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A mass is placed on a frictionless, horizontal table. A spring (k = 100 N/m), which can be stretched or compressed, is placed on the table. A 5.00-kg mass is attached to one end of the spring, the other end is anchored to the wall. The equilibrium position is marked at zero. A student moves the mass out to x = 4.0 cm and releases it from rest. The mass oscillates in SHM. (a) Determine the equations of motion. (b) Find the position, velocity, and acceleration of the mass at time t = 3.00 s.

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First, since there are no non-conservative forces acting on the system, the amplitude AA of the movement will be equal to the initial displacement (4.0 cm)\left(4.0 \text{ cm}\right). We can also calculate the angular frequency ω\omega with the given force constant kk and the mass mm. Once we have calculated ω\omega, we continue to write the equation for the general position x(t)x\left(t\right) of the object in simple harmonic motion. Then, we differentiate x(t)x\left(t\right) in respect to time to obtain v(t)v\left(t\right). Finally, we differentiate v(t)v\left(t\right) in respect to time to obtain a(t)a\left(t\right). Note that ϕ=0\phi=0 because the position is 4.0 cm4.0 \text{ cm} when t=0t=0.

A=4.0102 mω=km=1005.00=4.47 radsx(t)=Acos(ωt)=(4.0102 m)cos[(4.47 rads)t]v(t)=dxdt=Aωsin(ωt)=(4.0102 m)(4.47 rads)sin[(4.47 rads)t]=(1.79101ms)sin[(4.47 rads)t]a(t)=dvdt=Aω2cos(ωt)=(4.0102 m)(4.47 rads)2cos[(4.47 rads)t]=(7.99101 ms2)cos[(4.47 rads)t]\begin{align*} A&=4.0 \cdot 10^{-2} \text{ m} \\ \omega&=\sqrt{\dfrac{k}{m}} \\ &=\sqrt{\dfrac{100}{5.00}} \\ &=4.47 \text{ } \dfrac{\text{rad}}{\text{s}} \\ x\left(t\right)&=A \cos\left(\omega t\right) \\ &=\left(4.0 \cdot 10^{-2} \text{ m}\right) \cos\left[\left(4.47 \text{ } \dfrac{\text{rad}}{\text{s}}\right)t\right] \\ v\left(t\right)&=\dfrac{dx}{dt} \\ &=-A \omega \sin\left(\omega t\right) \\ &=-\left(4.0 \cdot 10^{-2} \text{ m}\right) \left(4.47 \text{ } \dfrac{\text{rad}}{\text{s}}\right)\sin\left[\left(4.47 \text{ } \dfrac{\text{rad}}{\text{s}}\right)t\right] \\ &=-\left(1.79 \cdot 10^{-1} \dfrac{\text{m}}{\text{s}}\right)\sin\left[\left(4.47 \text{ } \dfrac{\text{rad}}{\text{s}}\right)t\right] \\ a\left(t\right)&=\dfrac{dv}{dt} \\ &=-A \omega^2 \cos\left(\omega t\right) \\ &=-\left(4.0 \cdot 10^{-2} \text{ m}\right) \left(4.47 \text{ } \dfrac{\text{rad}}{\text{s}}\right)^2\cos\left[\left(4.47 \text{ } \dfrac{\text{rad}}{\text{s}}\right)t\right] \\ &=-\left(7.99 \cdot 10^{-1} \text{ } \dfrac{\text{m}}{\text{s}^2}\right)\cos\left[\left(4.47 \text{ } \dfrac{\text{rad}}{\text{s}}\right)t\right] \end{align*}

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