia depends on mass, since the more massive an object is, the more it resists a change in motion.
If an object is stationary, it won’t move unless something pushes or pulls it. Similarly, a moving object will continue to move at the same speed, in a straight line, and in the same direction, unless a force affects it. On Earth, a ball thrown horizontally through the air, if left to its own devices, will slow down and bend toward the ground. This is because the force of gravity pulls it towards the Earth and the air pushes against it, reducing its speed. In space, without gravity or air resistance, the ball would simply keep moving in a straight line at a constant speed.
The fact that a heavy object is more difficult to move than a light one demonstrates the relationship between inertia and mass. On Earth, gravity complicates the problem, but in space, things are clearer. Here, a massive object, like a cannonball, and a light object, like a tennis ball, are both weightless, but it still takes a much greater force to move a cannonball than a tennis ball. Likewise, it would take more force to stop or change the direction of a moving cannonball. Inertia can then be used to measure mass independently of gravity.
Examples of inertia
People encounter inertia every day. For example, someone driving a car will experience a force that will push them back against their seat when the car is accelerating; this is due to the driver’s resistance to the forward movement of the car. Similarly, as the car slows down, the driver is pushed forward, relative to the car, again due to his resistance to the change in motion. That’s why seat belts are an essential safety feature in cars. If the driver were to brake suddenly, the occupants would continue to move forward at their original speed and, without seat belts to restrain them, could be seriously injured.
The car’s own inertia is an important consideration for drivers. Explain why moving vehicles have a stopping distance that depends on the speed and mass of the vehicle. A car’s resistance to a change of motion also explains why the car will swerve out of control if the driver tries to swerve too fast: the vehicle will tend to keep moving in the same direction.
Rotational inertia
This is a similar concept, but applies to rotating objects. Again, the more mass an object has, the harder it is to spin, and the harder it is to stop it if it’s already spinning. The amount of resistance to a change in motion for a rotating object is known as the moment of inertia, usually given the symbol I. For a point on the surface of a rotating object, I is calculated as the mass times the square of the distance from the axis of rotation. Calculations for integer objects are more complicated.
When an object is moving in a straight line, its momentum is its mass times its velocity. For a rotating object, the equivalent is its angular momentum, which is I times its rotational speed. Angular momentum is always conserved, i.e. it remains the same even if one of the contributing factors changes. The change in one factor must be compensated for by a change in the other so that the angular momentum remains constant.
A good example is the huge increase in rotational speed when a star collapses under gravity into a neutron star. Stars normally rotate slowly, but when a neutron star forms, its diameter shrinks to a small fraction of its original value. This greatly reduces the moment of inertia at the star’s surface – as the distance from the axis of rotation is now much smaller – so its rotational speed must increase dramatically to maintain the same angular momentum. This is why neutron stars usually spin at many revolutions per second.
The origin of inertia
Isaac Newton, in formulating his laws of motion, assumed the existence of a fixed and absolute space against which all motion could be measured. In 1893, physicist Ernst Mach proposed that absolute space is meaningless and that any change in an object’s motion should be thought of as relative to distant stars. With Einstein’s theories of relativity, the idea of fixed space was effectively rejected, but it implies that the inertia of a nearby object is somehow affected by objects many light years away. Also, the effect appears to be instantaneous. A number of theories have been advanced, some involving exotic ideas such as influences traveling back in time, but, as of 2012, there appears to be no generally accepted explanation for the origin of the inertia.
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