# 2. Graphs of *y *=* a* sin *bx* and *y *=* a*
cos *bx*

by M. Bourne

### Interactive applet

Don't miss later on this page:

Spring period interactive

The variable *b* in both of the graph types

*y*=*a*sin*bx**y*=*a*cos*bx*

affects the **period** (or wavelength) of the graph. The **period** is the distance (or time) that it takes for the sine or cosine curve to begin repeating again.

## Graph Interactive - Period of a Sine Curve

Here's an applet that you can use to explore the concept of **period** and **frequency** of a sine curve.

Frequency is defined as `"frequency" = 1/"period"`. We'll see more on this below.

In this applet, a point on a circle rotates at a constant rate, and its height at time `t` traces out a sine curve.

### Things to Do

- At first you'll see a sine curve traced out as the circle rotates.
- Now, change the value of
*b*using the slider. If you increase*b*, the period for each cycle will go down and the frequency will increase. - Observe the number of cycles that you see between
*t*= 0 and*t*= 2π = 6.28. For*b*= 1 you see one cycle, for*b*= 2, you see 2 cycles, and so on. - Note the period of the curve is marked with a red indicator.

The units along the horizontal (time) axis are in radians. So *π* = 3.14 radians and 2*π* = 6.28 radians.

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## Did you notice?

- The variable `b` gives the number of cycles between `0` and `2pi`.
- Higher `b` gives higher frequency (and lower period).

### Formula for Period

The relationship between `b` and the **period** is given by:

`"Period"=(2pi)/b`

**Note:** As *b* gets larger, the period
decreases.

## Changing the Period

Now let's look at some still graphs to see what's going on.

The graph of *y *= 10 cos *x*, which we learned about in the last section, sine and cosine curves, is as follows.

As we learned, the **period** is `2pi`.

Next we see *y *= 10 cos 3*x*. Note the `3` inside the cosine term.

Notice that the period is different. (However, the amplitude is `10` in each example.)

This time the curve starts to repeat itself at `x=(2pi)/3`. This is consistent with the formula we met above:

`"Period"=(2pi)/b`

Now let's view the 2 curves on the same set of axes. Note that both graphs have an amplitude of `10` units, but their period is different.

## Interactive: Spring with mass

When you stretch (or compress) a spring then let go, it will vibrate back and forward. It will continue to do so if there are no other forces acting on it. (In reality, the spring slows down due to friction and the force of gravity.)

The vibration is **periodic**, and we can describe it using a sine or cosine curve.

The period of a spring's motion is affected by the stiffness of the spring (usually denoted by the variable *k*), and the mass on the end of the spring (*m*). You can investigate this property in the following interactive graph.

### Things to do

- Observe the curve we get as the spring vibrates. It's a cosine curve.
- Observe the period of the cosine curve.
- Vary the stiffness of the spring (`k`)and see the effect on the period.
- Vary the mass (`m`) and observe the effect on the period.
- What is the period when mass = stiffness (`m=k`)? Why?

For information, the period of a vibrating spring with stiffness *k* and with mass *m* on the end, is given by: `T=2 pi sqrt(m/k)`.

The equation of the cosine curve you'll see is `h = a cos sqrt(k/m)t`, where `h=` height at time `t`, and `a = ` amplitude of the motion.

In this applet, the position of `2pi` is fixed.

Copyright © www.intmath.com

Once again, a real spring would actually slow down as time goes on. Also, if we increase the mass, the spring will stretch out more, and if it's stiffer, it will stretch less. However, to keep things simple, the situation is idealized.

### Good to Know...

**Tip 1:** The number *b* tells us the number of cycles in each
2*π*.

For *y *= 10 cos *x*, there is **one** cycle
between `0` and 2*π* (because *b* = 1).

For *y *= 10 cos 3*x*, there are **3** cycles
between `0` and 2*π* (because *b* = 3).

**Tip 2: **Remember, we are now operating using RADIANS. Recall that:

2

π= 6.283185...

and that

2

π= 360°

We only use **radians** in this chapter.

For a reminder, go to: Radians

### Exercises

### Need Graph Paper?

**1.** Sketch 2 cycles of *y* = 3 cos 8*x*.

**2.** Sketch 2 cycles of *y* = cos 10*x*.

**3. **Sketch 2 cycles of *y* = 5 sin 2*π**x*.

4. Sketch 2 cycles of `y = 4\ sin\ x/3`

## Pistons don't exactly follow a sine path

The movement of a piston in an engine is often quoted as an example of a sine curve. It almost is, but only under certain conditions. See Curve Shape for the Movement of a Piston to investigate this further, using an interactive animation.

## Defining Sine Curves using Frequency

It is common in electronics to express the sin graph in terms of the frequency *f* as follows:

y= sin 2πft

This is very convenient, since we don't have to do any calculation to find the frequency (like we were doing above). The frequency, *f*, is normally measured in **cycles/second**, which is the same as Hertz (**Hz**).

The **period** of the curve (the time it takes to go from one crest to the next crest) can be found easily once we know the frequency:

`T=1/f`

The units for period are normally **seconds**.

### Example

Household voltage in the UK is alternating current, `240\ "V"` with frequency `50\ "Hz"`. What is the equation describing this voltage?

## Coming Next

In the next section we learn about phase shift.

See Graphs of y = a sin (bx + c).

Later, we learn some Applications of Trigonometric Graphs.

But first, let's see another application of frequency.

## Music Example

The frequency of a note in music depends on the **period** of the wave. If the frequency is high, the period is short; if the frequency is low, the period is longer.

A student recently asked me an interesting question. She wanted to know the frequencies of all the notes on a piano.

A piano is tuned to A = 440 Hz (cycles/second) and the other notes are evenly spaced, 12 notes to each octave. A note an octave higher than A = 440 Hz has twice the frequency (880 Hz) and an octave lower than A = 440 Hz has half the frequency (`220\ "Hz"`).

Click here to find out the frequencies of notes on a piano.

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