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.Continue Reading
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
Inertia is the tendency for an object to remain in motion or at rest unless it is acted upon by an outside force. In the field of physics, this concept is referred to as Newton's first law of motion.Full Answer >
Friction is a force that opposes motion, so it is unhelpful in all situations where the motion of an object is desired. A by-product of friction is that energy is wasted in the form of heat, which can cause problems in temperature-sensitive environments.Full Answer >
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 >
Newton's first law of motion states, "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it." An unbalanced (or unbalancing) force is another name for an external force.Full Answer >