Momentum & Kinetic Energy Conservation: 1kg Block & 2kg Block Collision

Click For Summary

Homework Help Overview

The problem involves a collision between a 1kg block and a 2kg block, where the 1kg block is initially at rest on a frictionless surface and connected to a spring. The 2kg block collides with the 1kg block, and they stick together. The discussion centers around the conservation of momentum and kinetic energy during the collision and the subsequent behavior of the blocks as the spring compresses.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are questioning whether momentum or kinetic energy is conserved during the collision and discussing the implications of the collision being perfectly inelastic. There are inquiries about the amount of spring compression and the speed of the blocks after compression.

Discussion Status

Some participants have provided insights regarding the conservation of momentum in collisions and the nature of energy conservation in this context. There is an ongoing exploration of the definitions and implications of energy conservation, particularly in relation to mechanical energy.

Contextual Notes

There is a mention of the collision being perfectly inelastic, which influences the conservation laws applicable to the scenario. The discussion also reflects on the need for clarity regarding the definitions of kinetic and mechanical energy.

xshezsciencex
Messages
6
Reaction score
0
A 1kg block is at rest on a horizontal frictionless surface touching the end of an unstretched spring of spring constant 200N/m. Tjhe other end of the spring is fixed to a wall. A 2kg block traveling at a speed v(initial)=4m/s collides with the 1kg block, and the two blocks stick together.

I am not sure if momentum or kinetic energy is conserved in this collision, if it is please explain.
When the blocks momentarily stop, by what amount is the spring compressed?
I understand that when the spring expands it will accelerate the coupled blocks from rest so what is the speed of the blocks when they are 0.20m to the left of the spring's end?
 
Physics news on Phys.org
xshezsciencex said:
I am not sure if momentum or kinetic energy is conserved in this collision, if it is please explain.

Hi xshezsciencex! :smile:

Momentum is always conserved in collisions.

Energy is never conserved unless the question says so!

In this case, the question says that the collision is perfectly inelastic (whcih is just about the opposite of conservation of energy), in that the two final velocities are the same.

So use conservation of momentum, plus vf1 = vf2.

(After the collision, when the spring is being compressed, energy is conserved, and of course momentum isn't)
 
tiny-tim said:
Energy is never conserved unless the question says so!

Perhaps putting "kinetic" at the beginning will make this phrase easier to swallow :wink:
 
Hi Borek! :smile:

"mechanical" energy, actually! :wink:
 
Good point. At least we agree that the statement was a little bit bold :-p
 

Similar threads

Find elastic energy in the compressed spring.
  • · Replies 12 ·
Replies
12
Views
3K
Problem with when to use Force and Energy. What compresses spring/
  • · Replies 3 ·
Replies
3
Views
2K
Time period of SHM & Energy conservation in pulley-spring-block system
  • · Replies 24 ·
Replies
24
Views
4K
Question about momentum and kinetic energy.
  • · Replies 5 ·
Replies
5
Views
2K
Conservation of momentum in a bullet-block-spring system
  • · Replies 6 ·
Replies
6
Views
3K
Conservation of energy for a series of elastic collisions
  • · Replies 2 ·
Replies
2
Views
1K
Conservation of Momentum Problem - Mechanical and Kinetic Energies
  • · Replies 2 ·
Replies
2
Views
1K
Confused about a conservation of energy problem
  • · Replies 4 ·
Replies
4
Views
2K
Kinetic Energy: Trial 1 vs. Trial 2
  • · Replies 5 ·
Replies
5
Views
2K
Dropping a Block on a Spring with Completely Inelastic Collision
  • · Replies 11 ·
Replies
11
Views
3K