Question

Newton's First Law: "If the vector sum of all the forces acting on a particle is zero then and only then
the particle remains unaccelerated" (i.e.,remains at rest or moves with constant velocity)
the sentence is true or false​

Answer 1

Answer:

yes True

Explanation:

First law of motion

If the (vector) sum of all the forces acting on a particle is zero then and only then the particle remains unaccelerated (i.e., remains at rest or moves with constant velocity).

We can say in vector notation

a = 0 if and only if resultant force F = 0

A frame of reference in which Newton's first law is valid is called an inertial frame of reference.

A frame of reference in whch Newton's first law is not valid is called a noninertial frame of reference. (Example: lamp in an elevator cabin whose cable had broken)

Example of lamp in an elevator who cable had broken:

In the cabin when on measures with reference to the cabin, the lamp hanging from the ceiling has no acceleration. Hence the forces acting on the lamp, its weight (W) and the tension in the string supporting it are balancing each other W = T.

But for an observer on the ground, lamp is accelerating with acceleration g, when he considers the forces acting on the lamp as w and T once again, W is not equal to T as lamp is accelarating. Both cannot be right at the same time, and it means in once of the frames Newton's first law is not applicable.

Answer 2

Answer:

a body continues its state of rest or of uniform motion unless a net force acts on it