Pulses: Amplitude and Length Revision Notes for Grade 10 NSC Matric Physical Sciences

Pulses: Amplitude and Length

What is a pulse?

A pulse is a single disturbance that travels through a medium. Think of it like creating a bump that moves along a rope when you flick one end - this bump is the pulse moving from where you created it to the other end.

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Key characteristics of pulses:

They are single disturbances (not continuous waves)
They travel through a medium from one place to another
The medium itself doesn't travel with the pulse - only the disturbance moves

Transverse pulses

A transverse pulse occurs when the particles in the medium move perpendicular (at right angles) to the direction the pulse is travelling.

For example, when you flick a rope horizontally, the rope moves up and down while the pulse travels horizontally along the rope. The particle motion (up-down) is perpendicular to the pulse direction (horizontal).

Understanding amplitude and pulse length

Two important measurements help us describe any pulse: its amplitude and its length.

Amplitude

Amplitude is the maximum distance the medium is displaced from its rest position. It tells us how "big" or "strong" the pulse is.

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Key points about amplitude:

Symbol: A
Unit: metres (m)
Measured from the rest position to the highest point of the pulse
Shows how far the medium moves from its normal, undisturbed position

Pulse length

Pulse length measures how long the pulse is - essentially the width of the disturbance as it travels through the medium.

The position of rest (also called the equilibrium position) is where the medium would naturally be if no pulse was present. This serves as our reference point for measuring amplitude.

Pulse behaviour over time

An important discovery about pulses is that their amplitude and length remain constant as they travel through a medium.

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Critical concept: Pulse properties remain constant

As a pulse travels through a medium:

The pulse keeps the same shape as it moves
The amplitude stays the same throughout the journey
The pulse length doesn't change
Only the position of the pulse changes with time

This constancy is crucial for understanding how energy and information can be transmitted through different media without losing their original characteristics.

Pulse speed

Pulse speed measures how fast a pulse travels through a medium. It's the distance covered by the pulse divided by the time taken.

Formula for pulse speed: $v = \frac{D}{t}$

Where:

$v$ = pulse speed (m·s⁻¹)
$D$ = distance travelled (m)
$t$ = time taken (s)

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Worked Example: Calculating pulse speed

Problem: A pulse covers a distance of 2 m in 4 s on a heavy rope. Calculate the pulse speed.

Solution:

Identify given information:
- Distance ( $D$ ) = 2 m
- Time ( $t$ ) = 4 s
- Find: pulse speed ( $v$ )
Apply the formula: $v = \frac{D}{t}$
Calculate: $v = \frac{2 \text{ m}}{4 \text{ s}} = 0.5{ m·s}^{-1}$
Final answer: The pulse speed is 0.5 m·s⁻¹

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Important note: Pulse speed depends on the properties of the medium (like tension, density, material type) and NOT on the amplitude or pulse length. A small pulse and a large pulse will travel at the same speed through the same medium.

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Key Points to Remember:

A pulse is a single disturbance that travels through a medium, unlike continuous waves
Transverse pulses have particle motion perpendicular to the direction of pulse travel
Amplitude measures the maximum displacement from the rest position - it shows pulse strength
Pulse length measures the width of the disturbance as it travels
Pulse properties remain constant - amplitude and length don't change as the pulse moves
Pulse speed = distance ÷ time and depends only on medium properties, not pulse characteristics

Pulses: Amplitude and Length (Grade 10 NSC Matric Physical Sciences): Revision Notes