a) ![x(t)=2.0 sin (10 t) [m]](https://tex.z-dn.net/?f=%20x%28t%29%3D2.0%20sin%20%2810%20t%29%20%5Bm%5D)
The equation which gives the position of a simple harmonic oscillator is:
![x(t)= A sin (\omega t)](https://tex.z-dn.net/?f=x%28t%29%3D%20A%20sin%20%28%5Comega%20t%29)
where
A is the amplitude
is the angular frequency, with k being the spring constant and m the mass
t is the time
Let's start by calculating the angular frequency:
![\omega=\sqrt{\frac{k}{m}}=\sqrt{\frac{50.0 N/m}{0.500 kg}}=10 rad/s](https://tex.z-dn.net/?f=%5Comega%3D%5Csqrt%7B%5Cfrac%7Bk%7D%7Bm%7D%7D%3D%5Csqrt%7B%5Cfrac%7B50.0%20N%2Fm%7D%7B0.500%20kg%7D%7D%3D10%20rad%2Fs)
The amplitude, A, can be found from the maximum velocity of the spring:
![v_{max}=\omega A\\A=\frac{v_{max}}{\omega}=\frac{20.0 m/s}{10 rad/s}=2 m](https://tex.z-dn.net/?f=v_%7Bmax%7D%3D%5Comega%20A%5C%5CA%3D%5Cfrac%7Bv_%7Bmax%7D%7D%7B%5Comega%7D%3D%5Cfrac%7B20.0%20m%2Fs%7D%7B10%20rad%2Fs%7D%3D2%20m)
So, the equation of motion is
![x(t)= 2.0 sin (10 t) [m]](https://tex.z-dn.net/?f=x%28t%29%3D%202.0%20sin%20%2810%20t%29%20%5Bm%5D)
b) t=0.10 s, t=0.52 s
The potential energy is given by:
![U(x)=\frac{1}{2}kx^2](https://tex.z-dn.net/?f=U%28x%29%3D%5Cfrac%7B1%7D%7B2%7Dkx%5E2)
While the kinetic energy is given by:
![K=\frac{1}{2}mv^2](https://tex.z-dn.net/?f=K%3D%5Cfrac%7B1%7D%7B2%7Dmv%5E2)
The velocity as a function of time t is:
![v(t)=v_{max} cos(\omega t)](https://tex.z-dn.net/?f=v%28t%29%3Dv_%7Bmax%7D%20cos%28%5Comega%20t%29)
The problem asks as the time t at which U=3K, so we have:
![\frac{1}{2}kx^2 = \frac{3}{2}mv^2\\kx^2 = 3mv^2\\k (A sin (\omega t))^2 = 3m (\omega A cos(\omega t))^2\\(tan(\omega t))^2=\frac{3m\omega^2}{k}](https://tex.z-dn.net/?f=%5Cfrac%7B1%7D%7B2%7Dkx%5E2%20%3D%20%5Cfrac%7B3%7D%7B2%7Dmv%5E2%5C%5Ckx%5E2%20%3D%203mv%5E2%5C%5Ck%20%28A%20sin%20%28%5Comega%20t%29%29%5E2%20%3D%203m%20%28%5Comega%20A%20cos%28%5Comega%20t%29%29%5E2%5C%5C%28tan%28%5Comega%20t%29%29%5E2%3D%5Cfrac%7B3m%5Comega%5E2%7D%7Bk%7D)
However,
, so we have
![(tan(\omega t))^2=\frac{3\omega^2}{\omega^2}=3\\tan(\omega t)=\pm \sqrt{3}\\](https://tex.z-dn.net/?f=%28tan%28%5Comega%20t%29%29%5E2%3D%5Cfrac%7B3%5Comega%5E2%7D%7B%5Comega%5E2%7D%3D3%5C%5Ctan%28%5Comega%20t%29%3D%5Cpm%20%5Csqrt%7B3%7D%5C%5C)
with two solutions:
![\omega t= \frac{\pi}{3}\\t=\frac{\pi}{3\omega}=\frac{\pi}{3(10 rad/s)}=0.10 s](https://tex.z-dn.net/?f=%5Comega%20t%3D%20%5Cfrac%7B%5Cpi%7D%7B3%7D%5C%5Ct%3D%5Cfrac%7B%5Cpi%7D%7B3%5Comega%7D%3D%5Cfrac%7B%5Cpi%7D%7B3%2810%20rad%2Fs%29%7D%3D0.10%20s)
![\omega t= \frac{5\pi}{3}\\t=\frac{5\pi}{3\omega}=\frac{5\pi}{3(10 rad/s)}=0.52 s](https://tex.z-dn.net/?f=%5Comega%20t%3D%20%5Cfrac%7B5%5Cpi%7D%7B3%7D%5C%5Ct%3D%5Cfrac%7B5%5Cpi%7D%7B3%5Comega%7D%3D%5Cfrac%7B5%5Cpi%7D%7B3%2810%20rad%2Fs%29%7D%3D0.52%20s)
c) 3 seconds.
When x=0, the equation of motion is:
![0=A sin (\omega t)](https://tex.z-dn.net/?f=0%3DA%20sin%20%28%5Comega%20t%29)
so, t=0.
When x=1.00 m, the equation of motion is:
![1=A sin(\omega t)\\sin(\omega t)=\frac{1}{A}=\frac{1}{2}\\\omega t= 30\\t=\frac{30}{\omega}=\frac{30}{10 rad/s}=3 s](https://tex.z-dn.net/?f=1%3DA%20sin%28%5Comega%20t%29%5C%5Csin%28%5Comega%20t%29%3D%5Cfrac%7B1%7D%7BA%7D%3D%5Cfrac%7B1%7D%7B2%7D%5C%5C%5Comega%20t%3D%2030%5C%5Ct%3D%5Cfrac%7B30%7D%7B%5Comega%7D%3D%5Cfrac%7B30%7D%7B10%20rad%2Fs%7D%3D3%20s)
So, the time needed is 3 seconds.
d) 0.097 m
The period of the oscillator in this problem is:
![T=\frac{2\pi}{\omega}=\frac{2\pi}{10 rad/s}=0.628 s](https://tex.z-dn.net/?f=T%3D%5Cfrac%7B2%5Cpi%7D%7B%5Comega%7D%3D%5Cfrac%7B2%5Cpi%7D%7B10%20rad%2Fs%7D%3D0.628%20s)
The period of a pendulum is:
![T=2 \pi \sqrt{\frac{L}{g}}](https://tex.z-dn.net/?f=T%3D2%20%5Cpi%20%5Csqrt%7B%5Cfrac%7BL%7D%7Bg%7D%7D)
where L is the length of the pendulum. By using T=0.628 s, we find
![L=\frac{T^2g}{(2\pi)^2}=\frac{(0.628 s)^2(9.8 m/s^2)}{(2\pi)^2}=0.097 m](https://tex.z-dn.net/?f=L%3D%5Cfrac%7BT%5E2g%7D%7B%282%5Cpi%29%5E2%7D%3D%5Cfrac%7B%280.628%20s%29%5E2%289.8%20m%2Fs%5E2%29%7D%7B%282%5Cpi%29%5E2%7D%3D0.097%20m)