Elastic potential energy of two blocks of mass

Click For Summary

Homework Help Overview

The discussion revolves around the elastic potential energy of two blocks of masses M and 3M on a frictionless surface, with a spring between them. After a cord holding the blocks is burned, one block moves with a known speed, prompting questions about the speed of the other block and the original potential energy in the spring.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants explore the factors that determine the potential energy in a spring, including compression/stretching and spring stiffness. There are discussions about conservation laws, specifically momentum and kinetic energy, and how they relate to potential energy. Some participants express confusion about calculating kinetic energy and its implications for potential energy.

Discussion Status

The discussion is active, with participants raising questions about the relationships between kinetic and potential energy, as well as conservation principles. Some guidance is offered regarding the conservation of energy, but there is no explicit consensus on the calculations or interpretations being discussed.

Contextual Notes

There is uncertainty regarding the initial conditions of the spring's compression or stretching, as well as the stiffness of the spring, which are not provided. Participants are also navigating the implications of their calculations and assumptions about energy conservation.

TrippingBilly
Messages
27
Reaction score
0
Two blocks of masses M and 3M are placed on a horizontal, frictionless surface. A light spring is attached to one of them, and the blocks are pushed together with the spring between them. A cord initially holding the blocks together is burned; after this, the block of mass 3M moves to the right with a speed of 1.50 m/s.
(a) What is the speed of the block of mass M?
(b) Find the original elastic potential energy in the spring if M = 0.350 kg.

I already have the answer to a, which is obviously 4.5m/s. But, I am clueless as to part b. Please help!
 
Physics news on Phys.org
What factors determine the potential energy stored in a spring and how do they relate to each other?
 
It depends on how much is compresses or stretches, but that isn't known and as far as I know can't be calculated. It also depends on the stiffness of the spring but that isn't given either.
 
TrippingBilly said:
It depends on how much is compresses or stretches, but that isn't known and as far as I know can't be calculated.
Okay, sorry a little misleading perhaps. What quantities must be conserved here?
 
Momentum is conserved, and kinetic energy is conserved, so that means that potential energy must be conserved too. and change in kinetic energy = - change in potential energy..
 
Last edited:
TrippingBilly said:
Momentum is conserved, and kinetic energy is conserved, so that means that potential energy must be conserved too.
Energy is conserved, it can however, be transformed from one form to another (i.e. from potential to kinetic).
 
I think my problem was only calculating the kinetic energy of the smaller mass, because I thought if i calculated it for the system they would offset each other but they dont. So, KE=(.5)(.35)(.45^2) - (.5)(.35)(3)(1.5^2) = 2.3625 so potential energy is the opposite of that?
 
TrippingBilly said:
I think my problem was only calculating the kinetic energy of the smaller mass, because I thought if i calculated it for the system they would offset each other but they dont. So, KE=(.5)(.35)(.45^2) - (.5)(.35)(3)(1.5^2) = 2.3625 so potential energy is the opposite of that?
Note that kinetic energy is not a vector quantity, it is scalar. You should therefore add the individual kinetic energies to obtain the potential.
 

Similar threads

Find elastic energy in the compressed spring.
  • · Replies 12 ·
Replies
12
Views
3K
Why can we move the spring with constant speed when we apply a force?
  • · Replies 29 ·
Replies
29
Views
3K
Body attached to a block by a spring, shaped like an inclined plane
  • · Replies 1 ·
Replies
1
Views
1K
Distribution of Energy when work is done on a system of 2 masses connected by a spring
  • · Replies 17 ·
Replies
17
Views
2K
Newton's law problem: Pushing 2 stacked blocks on a horizontal table
  • · Replies 13 ·
Replies
13
Views
3K
Conservation of energy for a block on an incline plane
  • · Replies 1 ·
Replies
1
Views
2K
Question about springs and rotational/translational energy
  • · Replies 4 ·
Replies
4
Views
2K
Dynamics of a Block on top of a slab with friction between them
  • · Replies 33 ·
2
Replies
33
Views
2K
Springs and Splitting Mass Concept Confusion
  • · Replies 1 ·
Replies
1
Views
2K
A block of mass m is dropped onto the top of a vertical spring....
  • · Replies 8 ·
Replies
8
Views
6K