An instrument box placed on a table is given a push. It is observed that it stops its motion after some time. What is the force that stopped its motion?
A. Gravitational force
B. Tension force
C. Friction force
D. Mechanical force

Hint: When a body accelerates in a particular direction, its velocity increases in that direction. Therefore, in this case, the box should have travelled with increasing velocity, however it is seen that the body comes to rest after some time. Therefore, it is understood that there is an opposing force acting the box.

Complete answer:
In the given case, it is said that an instrument box is placed on a table. Let us assume that the plane of the table is along the horizontal plane (i.e. parallel to the ground). Then it is said that the instrument box is given a push. It is observed that the box stops after some time.From Newton’s laws of motion we know that when we push a body, we exert a force on the body. Due to this force, the body accelerates in the direction of the applied force.

When a body accelerates in a particular direction, its velocity increases in that direction.Therefore, in this case, the box should have travelled with increasing velocity, however it is seen that the body comes to rest after some time.When a body accelerates in a particular direction, its velocity increases in that direction. Therefore, in this case, the box should have travelled with increasing velocity, however it is seen that the body comes to rest after some time.

This can happen only when some other force is applied on the box, in the direction opposite to the previously applied force. In this case, the force that stops the box must be in the opposite direction of the motion of the box and parallel to the surface of the table. The force that comes into play when there is a relative motion between two surfaces is called frictional force. Therefore, when the box moves forward relative to the table, the friction exerts an opposing force and the box comes to rest.

Hence, the correct option is C.

Note: In this case, there is one more that acts on the box and that is the gravitational force exerted by the earth. The direction of gravitational force on any object is always downwards (along the negative y-axis). Therefore, the gravitational force on the box is acting downwards. However, the box is in contact with the table, due to which the normal force exerted by the table will balance the gravitational force.