Question

A water tanker filled up to $\dfrac{2}{3}$rd of its height is moving with a uniform speed. On sudden application of the brake, the water in the tank would:
A. move backward
B. move forward
C. be unaffected
D. rise upwards

Answer 1

Three laws which were proposed by newton are used everywhere. These are the basics for the problems which deal with forces. First law tells us about inertia while second law tells us about the force and third law tells us about the action and reaction pair. We use one of these laws to solve this.

Complete answer:
According to Newton's first law “A body keeps on maintaining its state of rest or motion until and unless it is acted upon by an external force.” This property of the body is called inertia which means resistance to change. Inertia can be like inertia of rest or inertia of motion or inertia of direction.
Now coming to the question asked, a water tanker filled with water is moving with uniform speed which means the body is in equilibrium. That means no external force is acting on the body. Now they had applied the brakes very suddenly then deceleration will happen. Until brakes were applied the water tanker was kept on moving which means water was also in motion. But when brakes are applied, the tank will stop but the water will always try to move in the previous direction due to the property of inertia. Hence water will move forward when brakes are applied.

Hence answer would be option B.

Note:
In daily life also we see many examples of this first law. When sudden brake is applied to the moving bus then the passengers will fall forward this is due to inertia and when the bus at rest is started suddenly then the passengers will fall back this is also due to inertia.