Bullet striking a block on a frictionless surface

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Homework Help Overview

The discussion revolves around a problem involving a bullet striking a block on a frictionless surface, specifically focusing on the application of conservation of momentum and energy principles. Participants are exploring how to determine the combined velocity of the bullet and block after the collision.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the use of conservation of momentum to find the final velocity after the collision. There are questions about how to apply conservation of energy in this context, particularly regarding the role of gravitational potential energy and kinetic energy transformations. Some participants express confusion about the relevance of gravity in this scenario.

Discussion Status

The discussion is active, with participants questioning the assumptions made regarding energy conservation and the implications of the collision being inelastic. There is a recognition that kinetic energy is not conserved, and some guidance has been offered regarding the transformation of energy into heat. However, there is no explicit consensus on how to proceed with the problem.

Contextual Notes

Participants are grappling with the implications of energy conservation in an inelastic collision and the definitions of potential and kinetic energy in the context of the problem. There are mentions of multiple unknowns and the need for clarity on the variables involved.

Dustinsfl
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Homework Statement


A 10g bullet with velocity 1000m/s strikes a 100g block at rest. What is their combined velocity? Can you work the problem with conservation of momentum? Energy?


Homework Equations





The Attempt at a Solution


For conservation of momentum, we have
$$
10g\cdot 1000m/s + 100g\cdot 0m/s = 110gv_f\Rightarrow v_f = \frac{1000}{11}m/s.
$$
How is this done with the conservation of energy? The KE right after impact is ##55g\cdot v_f^2 = ?##
Am I using the 0 PE level of the block? If that is the case, I have ##(m+M)gh=110g\cdot 9.8m/s^2\cdot h##. What is ##h##? I have two unknowns then.
 
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Dustinsfl said:

Homework Statement


A 10g bullet with velocity 1000m/s strikes a 100g block at rest. What is their combined velocity? Can you work the problem with conservation of momentum? Energy?


Homework Equations





The Attempt at a Solution


For conservation of momentum, we have
$$
10g\cdot 1000m/s + 100g\cdot 0m/s = 110gv_f\Rightarrow v_f = \frac{1000}{11}m/s.
$$
How is this done with the conservation of energy? The KE right after impact is ##55g\cdot v_f^2 = ?##
Am I using the 0 PE level of the block? If that is the case, I have ##(m+M)gh=110g\cdot 9.8m/s^2\cdot h##. What is ##h##? I have two unknowns then.

Why are you bringing gravity into this? What happens if the block is sliding on a long flat surface after it is struck by the bullet?

What happens to the KE of the bullet after it strikes the block?

What happens to the KE of the block after it is struck by the bullet?
 
SteamKing said:
Why are you bringing gravity into this? What happens if the block is sliding on a long flat surface after it is struck by the bullet?

What happens to the KE of the bullet after it strikes the block?

What happens to the KE of the block after it is struck by the bullet?

It is an inelastic collision so the KE is ##55gv_f^2## directly after impact. I already stated this though.
 
Dustinsfl said:
How is this done with the conservation of energy? The KE right after impact is ##55g\cdot v_f^2 = ?##
Am I using the 0 PE level of the block? If that is the case, I have ##(m+M)gh=110g\cdot 9.8m/s^2\cdot h##. What is ##h##? I have two unknowns then.

The mechanical energy (kinetic+potential energy) is not conserved. The potential energy does not change, the kinetic energy decreases, as part of the initial energy transforms to heat.

ehild
 
ehild said:
The mechanical energy (kinetic+potential energy) is not conserved. The potential energy does not change, the kinetic energy decreases, as part of the initial energy transforms to heat.

ehild

How am I supposed to use this information here?
 
Dustinsfl said:
How am I supposed to use this information here?

It means you cannot solve the problem assuming conservation of energy. You have to calculate the total energy after the collision, in joules. The total energy is solely kinetic. What does "g" mean in your formula?


ehild
 

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