Physics · System Of Particles And Rotational Motion · NEET
It is an ideal model. A real solid bends or stretches a tiny amount under force. In a rigid body we assume this change is zero, so the distance between any two particles is always fixed. NEET problems (rods, discs, flywheels) treat bodies as perfectly rigid unless told otherwise, which keeps the maths simple.
In pure translational motion every particle of the body has the same velocity at any instant. A block sliding straight down a smooth incline is a good example: all points move together with equal velocity. The whole body can then be treated as a single point (the centre of mass).
When a body is fixed along a straight line (the axis of rotation), it can only rotate. Then every particle moves in a circle whose plane is perpendicular to the axis and whose centre lies on the axis. Particles on the axis stay still (r = 0), while particles far from the axis move fastest, since v = rω.
No. In rotation the linear velocity depends on distance from the axis through v = rω. All particles share the same angular velocity ω, but their linear speeds differ. This is the key contrast with translation, where linear velocity is the same for all particles.
Yes. A cylinder rolling down an incline both moves forward (translation of its centre of mass) and spins (rotation about its axis). The general motion of a rigid body is a combination of translation of the centre of mass plus rotation about the centre of mass.
A top spins, but its axis itself slowly moves (precesses) about the vertical, so the axis is not fixed in direction. For NEET, unless stated otherwise, rotation means rotation about a fixed axis, where the axis keeps the same position and direction.
Which of the following statements are correct? (a) The centre of mass of a body always coincides with its centre of gravity. (b) The centre of mass is the point where the total gravitational torque on the body is zero. (c) A couple on a body produces both translational and rotational motion. (d) Mechanical advantage greater than one means a small effort can lift a large load.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A rigid body is a body in which the distance between every pair of particles remains constant, no matter what force acts on it. It does not deform, bend or stretch.
In translation all particles have the same velocity and the body moves as a whole. In rotation each particle moves in a circle about a fixed axis and their linear speeds differ (v = rω), though angular velocity ω is common.
Translation: a block sliding straight down a smooth incline. Rotation: a ceiling fan or a wheel spinning about a fixed axle. Rolling of a cylinder is a combination of both.
Net external force causes translation of the centre of mass, while net external torque causes rotation. A couple has zero net force but non-zero torque, so it gives pure rotation.
Most rotational motion questions (moment of inertia, torque, angular momentum, equilibrium) assume a rigid body. Understanding it lets you treat a solid as a fixed system of particles and apply v = rω and centre-of-mass rules correctly.
