Physics gurus- do I have all the necessary variables?

[Pandamonium]

I'm four years removed from physics and my sister has a problem set due by midnight tomorrow. I used to be good at physics, but this one is a stumper. I figured someone on ATOT might know.

Here's the question:

A 5.00 g bullet is fired horizontally into a 1.00 kg wooden block resting on a horizontal surface. The coefficient of kinetic friction between block and surface is 0.20. The bullet remains embedded in the block, which is observed to slide 0.30 m along the surface before stopping. What was the initial speed of the bullet?

I thought to use the info to get an acceleration for the combined box-bullet, but I don't have enough information to do that. My memory is hazy, but I think most of the problems I did like this one did not involve friction.

 

[Hyperion042]

Conservation of energy:
1/2 m_bullet v^2 = (m_bullet + m_block) g d u
v = sqrt ( 2 * (m_bullet + m_block) ( 9.8 m/s^2 ) ( 0.3 m ) ( 0.20 ) / m_bullet )
m_bullet = 0.005 kg, m_block = 1.00 kg. Solve.

 

[Eeezee]

You have enough information

 

[FleshLight]

Are you using the Serway Jewett book? I remember this exact problem.

Conservation of Momentum FTW

 

[Pandamonium]

How is energy conserved in a completely inelastic collision? I've never seen mgxu = E, but I guess F(fr)=Nu=mgu and W=Fd combine to give W=mgud. So I guess it applies to a friction system. What gets me is how that can be applied in an inelastic problem.

The relevant equations I'm thinking include:
E conservation, P conservation, kinematics.

I can't use E cons. due to the type of collision.
I don't know how to use P cons. because I don't know the V(box+bullet) at the instant movement begins (or how much E is lost to get the box+bullet to overcome u(s))
I can't figure out a way of finding velocity without a time for the kinematics.

What does that leave me with?

Edit: the text is "Sears and Zemansky's University Physics, with Modern Physics" 11E, by Young & Freedman. My sister's gone to bed, but I'm determined to figure this one out =P She left her online grader up, and the answer given by Hyperion's solution is either wrong, or off by sigfigs. I haven't dealt with sigfigs since organic lab, and I don't feel like looking up the rules right now.

 

[Mo0o]

momentum is conserved. Find the velocity of the block+bullet right after impact by using the acceleration due to friction and how far it traveled to stop. Then use conservation of momentum to find velocity of bullet

 

[Pandamonium]

Originally posted by: Mo0o
momentum is conserved. Find the velocity of the block+bullet right after impact by using the acceleration due to friction and how far it traveled to stop. Then use conservation of momentum to find velocity of bullet

I wanted to do that, but I can't think of a method to calculate the acceleration of the box+bullet without a time, which isn't given in the problem.

Edit: Eeezee: do you have any tips for where to begin with this one?

 

[Mo0o]

Originally posted by: Pandamonium

Originally posted by: Mo0o
momentum is conserved. Find the velocity of the block+bullet right after impact by using the acceleration due to friction and how far it traveled to stop. Then use conservation of momentum to find velocity of bullet

I wanted to do that, but I can't think of a method to calculate the acceleration of the box+bullet without a time, which isn't given in the problem.

Vf^2=Vi^2 + 2a(X)

You should relearn your kinematic equations.

 

[Pandamonium]

Originally posted by: Mo0o

Originally posted by: Pandamonium

Originally posted by: Mo0o
momentum is conserved. Find the velocity of the block+bullet right after impact by using the acceleration due to friction and how far it traveled to stop. Then use conservation of momentum to find velocity of bullet

I wanted to do that, but I can't think of a method to calculate the acceleration of the box+bullet without a time, which isn't given in the problem.

Vf^2=Vi^2 + 2a(X)

You should relearn your kinematic equations.

I don't have Vi.

 

[Mo0o]

Originally posted by: Pandamonium

Originally posted by: Mo0o

Originally posted by: Pandamonium

Originally posted by: Mo0o
momentum is conserved. Find the velocity of the block+bullet right after impact by using the acceleration due to friction and how far it traveled to stop. Then use conservation of momentum to find velocity of bullet

I wanted to do that, but I can't think of a method to calculate the acceleration of the box+bullet without a time, which isn't given in the problem.

Vf^2=Vi^2 + 2a(X)

You should relearn your kinematic equations.

I don't have Vi.

That's what you're finding for the block+bullet situation. Vf is 0, after friction as stopped it

 

[Pandamonium]

That equation has unknowns in Vi and a. If I had one other relevant variable, I could figure out everything else from the kinematics. But I don't know how to begin.

 

[Mo0o]

Originally posted by: Pandamonium
That equation has unknowns in Vi and a. If I had one other relevant variable, I could figure out everything else from the kinematics. But I don't know how to begin.

You do know a. its the force of friction / mass of bullet+block

 

[Pandamonium]

Ahhh there it is. I never think to write out the "=ma" part of F=ma. I had F(fr)=umg and left it at that. I think the last time I did anything w/ physics was before my mcat, but I guess old habits die hard.

Edit: thanks!

 

[Mo0o]

Originally posted by: Pandamonium
Ahhh there it is. I never think to write out the "=ma" part of F=ma. I had F(fr)=umg and left it at that. I think the last time I did anything w/ physics was before my mcat, but I guess old habits die hard.

Edit: thanks!

What did you get on the MCAT? and what med school did you end up with?

 

[Vegitto]

What level is this? I got the exact same question (except with a BB instead of a bullet) on my physics test last week. I'm in high school, 5th grade (I don't know what American grade that is).

 

[Eeezee]

Originally posted by: Pandamonium

Originally posted by: Mo0o
momentum is conserved. Find the velocity of the block+bullet right after impact by using the acceleration due to friction and how far it traveled to stop. Then use conservation of momentum to find velocity of bullet

I wanted to do that, but I can't think of a method to calculate the acceleration of the box+bullet without a time, which isn't given in the problem.

Edit: Eeezee: do you have any tips for where to begin with this one?

Sorry, I wandered off for many hours and didn't check back until just now. It looks like you're headed down the right track though from the advice of others

 

[Eeezee]

Originally posted by: Vegitto
What level is this? I got the exact same question (except with a BB instead of a bullet) on my physics test last week. I'm in high school, 5th grade (I don't know what American grade that is).

I'd be curious to compare actually

In America, you enter high school at 9th grade as a freshman, your 2nd year is 10th grade (sophomore year), 3rd year (junior year)is 11th grade, and final year (senior year) is 12th grade. If you repeat the 12th grade, you're a "super senior"

 

[Vegitto]

Originally posted by: Eeezee

Originally posted by: Vegitto
What level is this? I got the exact same question (except with a BB instead of a bullet) on my physics test last week. I'm in high school, 5th grade (I don't know what American grade that is).

I'd be curious to compare actually

In America, you enter high school at 9th grade as a freshman, your 2nd year is 10th grade (sophomore year), 3rd year (junior year)is 11th grade, and final year (senior year) is 12th grade. If you repeat the 12th grade, you're a "super senior"

Hmm.. Our high school career takes 6 years, I'm in the fifth.. I'm 15, although most people in my class are 16/17. We've got one guy who's 18, but he's an idiot .