Frame of reference, rest and motion
Motion is relative: every description of position and velocity needs a frame of reference.
In this lesson
- Motion is relative: every description of position and velocity needs a frame of reference.
Is a parked car moving? Yes — relative to the Sun it is. Motion has no meaning without a frame.
A frame of reference is an origin plus axes (and a clock) that you use to measure positions and times. A body is 'at rest' or 'in motion' only with respect to a chosen frame — there is no absolute rest.
For JEE problems, the ground frame is the default, but switching frames (observing from a moving car, a lift, or another projectile) often turns a hard problem into a one-line answer.
Position of A relative to B
Worked example
The acceleration of a body starting from rest is a = 2 m/s² for 4 s, then zero for 2 s. Find its velocity at t = 6 s.
- Area under the a–t graph = change in velocity.
- Area = 2 m/s² × 4 s = 8 m/s (the last 2 s add nothing).
Answer: 8
Transcript (0 min)
WEBVTT 1 00:00:00.000 --> 00:00:05.000 Frame of reference, rest and motion — FemtoLearn. 2 00:00:05.000 --> 00:00:15.000 Motion is relative: every description of position and velocity needs a frame of reference.
Frequently asked
Is rest absolute?
No. A body at rest in one frame is in motion in another; only relative rest exists.
Practice
Free · 4 questions with full solutions- Q1 · Numerical · difficulty 2/5
The acceleration of a body starting from rest is a = 2 m/s² for 4 s, then zero for 2 s. Find its velocity at t = 6 s.
- Q2 · Numerical · difficulty 2/5
A body starts from rest with a = 4 m/s². Find the displacement in the 5th second.
- Q3 · Numerical · difficulty 2/5
A body starts from rest and accelerates uniformly at 9.5 m/s². Find the displacement in the 8th second of its motion.
- Q4 · Numerical · difficulty 2/5
A stone is dropped from rest from a height of 10 m. Taking g = 10 m/s², find the time it takes to reach the ground (neglect air resistance).