When the cutting edge of a knife is put against a fast rotating stone to sharpen it, sparks are seen to fly. Explain the reason.

When a kinife is sharpened with the help of a rotation grinding stone, the speerk always travel tangentiall to it .Why ?

When a burning stick of incense is moved fast in a circle, a circle of red light is seen. Give reason

Internal micrometer is a measuring intrument used to measure internal diameter (ID) of a large cylinder bore with high accuracy. Construction is shown in figure. There is one fixed rod B (to the right in figure) and one moved rod A (to the left in figure). It is based on the particle of advancement of a screw when it is rotated in a nut with internal threads. Main scale reading can be directly seen on the hub which is fixed with respect to rod B. When the cap is rotated rod A moves in or cut depending on direction of rotation. The circular scale reading is seen by checking which division of circular scale coincide with the references line This is to be multiplied by LC to get circular scale reading. Least count = value of 1 circular scale division = ("pitch")/("number of division on circular scale") Length of rod A is chosen to match the ID(PQ) to be measured. Zero error is checked by taking reading between standard blocks fixed at normal value of ID to be measured. Zero error is positive if cap end is one the right of the main scale and negative it is on the left side. During zero setting of the above instrument, the end of the cap is on left side of the zero of main scale (i.e. zero of main scale is not visible) and 41^(th) division of circular scale coincides with the reference line, the zero error is -

When a burning stick of incense is moved fast in a circle, a circle of rod light is seen. Why does this happen? Explain.