[Linear momentum] When to use which equation.

  • Thread starter hiuting
  • Start date
1. The problem statement, all variables and given/known data
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
(mbullet)(vinitial) = (mbullet + mblock)(vfinal)

but it turns out that I must use
b207134634.jpg


why do i have to use this one?

2. Relevant equations



3. The attempt at a solution
 

itr

43
0
eqn w/ velocities you use
 
157
0
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
 
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??
 
157
0
Yes, this is true because they stick together to form a system. They must have the same velocity.
 

Want to reply to this thread?

"[Linear momentum] When to use which equation." You must log in or register to reply here.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Top Threads

Top