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# Thread: Reps for physics help

1. ## Reps for physics help

Collision problem, and I can't figure out how they got this answer.

You have been asked to design a 'ballistic spring system' to measure the speed of bullets. A bullet of mass m1 is fired into a bullet of mass M2. The block, with the embedded bullet, then slides across a frictionless table and collides with a horizontal spring whose spring constant is k. The opposite end of the spring is anchored to a wall. The spring's maximum compression d is measured.

Find an expression for the bullet's speed VB in terms of m1, M2, k, and d.

The back of the book says

VB= sqrt((m1+m2)*kd^2)/m^2)

2. Lol I still can't pass optics

3. Physics makes me hate school.

4. sounds to me like you just have to plug in the numbers

5. Originally Posted by .Excalibur
sounds to me like you just have to plug in the numbers
No, it wants the general formula using variables. I figured it out though. Next question, which I just can't remember how to do, is how to find energy lost in a collision.

edit: The answer I gave at the bottom of the post is what the book said the answer was. I think I figured out why.

6. Originally Posted by Father of Cajun
No, it wants the general formula using variables. I figured it out though. Next question, which I just can't remember how to do, is how to find energy lost in a collision.

edit: The answer I gave at the bottom of the post is what the book said the answer was. I think I figured out why.
Energy lost in a collision?

As in measure the amount of energy the object had before impact?

Energy lost in a collision?

As in measure the amount of energy the object had before impact?
Sort of. According to http://www.mcasco.com/qa_icel.html

It's simply 1/2 m*v^2 of the objects before minus 1/2m*v^2 of the object after.

Given that the speed of the bullet is 200 m/s and the mass of the bullet is 5 g. the mass of the block it hits is 2 kg

1/2(.005 kg)(200m/s)^2=100 N

after impact, 1/2(2.005 kg)(.499 m/s)^2=.24N

never mind. That's the right answer<_<. Thanks guys.

8. Originally Posted by Father of Cajun
Collision problem, and I can't figure out how they got this answer.

You have been asked to design a 'ballistic spring system' to measure the speed of bullets. A bullet of mass m1 is fired into a bullet of mass M2. The block, with the embedded bullet, then slides across a frictionless table and collides with a horizontal spring whose spring constant is k. The opposite end of the spring is anchored to a wall. The spring's maximum compression d is measured.

Find an expression for the bullet's speed VB in terms of m1, M2, k, and d.

The back of the book says

VB= sqrt((m1+m2)*kd^2)/m^2)
if thats what college level looks like then i am phucked.

9. Originally Posted by MistuuhMitch
if thats what college level looks like then i am phucked.
Tbh, this is one of the easier ones, but I'm on Fluids and stuff now and I was just having a brainfart with this.

10. if you tell me what text book your using, chapter and problem, i can look it up

if you tell me what text book your using, chapter and problem, i can look it up
:O Physics: A Strategic Approach by Randall D. Knight,2nd edition.

I'll PM you with a problem in a minute. I figured that one out and I'm working through some more now.

12. "College Physics: A Strategic Approach (2nd) textbook problems explained step-by-step"

cramster ftw, pm away

"College Physics: A Strategic Approach (2nd) textbook problems explained step-by-step"

cramster ftw, pm away
Thanks man I will for sure, just let me find one I can't figure out.

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