Question

Newton first law of motion is NOT applicable if ________ 
1. object is at rest
2. object is moving with uniform velocity
3. object is moving with uniform acceleration
4. none of the above

Answer 1

Correct Answer - Option 3 : object is moving with uniform acceleration

CONCEPT

• Newton’s First Law: It states that a body continues in its state of rest or uniform motion in a straight line until and unless an unbalanced force acts on it.
  • It is valid only in an inertial frame of reference.
  • Newton’s First Law is also known as the law of inertia.
• Inertia is the property of the inability of a body to change its position of rest or uniform motion in a straight line.
  • Mass is the measure of the inertia of a body.
• Inertial Frame: A frame of rest or uniform motion. Ex- A bus moving with constant acceleration.
• Non-inertial frame: A-frame which is accelerating either in a linear fashion or rotating around its axis. Ex- A car turning with constant speed at corners.

EXPLANATION:

• Newton’s First law of motion says about the state of rest or uniform motion and is valid on only Inertial Frame.
• Thus newton’s laws are not valid in a non-inertial frame or object is moving with uniform acceleration
• For example, a man inside the accelerating car will feel force exerted on him although no external force is applied hence first law is not valid for accelerating frame of reference.

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• Newton’s first law always seen wrong in an accelerating frame because An object is a rest at an inertial frame can see accelerating in a non-inertial frame even though no force acting on it.
• For Example: Suppose a man standing in a truck, and the truck is moving with acceleration ‘a’ and another person is standing still on the ground.
  • Case 1: Still person on the ground sees a man standing still but the truck moves forward and person in it.
  • Case 2: Man in bus sees the still person on the ground move with an acceleration ‘a’ backward where no force is visible.