Confirm Physics Textbook Error: Trampolinist Spring Constant

[Ukitake Jyuushirou]

hello

i think i may have found an error in one of the answer in my physics textbook but i want to 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

 

[Doc Al]

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.

 

[Ukitake Jyuushirou]

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.

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

 

[HallsofIvy]

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

 

[Doc Al]

So far, so good. Now check your arithmetic.

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

 

[Ukitake Jyuushirou]

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

edited...thanks for telling

 

[Ukitake Jyuushirou]

So far, so good. Now check your arithmetic.

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

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"...