Question

Consider that a car is moving forward and the tyres start rotating. The direction of the moment caused due to the rotation of tyres will be
A. Inside the car
B. Outside of car
C. In the forward direction
D. In the backward direction

Answer 1

When a force is applied such that it is not having equal and opposite force directly along the line of action, a moment is developed. The magnitude of the moment is given as the product of the force (F) and the moment arm (d).Mathematical expression is $m = F \times d$ .

Complete answer:
The direction of the moment of force can be given by the right-hand thumb rule. This rule says that given two vectors, point the thumb towards one vector and the direction of curl of the fingers would give the direction of the other vector. Curl your fingers in the direction of rotation of tyres. You will find that the thumb will point inwards. Thus, the direction of the moment of force is inwards.

Hence option A is correct.

Note: The moment tells you the tendency of a body under the influence of a force to rotate about a point or about an axis. It does not relate in any way to the tendency to translate. In the above question, the direction was found out using the right-hand thumb rule. This is a general rule which can be used to find the direction of many vectors given that we know what quantity signifies the direction of the thumb and what signifies the direction of curl of fingers.