[Linear momentum] When to use which equation.

[hiuting]

Homework Statement

I'm confused about when to use which formula.
I thought for inelastic collisions,
for example, when a bullet is shot into a block,
I should use
(m_bullet)(v_initial) = (m_bullet + m_block)(v_final)

but it turns out that I must use

why do i have to use this one?

Homework Equations

The Attempt at a Solution

 

[itr]

eqn w/ velocities you use

 

[kjohnson]

They should really be the same equation. (m1)(vi)+(m2)(vi)=(m1)(vf)+(m2)(vf) will reduce to the top equation you wrote if the block "m2" is initally at rest (vi=0) and upon impact they stick and form a system. Thus they have a combined mass (m1+m2) and the same final velocity (vf). So it reduces to:

(m1)(vi)bullet = (m1+m2)(vf)system

 

[hiuting]

They should really be the same equation. (m1)(vi)+(m2)(vi)=(m1)(vf)+(m2)(vf) will reduce to the top equation you wrote if the block "m2" is initally at rest (vi=0) and upon impact they stick and form a system. Thus they have a combined mass (m1+m2) and the same final velocity (vf). So it reduces to:

(m1)(vi)bullet = (m1+m2)(vf)system

then v1 final and v2 final are the same??

 

[kjohnson]

Yes, this is true because they stick together to form a system. They must have the same velocity.