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```
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...}

```
9 years ago

Sudheesh Singanamalla
114 Points
```							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

```
9 years ago
vishal bhatnagar
14 Points
```							first you have to choose your coordinate axis and then work according to your convention.
```
9 years ago
509 Points
```							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)
```
9 years ago
iit jee
44 Points
```							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

```
9 years ago
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### Course Features

• 110 Video Lectures
• Revision Notes
• Test paper with Video Solution
• Mind Map
• Study Planner
• NCERT Solutions
• Discussion Forum
• Previous Year Exam Questions