How Does Bullet Velocity Change After Passing Through Different Block Widths?

  • Thread starter Thread starter Verrine
  • Start date Start date
  • Tags Tags
    Block Bullet
AI Thread Summary
The discussion revolves around calculating the bullet's velocity after passing through two blocks of different widths. Initially, the bullet travels at 200 m/s through a block and stops after covering a distance D. When considering a second block of half the width, the calculations initially led to an incorrect conclusion that the bullet's velocity increased to 244 m/s. After clarification and further calculations, it was determined that the bullet's velocity after passing through the second block is actually 141 m/s. The final consensus emphasizes the importance of understanding kinetic energy and the work done by forces in these scenarios.
Verrine
Messages
8
Reaction score
0

Homework Statement



A bullet goes through a block and covers a distance D with a velocity of V0=200m/s. Calculate the velocity that the same bullet has AFTER passing through another block of the same material but which has half the width of the initial block.

Homework Equations

The Attempt at a Solution

 
Physics news on Phys.org
Verrine said:

Homework Statement



A bullet goes through a block and covers a distance D with a velocity of V0=200m/s. Calculate the velocity that the same bullet has AFTER passing through another block of the same material but which has half the width of the initial block.

Homework Equations

The Attempt at a Solution


You have to make an attempt at solving this problem yourself. It is your homework, after all! That said, are you sure that's all the information you have?
 
PeroK said:
You have to make an attempt at solving this problem yourself. It is your homework, after all! That said, are you sure that's all the information you have?
Ok sure. I have an attempt, the problem is that my answer doesn't coincide with the answer in the textbook.
For the first block I used the fact that the variation of kinetic energy is equal with the work force, so: 0-m*V0^2/2 =Fr *d => Fr=m*V0^2/2d
For the second block: mV^2/2 - mV0^2/2= Fr * d/2, so V=244 m/s
But the answer is wrong ;(. And yes, this is all they give me.
 
Verrine said:
Ok sure. I have an attempt, the problem is that my answer doesn't coincide with the answer in the textbook.
For the first block I used the fact that the variation of kinetic energy is equal with the work force, so: 0-m*V0^2/2 =Fr *d => Fr=m*V0^2/2d
For the second block: mV^2/2 - mV0^2/2= Fr * d/2, so V=244 m/s
But the answer is wrong ;(. And yes, this is all they give me.

Is ##D## the thickness of the block?

Are you to assume that the bullet goes through the second block after the first? Or, that it goes through the second block in a separate experiment?

Where did you get the final velocity of ##0## from?

I'm not sure I follow your calculations, but you seem to have the bullet speeding up as a result of going through the second block!?
 
PeroK said:
Is ##D## the thickness of the block?

Are you to assume that the bullet goes through the second block after the first? Or, that it goes through the second block in a separate experiment?

Where did you get the final velocity of ##0## from?

I'm not sure I follow your calculations, but you seem to have the bullet speeding up as a result of going through the second block!?

It goes through the second block in a separate experiment. To be sincere, I don't know myself what D actually stands for. What they say is that the bullet passes through the first block and it covers a certain distance, which is D. I also saw the D as the thickness of the block, but I am not entirely sure if that is also how they see it :(. Sorry

For the calculus: we had mV^2/2 - mV0^2/2=Fr*d/2 => mV^2/2=mV0^2/2 + mV0^2/2 * d/2 => V^2=6V0^2 /4 => v=244 m/s

Also about the 0 velocity, I thought that after the bullet covers that distance, it stops, so the energy is 0.
 
perhaps you should post the question as the books state it.
 
gleem said:
perhaps you should post the question as the books state it.

The book is not in English. I had to translate it. But trust me, this is what it says.
 
Verrine said:
The book is not in English. I had to translate it. But trust me, this is what it says.

What's the answer? We might then work out what the question is!

My guess is ##141m/s##.
 
PeroK said:
What's the answer? We might then work out what the question is!

My guess is ##141m/s##.
Yes! This is the answer! Can you please explain how you did it? :D
 
  • #10
Verrine said:
Yes! This is the answer! Can you please explain how you did it? :D

Here's the question:

A bullet with an initial velocity of ##200m/s## is fired into a block and stops after a distance ##D##. If the same bullet is fired into a block of thickness ##D/2##, how fast is the bullet moving when it leaves the block?
 
  • Like
Likes beakerinabowtie, gleem and Verrine
  • #11
Verrine said:
It goes through the second block in a separate experiment. To be sincere, I don't know myself what D actually stands for. What they say is that the bullet passes through the first block and it covers a certain distance, which is D. I also saw the D as the thickness of the block, but I am not entirely sure if that is also how they see it :(. Sorry

For the calculus: we had mV^2/2 - mV0^2/2=Fr*d/2 => mV^2/2=mV0^2/2 + mV0^2/2 * d/2 => V^2=6V0^2 /4 => v=244 m/s

You need to think about why you got a final velocity greater than the initial velocity here.
 
  • #12
PeroK said:
You need to think about why you got a final velocity greater than the initial velocity here.

I simply tried to solve the equations with what formulas I knew... I actually have no idea what is going on here. The problem is confusing and I don't really see the connection between the velocity of the bullet and the thickness of the block. Sorry. If you could explain it step by step, I would be really grateful.
 
  • #13
Verrine said:
I simply tried to solve the equations with what formulas I knew... I actually have no idea what is going on here. The problem is confusing and I don't really see the connection between the velocity of the bullet and the thickness of the block. Sorry. If you could explain it step by step, I would be really grateful.

There are two ways to solve this. The better way is to look at the energy. Do you know about Kinetic Energy and the Work done by a Force?

If not, then you can also use the equations of motion.
 
  • #14
PeroK said:
There are two ways to solve this. The better way is to look at the energy. Do you know about Kinetic Energy and the Work done by a Force?

If not, then you can also use the equations of motion.
Yes, I know about Kinetic energy and work.
 
  • #15
Verrine said:
Yes, I know about Kinetic energy and work.

What's the initial KE of the bullet? And what force does work to stop it in this case?
 
  • #16
PeroK said:
What's the initial KE of the bullet? And what force does work to stop it in this case?
I think I got it. If I do mv^2/2 -mv0^2/2=Fr*d/2, then mv^2/2 =mv0^2/4. So v=v0 /rad 2 =v0*rad2 /2 =141m/s :):) Thank you . :D
 
  • #17
You might like to know where you went wrong initially.
Verrine said:
For the second block: mV^2/2 - mV0^2/2= Fr * d/2
Think about the signs there. Should V be more or less than V0? Will it gain or lose KE?
 
  • Like
Likes Verrine

Similar threads

Is My Physics Calculation on Bullet and Block Interaction Correct?
Replies
14
Views
4K
A bullet pushing a knot out of a wood block
Replies
13
Views
3K
What is the force imparted by the bullet on the block
Replies
8
Views
4K
Momentum problem -- Two bullets fired into a block
Replies
7
Views
2K
Calculating the velocity of of block that was crossed by a bullet
Replies
3
Views
2K
How can the kinetic energy ratio be calculated after a bullet collision?
Replies
6
Views
2K
Collisions - bullet shot in in block
Replies
1
Views
2K
How Fast Was the Bullet That Lodged in the Wooden Block?
Replies
6
Views
4K
Bullet passes through block - conservation of momentum
Replies
5
Views
3K
How Do You Calculate the Final Speed of a Bullet-Block System?
Replies
13
Views
2K

Hot Threads

Recent Insights

Back
Top