# Spring constant

1. Jul 14, 2006

### Ukitake Jyuushirou

hello

i think i may have found an error in one of the answer in my physics textbook but i wanna confirm it with the physics experts here

Question:

A trampolinist of 55kg bounces in the middle of a trampoline mat. She finds that she bounces 80 times in a min. What is the spring constant?

Ans at the back: 3.9kN/m

My ans: 3.5kN/m

Last edited: Jul 14, 2006
2. Jul 14, 2006

### Staff: Mentor

I assume you are to model this as simple harmonic motion of the trampolinist, in which case the book answer seems correct. Show how you arrived at your answer.

3. Jul 14, 2006

### Ukitake Jyuushirou

i work out the period by taking 80 divide by 60s which gives 1.33Hz
T = 1/f therefore T = 0.75s

using T = 2pi (square root m/k)
0.75 = 2pi (square root 50kg/k)
0.5625 = 39.488656 x 50/k
k = 3510 N/m

Last edited: Jul 14, 2006
4. Jul 14, 2006

### HallsofIvy

Staff Emeritus
3.9 kN/m2? A spring constant is in "force per distance", kN/m, not m^2.

5. Jul 14, 2006

### Staff: Mentor

So far, so good. Now check your arithmetic.

(Yeah, those units are wacked. Good catch by Halls.)

6. Jul 14, 2006

### Ukitake Jyuushirou

edited...thanks for telling

7. Jul 14, 2006

### Ukitake Jyuushirou

D'OH!!! ur rite...i mixed up that qn with the one above it ...this one has 55kg not 50kg...

i wonder if there is a smiley for "bashing my own head with textbook"....