I read it in a textbook but didn't get it...
When knife is sharpened by pressing it against a grinding stone, the sparks fly off along the tangent to the grinding stone, on account of direction of inertia.
How the concept of direction of inertia is working here...could you be a little detailed??
Sparks are small pieces of a grinding stone heated to a high temperature due friction. When they break away from a grinding stone, they continue motion due to inertia.