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Translational motion is movement of an object without a change in its orientation relative to a fixed point, as opposed to rotational motion, in which the object is turning about an axis. In other words, an arrow painted on an object undergoing pure translational motion would continue pointing in the same direction; any rotation would cause the arrow to change direction. In the real world, most movement is a combination of the two. In space, for example, objects such as stars, planets and asteroids are constantly changing position relative to one another, but are also invariably rotating. An understanding of translational motion plays a key part in basic physics and in comprehending the behavior of moving objects in general, from atoms to galaxies.
In theory, pure translational motion need not involve travelling in a straight line. It is possible for an object to move in a curved path without changing its orientation; however, in most real-life situations, a change in direction would involve turning on an axis, in other words, rotation. In aeronautics, translational motion means movement along a straight line, forwards or backward, left or right and up or down. When an airplane is circling an airport, it is continually changing its orientation and undergoing some degree of rotation.
The study of translational motion is known as translational dynamics and uses a series of equations to analyze the movement of objects and how they are affected by various forces. The tools used to study movement include Newton’s laws of motion. The first law, for example, states that an object will not change its motion unless a force acts upon it, while the second law states that force is equal to mass multiplied by acceleration. Another way of saying this is that acceleration is equal to force divided by mass, which means that it is harder to change the translational motion of a massive object than a less massive one. The forces that can act on an object include gravity and friction.
On the molecular level, the temperature of a substance can be defined largely in terms of the translational motion of its atoms or molecules. Rotation also plays a role on molecular motion, but it is not important in terms of temperature. If heat is applied to a solid, electromagnetic energy is converted into kinetic energy in that its molecules will move about faster. This increases its temperature and may cause it to expand in volume. If enough heat is applied, the material will melt into a liquid state and finally boil to form a gas, as the average speed of the molecules increases.
The molecules in a substance subjected to heat behave in accordance with Newton’s laws of motion. Molecules with more mass require more force to increase their speed. Heavier substances will therefore usually require more heat to cause them to melt or boil. Other forces, however, can also act on molecules to restrain them, so this rule does not always hold true. Water, for example, has a higher boiling point than would be expected for its molecular weight because of the hydrogen bonds that hold the molecules together.
Most movement in the physical world is a combination of translational motion and rotational motion, in which the latter controls the direction on the axis while the former propels the object in that direction. The human body moves with a combination of these two types of motion. The limbs rotate on their joints, providing the impetus for directional movement, such as walking. Humans can walk in this way over varying slopes without changing their overall orientation.
Experiments have determined that combined translational and rotational motion is more efficient in terms of kinetic energy than translational alone. Pure translational motion creates constant friction against its surrounding surfaces, even the air, causing greater loss of kinetic energy and momentum over time. Adding rotational motion reduces the friction, allowing kinetic energy to persist for a longer period. For example, a wheel rolling along a surface demonstrates both types of movement and experiences far less friction than would be the case if it were pushed along without any rotation.