Motion is relative to an observer or to an object. For example, a woman driving a car is not in motion relative to the car, but she is in motion relative to an observer standing on the side of the road.Know More
A similar example is a cat sleeping on a rug. The cat appears to be at rest to an observer on Earth, but relative to an object at a fixed point in space, the cat is moving as fast as the Earth turns. As these examples show, discussions of motion must include a point of reference, especially when calculating measurements such as speed. For example, if a train is traveling 90 miles per hour and a man walks from the back of a train car to the front, how fast is the man moving? The answer depends on the point of reference. To another passenger in the train, the man is moving at a rate much less than 90 miles per hour. To an observer outside of the train, however, the man is moving at a rate slightly more than 90 miles per hour.
Most people consider motion relative to a fixed point on the Earth. Physicists consider that the universe has no fixed point, so all motion is relative to what is defined as a fixed point. In fact, this basic idea is part of Einstein's theory of relativity.Learn more about Motion & Mechanics
Unbalanced force occurs when two forces acting on an object are not equal in size, causing a change in motion. Unbalanced forces cause a still object to move or moving objects to slow down or speed up, stop or change direction.Full Answer >
The law of inertia is a fundamental physics principle used to describe how an object behaves in motion or at rest, particularly when forces are applied to it. The object can be anything that has mass.Full Answer >
Force and motion are related because exerting force on an object causes a change in motion. An example would be kicking a ball to propel it forward. Two examples of natural forces are gravity and magnetism.Full Answer >
The law of inertia states that 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. The law of inertia is sometimes referred to as Newton's first law of motion.Full Answer >