CBSE Class 11-science Answered

(1) First equation of Motion: v = u + a t

Consider a body of mass “m” having initial velocity “u”.

Let after time “t” its final velocity becomes “v” due to uniform acceleration “a”.

Now we know that:
Acceleration = Change in velocity / Time taken

Acceleration = (Final velocity - Initial velocity) / Time taken

  a = v - u / t
 a t = v - u

or  v = u + a t

This is the first equation of motion.

(2) Second equation of motion: s = u t + 1/2 at²

Let the distance travelled by the body be “s”.

We know that
Distance = Average velocity x Time

Also, Average velocity = (u + v) / 2

.:  Distance (t) = (u + v) / 2 t            …….eq.(1)

Again we know that:

v = u + at

Substituting this value of “v” in eq.(1), we get

s = (u + u + a t) / 2 t
s = (2 u + a t) / 2 t
s = (2 u t + a t²) / 2
s = (2 u t / 2) + (a t² / 2)

or  s = u t + (1/2) a t²

This is the 2nd equation of motion.

(3) Third equation of Motion:  v² = u² +2 a s

We know that

v = u + a t
v - u = a t

or  t = (v - u) / a                                    ………..eq.(2)

Also we know that

Distance = average velocity x Time

.: s = [(v + u) / 2] x [(v - u) / a]

s = (v² – u²) / 2 a

2 a s = v² – u²

or  v² = u² + 2 a s

This is the third equation of motion.