# Concept of Force Confusion

1. May 20, 2009

### mrlucky0

1. The problem statement, all variables and given/known data

Object A and B are placed on a spring. Object A has twice as much mass as object B. If the spring is depressed and released, propelling the objects into air, object A will:

answer: rise to the same height as object B

2. Relevant equations

3. The attempt at a solution

Based on my reasoning and math, I am confused why both objects would rise to the same height:

The force on both objects is the same, F. And 2*mass of B = mass of A

F = (mass of A)*(accl. A) = (mass of B)*(accl. B)
F = 2*(mass of B)*(accl. A) = (mass of B)*(accl. B)

cancel out (mass of B):
F = 2*(accl. A) = (accl. B)

Since the upward acceleration of B is twice the that of A, why would the height be the same?

2. May 20, 2009

### tyco05

You are only considering the upward forces due to the spring.

What about the force on each object due to gravity..... will it be the same???

Consider....

Last edited: May 20, 2009
3. May 20, 2009

### HallsofIvy

Staff Emeritus
I think the simplest way to do this problem is to use "conservation of energy". The spring has a specific amount of energy stored in it when compressed and, on release gives the same energy to A and B. They will rise to heights that give the same potential energy to both.

4. Jun 10, 2011

### get_physical

any one know how to approach this problem?

5. Jun 10, 2011