Trouble with equations used in SHM calculations

  • Thread starter vf_one
  • Start date
Hi everyone

I'm having trouble using the equations for SHM calculations. We got told to set them on radians but I forgot to when I was doing the problems and got some of the answers right. But when I tried to do them when the calculator was set in radians, the answers were totally wrong.

We are given a set of equations to use

v= -X√(k/m)sin(2πft)
a= -XK/mcos(2πft)

Do we set the calculator to radians when we use these equations?
Do we take into account the negative "-" sign in front of X for the velocity and acceleration equations?

I'll show you the working a question I did and maybe you can see where I went wrong.

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

A 64kg bungy jumper is displaced 13m from equilibrium downwards and then released. She bounces up and down with a period of 5.4s. When she is 5m abover her initial rest position and moving upwards, what is her velocity?

2. The attempt at a solution

I calculated this with my calculator set in degrees
5=13cos(2π x 1/5.4t)
t= 57.9s

v= -X√(k/m)sin(2πft)
v= -13√(86.6/64)sin(2π x 1/5.4 x 57.9)
v= -14m/s

The answer was 14m/s. Why was the negative sign ignored?

But when we were asked to calculate the acceleration at 5m displacement the answer is

a= -XK/mcos(2πft)
a= -13 x 86.6/64cos(2π x 1/5.4 x 57.9)
a= -6.77m/s2 Calculator still set in degrees

If anyone could help that would be very appreciated. Thanks

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