Question

Class 9th, Physics.
Chapter - Force & Laws of Motion.

An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no, provide a reason.

Explanation required. ​

Answer 1

★ Answer :

Given : net zero external unbalanced force is experienced by an object .

Yes, it is possible for the object to be travelling with zero velocity when non zero external unbalanced force is applied .

Reason :

Given :

$f_{external} = 0$

We know that ,

→ F = ma

$f_{external} = ma$

0 = ma

mass can never be 0 .

therefore,

acceleration = 0

We know that ,

→ When velocity is constant acceleration becomes 0 as acceleration is rate of change of velocity and there is no change in velocity when velocity is constant .

$a = \frac{v - u}{t}$

v = final velocity

u = initial velocity

$0 = \frac{v - u}{t}$

v - u = 0

v = u

hence , proved velocity is constant .

→ We can have any value of velocity unless and until both initial and final velocity is same.

Hence proved , that an object experiencing a net zero external unbalanced force can have non - zero velocity .

Answer 2

simply you can understand that " net zero external unbalanced force "means the sum(resultant) of forces acting on the object is zero . From Newton's first law of motion that a body will remain moving with same velocity if there is no external force or sum of external force acting on it is zero.

No condition required to place on magnitude or in velocity direction,,the body will move with same velocity and in the same direction.

The condition is only that, external forces should be zero.