Damped Oscillation 12-19-2020 (Part 1)

Classical Mechanics Level pending

A block of mass m m is connected to one end of a spring of force constant k k and natural length L 0 L_0 . The other end of the spring is fixed in place at the origin ( x , y ) = ( 0 , 0 ) (x,y) = (0,0) . At time t = 0 t = 0 , the block is at position ( x , y ) = ( 1 , 0 ) (x,y) = (1,0) with velocity ( x ˙ , y ˙ ) = ( 10 , 10 ) (\dot{x}, \dot{y}) = (10,10) . The ambient gravitational acceleration g g is downward, which corresponds to the y -y direction. As the block moves, it is subjected to damping forces, such that the total force on it is:

F = F s + F g + F D = F s + F g D v \vec{F} = \vec{F}_s + \vec{F}_g + \vec{F}_D \\ = \vec{F}_s + \vec{F}_g -D \vec{v}

In the above equation, F s \vec{F}_s is the spring force exerted on the block, F g \vec{F}_g is the gravity force exerted on the block, F D \vec{F}_D is the damping force exerted on the block, v \vec{v} is the velocity of the block, and D D is a damping constant.

When the block comes to rest in its equilibrium position, how far away is it from the origin?

Details and Assumptions:
1) m = 1 m = 1
2) g = 10 g = 10
3) k = 5 k = 5
4) L 0 = 1 L_0 = 1
5) D = 0.5 D = 0.5
6) Assume standard S I SI units for all quantities


The answer is 3.0.

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1 solution

Karan Chatrath
Dec 19, 2020

At equilibrium, the damping force becomes zero as the mass is at rest, which also means that the mass does not accelerate anymore. Essentially, the spring force and gravity force balance each other yielding:

m g = K ( y L o ) y = m g K + L o = 3 mg = K(\lvert y \rvert - L_o) \implies \lvert y \rvert = \frac{mg}{K} + L_o = \boxed{3}

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