# an elastic ball hits a wall and rebounds with same velocity, what is its change in momentum?????[pls tell me which velocty has to be taken as negative- intial or final and why??? pls explain...}

Sudheesh Singanamalla
114 Points
11 years ago

Dear shivam ,

there are some conventions to be noted :

1. If a body travels in the positive 'x' direction , it is taken as positive

2. If a body travels in the negative 'x' direction , it is taken as negative

the same is the case with velocity.

here the answer will differ according to the figure drawn.

consider this figure

initial momentum = P1 = mv

final momentum after hitting the wall =P2 = m (-v) = -mv

change in momentum = P2-P1 = -mv - mv = -2mv

therefore magnitude of change in momentum = 2mv

vishal bhatnagar
14 Points
11 years ago

first you have to choose your coordinate axis and then work according to your convention.

509 Points
11 years ago

consider a ball is moving along +ve x axis & a wall is placed in yz plane...

initial velocity = v = u (i)                          (i is unit vector along +x axis)

intial momentam = mv = pi = mv (i)     ..................1

now when ball hits the wall it rebounds & its velocity gets reversed ..

final velocity = u (-i)

final momentam = mv (-i)          ....................2

change in momentam = initial - final momentam

= mu (i) - mu (-i)

= 2mu (i)

iit jee
44 Points
11 years ago

hi,

if the mass of ball is m and initial velocity =v

then change in momentum is 2mv!!!!!!!!!!!!!!!

final velocity must be taken negetive!!!!!!!!!!!!!!

kedar joshi