- #1

keroberous

- 15

- 1

- Homework Statement
- I can never quite remember when solving problems with elastic materials whether I should be using Hooke's Law or conservation of energy. Below are two sample problems.

- Relevant Equations
- Hooke's Law: ##F=kx##

Elastic Potential Energy: ##E_e=\frac{1}{2}kx^2##

Gravitational Potential Energy: ##E_g=mgh##

Here are the two questions I want to compare:

1. A student of mass 62 kg stands on an upholstered chair containing springs, each of force constant 2.4 × 10

2. A 0.20-kg ball attached to a vertical spring of force constant 28 N/m is released from rest from the unstretched equilibrium position of the spring. Determine how far the ball falls, under negligible air resistance, before being brought to a momentary stop by the spring.

I can get the right answer for both questions, by using Hooke's law for number 1 and conservation of energy for question 2. My question is if I didn't know what the answers were, how would I choose the method? By using the incorrect method I'm off by a factor of two in both cases (bigger in 1 and smaller in 2).

1. A student of mass 62 kg stands on an upholstered chair containing springs, each of force constant 2.4 × 10

^{3}N/m. If the student is supported equally by six springs, what is the compression of each spring?2. A 0.20-kg ball attached to a vertical spring of force constant 28 N/m is released from rest from the unstretched equilibrium position of the spring. Determine how far the ball falls, under negligible air resistance, before being brought to a momentary stop by the spring.

I can get the right answer for both questions, by using Hooke's law for number 1 and conservation of energy for question 2. My question is if I didn't know what the answers were, how would I choose the method? By using the incorrect method I'm off by a factor of two in both cases (bigger in 1 and smaller in 2).