# Momentum and energy in collisions, bullet problem

#### jchojnac

1. Homework Statement

A bullet of mass 0.125 kg traveling horizontally at a speed of 250 m/s embeds itself in a block of mass 3.5 kg that is sitting at rest on a nearly frictionless surface.
(a) What is the speed of the block after the bullet embeds itself in the block?

v = ? m/s

(b) Calculate the kinetic energy of the bullet plus the block before the collision:

Ki = ? J

(c) Calculate the kinetic energy of the bullet plus the block after the collision:

Kf = ? J

d) Was this collision elastic or inelastic?
inelastic

(e) Calculate the rise in thermal energy of the bullet plus block as a result of the collision:

Ethermal,bullet + Ethermal,block = ? J

(f) Which of the following statements are true?
The bullet and block got hot because Q was large.
Because the process was very fast, Q was negligible.
Q was negative during the collision because the bullet and block were hotter than the surroundings.

2. Homework Equations
delta p1+delta p2=0
delta K=0

3. The Attempt at a Solution
For a.) i tried 2(m/M)v but it was wrong.

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#### LowlyPion

Homework Helper
1. Homework Statement

A bullet of mass 0.125 kg traveling horizontally at a speed of 250 m/s embeds itself in a block of mass 3.5 kg that is sitting at rest on a nearly frictionless surface.
(a) What is the speed of the block after the bullet embeds itself in the block?
v = ? m/s
(b) Calculate the kinetic energy of the bullet plus the block before the collision:
Ki = ? J
(c) Calculate the kinetic energy of the bullet plus the block after the collision:
Kf = ? J
d) Was this collision elastic or inelastic?
inelastic

(e) Calculate the rise in thermal energy of the bullet plus block as a result of the collision:

Ethermal,bullet + Ethermal,block = ? J

(f) Which of the following statements are true?
The bullet and block got hot because Q was large.
Because the process was very fast, Q was negligible.
Q was negative during the collision because the bullet and block were hotter than the surroundings.

2. Homework Equations
delta p1+delta p2=0
delta K=0

3. The Attempt at a Solution
For a.) i tried 2(m/M)v but it was wrong.
What is the equation for an inelastic collision then?

Does this help?
http://en.wikipedia.org/wiki/Inelastic_collision#Perfectly_inelastic_collision

yea thanks

#### jchojnac

for c.) wouldn't the final kinetic energy be 1/2*m*v^2 of just the block since the final speed of the bullet is 0?

#### LowlyPion

Homework Helper
for c.) wouldn't the final kinetic energy be 1/2*m*v^2 of just the block since the final speed of the bullet is 0?
In an inelastic collision the bullet will have the same velocity as the block. It is embedded isn't it? So the final kinetic energy will be the sum of the masses times the final velocity.

#### jchojnac

i tried (m1+m2)*vf...(.125+3.5)*8.62 and got 31.2475. It said it was wrong. I calculated the final velocity and got it right. What's wrong?

#### sriceb01

i tried (m1+m2)*vf...(.125+3.5)*8.62 and got 31.2475. It said it was wrong. I calculated the final velocity and got it right. What's wrong?
Try 1/2 *(m1+m2)*vf^2.....this should work

#### LowlyPion

Homework Helper
i tried (m1+m2)*vf...(.125+3.5)*8.62 and got 31.2475. It said it was wrong. I calculated the final velocity and got it right. What's wrong?
Isn't c) asking for 1/2 m*v2?

1/2*3.65*(8.62)2

#### sriceb01

For part e) take the difference between the kinetic energy of the bullet plus the block before the collision and the kinetic energy of the bullet plus the block after the collision to get the answer.
f) it happens so fast, that q is negligible.

yea i got it now

#### fball558

hey, are you in 172??
is this on the webassign?
what did you get for the first problem? the one with different masses? i have a posting on here, i tried many different things but cant get it.
can you help me on it??

#### sriceb01

Yea man, Im in 172 and on webassign, im stuck at the same spot you are.

#### fball558

problem one answers are

|1,final| < |1,initial|

If m2 >> m1, then the final speed of object 1 is greater than the final speed of object 2.

1,initial = 1,final + 2,final

is that where you were stuck?
i guess we had to assume it was elastic where M1 bounces off of M2.
not sure how the expect us to know that with what is given but oh well.

#### sriceb01

No, sorry im mean on question #3 where you got stuck...I got #1

#### fball558

Question #3 i found the Vfinal by using
P1(inital) + P2(inital) = P1(final) + P2(final)
you know all but P2(final) just solve for it by doing m*v (find the P) for each
remember your answer is in P so use P= mv to get v (just divide by mass)

not sure how to find E internal. im still stuck on that.

lastly the box at the bottom just click all the boxes with Q = 0 (should be 2 boxes)

#### sriceb01

alrite man thanks!

#### sriceb01

the only thing im stuck on is the e internal

#### fball558

no problem.
see you on here tomorrow or thursday for the hw due then lol
its worth 67 points but not sure how hard it is.
good luck on it lol

#### fball558

yeah i did not get that. got everything else so 21/22 is fine with me

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