Why is it important to define a frame of reference?
a. an observer cannot be moving.
b. motion must be defined relative to something.
c. it's impossible to measure motion.
d. a stationary observer is always moving.

It is important to define a frame of reference because motion must be defined relative to something. Hence option(b) is correct.

Further Explanation:

Frame of reference is a framework that is used for observation of physical phenomena. It consists of an observer, a coordinate system and positions with respect to the coordinate system.

(a)

Let us take an example of a trolley moving with velocity and a man on the ground at rest.

Since, the man's velocity is zero, the relative velocity of trolley with respect to the man is .

Now, if a man starts to run with speed and speed of trolley increases to two times of initial velocity i.e. , the relative velocity of trolley with respect to man is .

Here, in both the conditions, the relative velocity of trolley with respect to time is same.

Therefore, it is wrong to say that an observer cannot be moving. Hence, option(a) is incorrect.

(b)

Let us take an example of a man on the ground at rest and a trolley moving with the velocity having an object placed in it. The object will also moves with the velocity .

For a man, trolley is moving with velocity but for an object trolley is at rest because there is no relative motion between object and trolley.

Therefore, it is right to say that motion must be defined relative to something. Hence, option(b) is correct.

(c)

Let us take an example of a first object moving with velocity and a second object moving with velocity in the opposite direction and a third object is at rest.

The relative velocity of first object with respect to second object is and the relative velocity of first object with respect to third object is .

Here, the relative velocity of an object is different with respect to different object but all the motions are measurable.

Therefore, it is wrong to say that it's impossible to measure motion. Hence, option(c) is incorrect.

(d)

Let us take an example of two trains moving with same velocity in same direction.

Consider a man sitting in one train observes a man running in other train, then the man sitting is said to be stationary observer for the man running. Here, the man is stationary observer while in motion.

Now, if a man standing on the ground at rest will see the man on train, he will be called stationary observer. Here, the man is stationary observer while not in motion.

Therefore, it is wrong to say that a stationary observer is always moving. Hence, option(d) is incorrect.

Learn more:

1. Motion on a rough surface: link

2. Motion under the gravitational force: link

3. Principle of conservation of momentum: link

Answer Details:

Grade: High School

Subject: Physics

Chapter: Kinematics

Keywords:

Frame, reference, important, observer, motion, relative, velocity, speed, moving, measure, stationary, example, trolley, man and train.

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