Question

A boy is whirling a stone tied with a string in a horizontal circular path. If the string breaks, the stone
A. will continue to move in the circular path
B. will move along a straight line towards the centre of the circular path
C. will move along a straight line tangential to the circular path
D. will move along a straight line perpendicular to the circular path away from the boy

Answer 1

First law of Newton plays a significant role in this question. Concepts of classical mechanics can also be used. Having an idea of centripetal force can help to solve the question directly. Try to imagine a simple case of whirling stone tied to a string and how centripetal force will act on it. Centripetal force helps to keep an object in a curved path.

Complete answer:
When the boy is whirling a stone which is tied with a string covering a circular path, then the centripetal force starts acting towards the center of the circular path. According to the question, while writing the stone the string breaks. The moment the string breaks the centripetal force acting towards the center ceases and hence no force acts on the system. So by Newton’s first law of motion the stone will continue in a straight line and will fly off along the tangent of the circular path.
Therefore option C is correct.

Additional information:
Newton’s first law: Object will continue to rest or in motion unless external force is applied to it.
Newton's Second law: The applied force on an object is equal to the product of acceleration and mass of the object.
Newton’s third law: When two objects interact with each other, then the force acting between them is equal in magnitude and opposite in direction.
Centripetal force: The force which is used to keep an object in a curved path is known as centripetal force. Centripetal force has the tendency to accelerate the object by changing its direction.

Note:
Newton’s law should be studied properly and we should know the conceptual tricks to solve this type of question. Centripetal force depends on the velocity and radius of the rotating object. The formula for centripetal force is given by $\dfrac{m{{v}^{2}}}{r}$.