Question

an unbalanced external force is needed to initiate the motion from state of rest but no unbalanced net force is needed to sustain the uniform motion . justify the statement fo class 9

Answer 1

Unbalanced forces:
When two forces are acting in opposite direction of an object which causes change in state of motion of object.
The two forces are not equal in magnitude.
Hence where the force is more in that direction, the object will move.
So an unbalanced external force is needed to set an object in motion.
According Newton's FIRST law every object continues to be in state of rest or uniform motion unless a net force acts on it.
So, once objects moves it continues its motion. Always an external force may not be needed to stop the motion.sometimes due to friction the object May travel certain distance and stops.

Answer 2

Explanation:

for justifying this statement you need to know about 2 things

1.unbalanced forces

2. Newton's first law of motion

When two forces are acting in opposite direction of an object which causes change in the state of motion of an object these are unbalanced forces .

The two unbalanced forces are not equal in magnitude hence the object moves in the direction of the force which has a greater magnitude .

So an unbalanced external force is required to bring the object in a state of motion

According to Newton's law of motion an object continues to remain in a state of rest or of uniform motion untill an external unbalanced force acts on it

So once object moves it continues in a state of motion .

Therefore if we apply the unbalanced force the object would come to a rest

Therefore no unbalanced force is required to sustain the uniform motion of the object

hope this helps