1.
3. An object of mass M is hanging by a light spring of force constant k from the ceiling, as
shown in Fig.3. A stnall ball of mass m which moves vertically upward colides with the object.
After the collision, the objoct and the small ball stick together and oscillate in simple harmonic
motion. Bcfore the collision, the object is at rest. The speed of the small ball just beore the
collision is denoted as v. The acceleration of gravity is denoted as g. Answer the following
questions. Fig. 3 (1) Find the amount of stretch of the spring from its natural length before the
collision (n) Mk (d) Mgk (e) A2 Mg (2) Find the speed of the object and the small ball just after
the small ball collides with the object and they stick together (a)Mv (c)M-m,v (b) (f) Minu (3)
Find the amount of decrease of the sum of kinetic energies of the small ball and the object,
before and after the smali ball collides with the object. (b) ^M (4) Find the period of the simple
harmonic oscillation after the small ball and the object stick together (5) During the simple
harmonic oscillation of the small ball and the object, which arc stuck together, the spring is at its
natural length wen the object is at its highest position. Find the kinetic energy of the small ball
just before it collides with tihe object. kTm (e)M 2 m 2km
Solution
(1) When the mass is hanged from the spring the spring get stretched under the weight Mg of the
object, so by Hooke's law, force = k × extension
Or, Mg = k × extension
Thus, extension = Mg/k. Option (e).
(2) Let the speed of the system of mass+object after collision be V, then, using conservation of
linear momentum,
M.0 + m.v = (M+m).V (as the object is at rest => mv = (M+m)V before collision)
Thus speed after collision,V = m/M+m v. Option (c).
(3) KE before collision = 1/2 mv^2 + 0 (as object is at = 1/2 mv^2 rest before collision)
KE after collision = 1/2 (M+m)V^2
= 1/2 (M+m){mv/(M+m)}^2

2.
= 1/2 m^2v^2/M+m
Decrease of KE = 1/2 mv^2 - 1/2 m^2v^2/M+m
= 1/2 mv^2 [1 - m/M+m]
= 1/2 Mm/M+m.v^2 , Option (d).