Angular velocity and the radian
In a nutshell
Angular velocity is the rate of change of angular displacement with respect to time. Linear velocity is the rate of change of linear displacement with respect to time. The number of times an object rotates in a second is given by its frequency and how long it takes to complete one full rotation is given by the time period. Angular velocity is measured in radians per second.
Equations
description | equation |
Angular velocity
| ω=2πfω=T2πω=ΔtΔθω=rv | Conversion from degrees to radians | θ(rad)=180π×θ(°) | Conversion from radians to degrees | θ(°)=π180×θ(rad) | Linear velocity | | |
Variable definitions
quantity name | symbol | derived unit | si baseunits |
angular displacement | | | |
| | | |
linear velocity | | | |
angular velocity | | | |
| | | |
frequency | | | |
The radian
Consider a circle with a radius of r. If the arc length is equal to one radius r, then the angle subtended from the centre of the circle will be equal to 1 radian, or 1 rad.
In 180° there is a little over 3 rad. In fact there are 3.14159265359... rad which is exactly equal to π.
Note: You need to remember that π rad=180° for your exams as you are not given the conversion and it comes up a lot in circular motion!
To convert from degrees to radians you can use the following equation:
θ(rad)=180π×θ(°)
To convert from radians to degrees you can use this equation:
θ(°)=π180×θ(rad)
Example
Convert 260° into radians.
First, write out the values given and check they are in the correct form:
θ°=260
Next, write down the equations needed and rearrange if necessary:
θ(rad)=180π×θ(°)
Then, substitute in values into the equation:
θ(rad)=180π×260θ(rad)=4.5379...
Make sure to include units and round the lowest number of significant figures of values given by the question:
θ(rad)=4.5 rad
The angle in radians is 4.5 rad.
Angular Velocity
Angular velocity ω is defined as the rate of change of angular displacement θ with time t:
ω=tθ
Time period
The time period T is defined as the time taken for one complete revolution of the circle. This is measured in seconds s.
Frequency
The frequency f, is defined as how many revolutions are occurring per second and is measured in hertz Hz.
Time period and frequency are related by the equations:
f=T1T=f1
Considering the angular velocity for a complete circle, an object would turn through 2π rad in a time of T s.
ω=T2π
Substituting in the equation relating frequency and time period, the equation can be also be written in terms of frequency:
ω=2πf
Example
A circular disk is rotating 45 times per second. What is the angular velocity of the disc?
First, write down the values given and check they are in the correct form:
f=45Hz
Next, write down the equations needed and rearrange if necessary:
ω=2πf
Then, substitute the values into the equation:
ω=2π×45ω=282.7433...
Make sure to include units and round the lowest number of significant figures of the values given by the question:
angular velocity=280rads−1
The angular velocity is 280rads−1.
Linear velocity
Linear velocity is defined as the velocity of an object rotating in uniform circular motion with respect to a linear displacement as opposed to an angular displacement.
The equation for linear velocity is given as:
v=ts
For a complete revolution, the displacement will be equal to the circumference and the time will be equal to the time period:
v=T2πr
As ω=T2π, the following substitution can be made:
v=ωr
Example
An object with a velocity of 14ms−1 is rotating 80cm about a point. What is the time period of the object?
First, write down the values given and check they are in the correct form:
v=14ms−1r=80cm=0.8m
Next, write down the equations needed:
v=ωrω=T2π
Substitute in T2π for ω and rearrange for T:
v=T2π×rT=v2πr
Then, substitute the values into the equation:
T=142π×0.8T=0.3590...
Make sure to include units and round the lowest number of significant figures of the values given by the question:
Time period=0.36s
The time period is 0.36s.