Mechanical motion is any change in the position of one body in relation to another. It is the simplest and most fundamental form of motion we observe in the physical world, and it is a core part of the broader study of motion in physics.
Physics covers a very wide scope — from tiny subatomic particles to planets and galaxies. All bodies around us, whether stars, planets, tiny atomic particles, or their constituents, are in a state of continuous motion. Studying this motion in full detail can be quite complex.
The simplest form of motion we find around us is the displacement of one object or body with respect to another — this is what we call mechanical motion.
We define mechanical motion as any change in the position of one body in relation to another.
The operation of most mechanisms, machines, and transportation is based on mechanical motion. Mechanical Motion is also a component of more complex, nonmechanical processes. Heat phenomena, light radiation, and nuclear reactions are examples of such complex motions. Motion is a fundamental part of the physical world and is present in objects large and small.
Types of Mechanical Motion
There are two fundamental types of motion in mechanics: translational motion and rotational motion.
1. Translational Motion
- In translational motion, all points of the moving body travel in an identical manner.
- If a body has translational motion, a straight line connecting any two points on it is displaced parallel to itself. This is shown in the figure below.
- So, when a body is in translational motion, its motion can be sufficiently described by the motion of just one of its points.
- Translational motion can itself be uniform or non-uniform, depending on whether the body’s speed stays constant — see our guide to uniform motion for more. When this straight-line displacement happens along a single straight path, it is specifically called rectilinear motion.
2. Rotational Motion
- Rotational motion is a type of motion in which all the points of the body describe circles lying in parallel planes.
- The centre of all these circles lies on the rotational axis — a single straight line about which the body rotates.
- This is shown in the figure below, where a disk rotates about the axis of rotation passing through point $O$.
- The figure also shows circles lying in parallel planes, which depict the motion of points $A$, $B$, and $C$ respectively.
IMPORTANT NOTE
- Translational motion: every point of the body has the same displacement, velocity, and direction.
- Rotational motion: every point of the body moves in a circle about a fixed axis, so different points can have different speeds depending on their distance from the axis.
- A real body — for example, a rolling wheel — can show translational and rotational motion at the same time.
Point Object (Mass Point) in Mechanics
- While solving many problems in mechanics, it is convenient to disregard the size of the body and instead treat it as a point object (or simply a mass point) — an idealization of a body in which its size is neglected. For a deeper look at this idealization, see our detailed article on the point object concept.
- This simplification is useful when the size and shape of the body are not important for solving the problem at hand.
- It is important to note that the concept of a point object is abstract. We assume the body is a point object for convenience; in reality, every body has a finite size.
When Should an Object Be Considered a Point Object?
Let us look at when to treat an object as a point object, and when its shape and size must be taken into account.
- In some situations, we can treat an object as a point mass or particle without considering its size and shape.
- For example, to calculate the time it takes for an airplane to fly from New Delhi to Mumbai, we don’t need to consider the motion of its different parts.
- However, for other phenomena like take-off and landing, we need to consider the airplane’s size and shape.
- Similarly, we can treat planets like particles when studying their orbital motion around the Sun.
- But for other phenomena, like day and night or the seasons, we need to consider factors like the planet’s size and rotation.
- So, whether we can treat an object as a point mass depends on the problem at hand, not on the actual size of the object.
| Situation | Treat as Point Object? | Why |
|---|---|---|
| Airplane flying Delhi to Mumbai | Yes | Only its overall position along the route matters |
| Airplane during take-off/landing | No | Its size, shape, and orientation affect the motion |
| Planet’s orbital motion around the Sun | Yes | Only its position in the orbit matters |
| Planet’s day-night cycle or seasons | No | Its size, shape, and axial tilt matter |
Frequently Asked Questions
What is mechanical motion in physics?
Mechanical motion is any change in the position of one body in relation to another. It is the simplest form of motion and underlies the working of most mechanisms, machines, and modes of transportation.
What are the two types of mechanical motion?
The two fundamental types of mechanical motion are translational motion, in which all points of a body travel identically, and rotational motion, in which all points describe circles in parallel planes about a fixed axis.
What is the difference between translational motion and rotational motion?
In translational motion, a straight line joining any two points of the body stays parallel to itself as the body moves. In rotational motion, every point of the body moves in a circle around a fixed axis, so the body’s orientation keeps changing.
What is a point object or mass point in physics?
A point object, or mass point, is an idealized body whose size is neglected and whose entire mass is assumed to be concentrated at a single point. It is used to simplify problems where the body’s shape and size do not affect the result.
When can a body be treated as a point object?
A body can be treated as a point object when only its overall position matters for the problem, and its size, shape, and internal motion do not affect the answer — for example, a planet orbiting the Sun. It cannot be treated as a point object when its size or shape directly affects the phenomenon being studied, such as an airplane during take-off.
Can a body show both translational and rotational motion at the same time?
Yes. Many real bodies, such as a rolling wheel, undergo translational and rotational motion simultaneously — the wheel’s centre moves forward while every point on the rim also moves in a circle about the axle.
