Up and down and up and down and...

Not every oscillation in nature is a harmonic oscillation - in this problem, we will examine a non-harmonic oscillation. Suppose we had a rubber ball with a perfect coefficient of restitution so that, when dropped, it would always return to the same height. The period of the bouncing is the time T T between successive bounces and the amplitude A A of this non-linear oscillation is the maximum height of the ball above the floor. For a harmonic oscillator, the period and amplitude are independent, but not so here. Instead, there's a relation between them: T T proportional to A w A^w . Find w w .


The answer is 0.5.

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4 solutions

Tanishq Aggarwal
Nov 24, 2013

Using a simple one-dimensional kinematic equation for an object with constant acceleration, we model the ball as a projectile with initial height A and initial velocity 0. Its position in the y-direction as a function of time is now

y ( t ) = A + 1 2 g t 2 y(t) = A + \frac{1}{2}gt^2

Where g is the gravitational acceleration (-9.8 m/s^2). To find the period T, we need to find when the height is zero, so

0 = A + 1 2 g t 2 0 = A + \frac{1}{2}gt^2

T 2 = 2 A g T^2 = \frac{-2A}{g}

From this equation, it's clear that T 2 T^2 is proportional to A, so T T is proportional to A 1 2 A^{\frac{1}{2}} .

Since the descent of the ball is essentially the reverse of the ascent. This means that we only need to consider the descent to find w w . We need to use suvat to find w w :

Using:

s = u t + 1 2 a t 2 s = u \cdot t + \frac{1}{2} \cdot a \cdot t^2

Let:

s = A , t = 1 2 T s = A, t = \frac{1}{2}T and u = 0 m s 1 u = 0ms^{-1}

This Gives:

A = 1 2 a ( T 2 ) 2 A = \frac{1}{2} \cdot a \cdot (\frac{T}{2})^2

A = 1 2 a 1 4 T 2 A = \frac{1}{2} \cdot a \cdot \frac{1}{4} \cdot T^2

A = 1 8 a T 2 A = \frac{1}{8} \cdot a \cdot T^2

Since 1 8 a \frac{1}{8} \cdot a is a constant:

A T 2 A \propto T^2

T A 0.5 T \propto A^{0.5}

Therefore, w = 0.5 \boxed{w = 0.5}

Michael Tong
Nov 24, 2013

Since mechanical energy is conserved, at the top of the "bounce" the potential energy is equal to the kinetic energy at the bottom of the bounce. The amplitude A is equal to the relative height, so m g A = 1 2 m v 2 v = 2 g A mgA = \frac{1}{2}mv^2 \rightarrow v = \sqrt{2gA} . Thus, at the bottom the oscillator goes upward at such a speed. The amount of time it takes for it to return is reduced to a simple kinematics problem, and using the fact that v f = v 0 g t v_f = v_0 - gt and that v f = v 0 v_f = -v_0 , we come to t = 2 v 0 g = 2 2 g A g t = \frac{2v_0}{g} = \frac {2\sqrt{2gA}}{g} . Thus, T T is proportional to A 1 2 A^{\frac{1}{2}} .

Daniel Alfaro
Nov 29, 2013

A 2 = v i + 1 2 g ( T 2 ) 2 A = g T 2 4 A 1 2 = g 2 T \frac { A }{ 2 } ={ v }_{ i }+\frac { 1 }{ 2 } g{ (\frac { T }{ 2 } ) }^{ 2 }\Rightarrow A=g\frac { { T }^{ 2 } }{ 4 } \Rightarrow { A }^{ \frac { 1 }{ 2 } }=\frac { \sqrt { g } }{ 2 } T

w = 0.5 w=0.5

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