The displacement of an elastic wave is given by the function $y = 3\sin\omega t + 4\cos\omega t$. where $y$ is in cm and $t$ is in second. Calculate the resultant amplitude.

The displacement of an elastic wave is given by the function y = 3sinω t + 4cosω t. where y is in cm and t is

NCERT Exemplar · Q19

y = 3\sin\omega t + 4\cos\omega t

Sikkim CbseShort· 2mImportance★★★★★est

76% · 44/58 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Simple Harmonic Motion

Simple Harmonic Motion: The Natural Rhythm of Things

Imagine a ball placed at the bottom of a perfectly smooth, U-shaped bowl. If you give it a gentle push, what happens? It rolls up one side, slows down, stops for an instant, then rolls back down, past the bottom, up the other side, stops, and returns. Left alone, it keeps doing this — back and forth, back and forth — in a steady, repeating rhythm.

That rhythm is the heart of Simple Harmonic Motion (SHM). It's the most fundamental kind of oscillatory (back-and-forth) motion in physics.

The Intuition: A Restoring Force That Fights Displacement

The key idea is this: the further you push the object from its resting (equilibrium) position, the stronger the force that tries to pull it back.

In the bowl, when the ball is at the bottom (equilibrium), gravity pulls straight down, and the bowl pushes straight up — no sideways force. But when you push the ball up the side, gravity now has a component that pulls it down the slope. The higher up the side you push it, the steeper the slope, and the stronger that pull-back force becomes.

This is a restoring force — it always points toward equilibrium. And crucially, in SHM, this restoring force is directly proportional to the displacement from equilibrium. Double the displacement, double the restoring force.

F = -k x

The Precise Statement

Simple Harmonic Motion is the motion of an object where the restoring force is directly proportional to the displacement from equilibrium and acts in the opposite direction.

That's it. That single condition — $F = -kx$ — is the entire definition. Everything else (the sine waves, the formulas for period and frequency) follows mathematically from this one law.

What Does This Motion Look Like?

If you track the ball's position over time, you get a beautiful, smooth wave — a sine wave (or cosine wave). It's the same shape as the shadow of a spinning wheel cast on a wall.

The motion has three key descriptors:

Amplitude ( $A$ ): The maximum displacement from equilibrium. How far you initially pushed the ball up the side of the bowl.
Period ( $T$ ): The time it takes to complete one full back-and-forth cycle (e.g., from the leftmost point, back to the leftmost point).
Frequency ( $f$ ): How many cycles happen per second. $f = 1/T$ .

Note

A remarkable fact: for a given system (fixed $k$ and fixed mass $m$ ), the period and frequency do not depend on the amplitude. A big push and a tiny push take exactly the same time to complete one cycle. This is called isochronism — and it's why pendulums were used to keep time in clocks.

The Mathematical Description (Derived from $F = -kx$ )

Using Newton's second law ( $F = ma$ ) and the definition of acceleration ( $a = \frac{d^2x}{dt^2}$ ), the condition $F = -kx$ becomes:

m \frac{d^2x}{dt^2} = -k x

This is a differential equation. Its solution — the position as a function of time — is:

x(t) = A \cos(\omega t + \phi)

Where:

$\omega = \sqrt{\frac{k}{m}}$ is the angular frequency (radians per second). It tells you how fast the oscillation is.
$\phi$ is the phase constant (determines where in the cycle you start measuring time). …

Start your 14-day free trial to unlock the full solution →

Two perpendicular harmonic components of the same frequency combine vectorially; the resultant amplitude is found by treating the sine and cosine coefficients as perpendicular vectors. Resultant amplitude = 5 cm.

Why This Works: Superposition of Perpendicular Components

When two harmonic oscillations of the same angular frequency $\omega$ are added, they interfere to produce a single harmonic motion at that frequency. The key insight is that $\sin\omega t$ and $\cos\omega t$ are perpendicular in phase space—they differ by $90°$ . Just as perpendicular vectors add by the Pythagorean theorem, so do these components.

The general principle: any linear combination $A\sin\omega t + B\cos\omega t$ can be rewritten as a single sinusoid $R\sin(\omega t + \phi)$ , where the resultant amplitude $R$ is the vector sum of the two perpendicular contributions.

Step-by-Step Solution

1. Identify the two harmonic components

The displacement is:

y = 3\sin\omega t + 4\cos\omega t

Here the coefficient of $\sin\omega t$ is $A = 3$ cm and the coefficient of $\cos\omega t$ is $B = 4$ cm.

2. Recognize the phase relationship

Since $\cos\omega t = \sin(\omega t + 90°)$ , the two terms oscillate at the same frequency but are $90°$ out of phase. This perpendicularity means we cannot simply add the amplitudes arithmetically; instead, they combine as perpendicular vectors.

3. Apply the vector addition formula

The resultant amplitude $R$ of two perpendicular oscillations with amplitudes $A$ and $B$ is:

R = \sqrt{A^2 + B^2}

Substituting our values:

$R = \sqrt{3^2 + 4^2} = \sqrt{9 + 16} = \sqrt{25} = 5 \text{ cm}$ …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ18 — A tuning fork A, marked 512 Hz, produces 5 beats per second, where sou…→Next questionQ20 — A sitar wire is replaced by another wire of same length and material b…

Q.The displacement of an elastic wave is given by the function y=3sin⁡ωt+4cos⁡ωty = 3\sin\omega t + 4\cos\omega ty=3sinωt+4cosωt. where yyy is in cm and ttt is in second. Calculate the resultant amplitude.

Why This Works: Superposition of Perpendicular Components

Step-by-Step Solution

🎓Unlock everything free for 14 days

Q.The displacement of an elastic wave is given by the function $y = 3\sin\omega t + 4\cos\omega t$ . where $y$ is in cm and $t$ is in second. Calculate the resultant amplitude.

Unlock everything free for 14 days