# Homework Help: Law of conservation of momentum problem?

1. Apr 25, 2012

### abrowaqas

1. The problem statement, all variables and given/known data

A man of mass 65kg is running at speed of 4.9m/s, jumps in to a rowboat of 88kg that is drifting without friction in the same direction at a speed of 1.2m/s. when the man is seated in the rowboat, what is the final velocity of the boat?

2. Relevant equations

Momentum before = momentum after

3. The attempt at a solution

If
m= mass of man = 65kg
v= velocity of man = 4.9m/s

m' = mass of rowboat = 88kg
v' = velocity of rowboat = 1.2m/s

V= required final velocity
let
M= mass of both rowboat and man = m+ m' = 153kg

now
law of conservation of momentum

momentum before = momentum after

i-e
mv = M(v'+V)

so

V= (mv-Mv')/M

= 0.88 m/s

is it right method ?

but how could the speed of rowboat is such less?

2. Apr 25, 2012

### tiny-tim

hi abrowaqas!
yes
noooo

3. Apr 25, 2012

### abrowaqas

P = momentum, m = mass and v= velocity, and P=m*v

So before the collision we have a total momentum of (65*4.9) + (88*1.2) = 424.1Kg/ms^-1

After the collision, the momentum must be 424.1. This is equal to the combined mass of the man and the boat (65+88=153kg) multiplied by the final velocity of the boat. So all we need to do is 424.1/153 to give us your answer of 2.77 m/s.

is this method right?

4. Apr 25, 2012

### tiny-tim

are you quoting someone?

yes, that method is correct

now rewrite your m M v V equation so that it's correct!​

5. Apr 26, 2012