When a disc rotates with uniform angular velocity, which of the following is not true?

To determine which statement is not true when a disc rotates with a uniform angular velocity, let us analyze the definition of uniform angular velocity and its relationship with other rotational parameters.

Uniform angular velocity means that the angular velocity vector remains constant over time. This implies two things:
1. The magnitude of the angular velocity (which is the speed of rotation) remains constant and is non-zero.
2. The direction of the angular velocity vector (which determines the orientation of the axis of rotation and the sense of rotation) remains unchanged.

Now, let us evaluate the given options:

1. The sense of rotation remains same: Since the direction of the angular velocity vector is constant, the sense of rotation (clockwise or counterclockwise) does not change. Therefore, this statement is true.

2. The orientation of the axis of rotation remains same: Since the direction of the angular velocity vector is constant, the axis of rotation remains fixed in space. Therefore, this statement is true.

3. The speed of rotation is non-zero and remains same: Since the angular velocity is uniform and the disc is rotating, its magnitude (speed of rotation) is constant and non-zero. Therefore, this statement is true.

4. The angular acceleration is non-zero and remains same: Angular acceleration, denoted by
$\alpha$
is defined as the rate of change of angular velocity with respect to time:

$\alpha =\frac{d\omega }{dt}$

Since the angular velocity
$\omega$
is constant (uniform), its derivative with respect to time is zero:

$\alpha =0$

Thus, the angular acceleration must be exactly zero, not non-zero. Therefore, this statement is false.

Since the question asks for the statement that is not true, the correct option is "The angular acceleration is non-zero and remains same."