Torque

Storyboard

If it is desired to modify the rotational state of the body, the angular momentum must be modified.

The speed with which this occurs is called torque defined as the variation of the angular momentum in time and is vectorial since the variation of the angular momentum is. This was defined by Newton in his second principle in the case of rotation.

>Model

ID:(599, 0)



Torque with constant moment of inertia

Description

If it is desired to modify the rotational state of the body, the angular momentum must be modified. The speed with which this occurs is called torque defined as the variation of the angular momentum in time and is vectorial since the variation of the angular momentum is. This was defined by Newton in his second principle in the case of rotation.

Variables

Symbol
Text
Variable
Value
Units
Calculate
MKS Value
MKS Units
$\theta$
theta
Angle
rad
$L$
L
Angular Momentum
kg m^2/s
$\omega$
omega
Angular Speed
rad/s
$\alpha_0$
alpha_0
Constant Angular Acceleration
rad/s^2
$\Delta\omega$
Domega
Difference in Angular Speeds
rad/s
$\Delta\theta$
Dtheta
Difference of Angles
rad
$\Delta s$
Ds
Distance traveled in a time
m
$F$
F
Force
N
$\theta_0$
theta_0
Initial Angle
rad
$L_0$
L_0
Initial Angular Momentum
kg m^2/s
$\omega_0$
omega_0
Initial Angular Speed
rad/s
$p_0$
p_0
Initial moment
kg m/s
$v_0$
v_0
Initial Speed
m/s
$a$
a
Instant acceleration
m/s^2
$p$
p
Moment
kg m/s
$I$
I
Moment of Inertia
kg m^2
$\Delta p$
Dp
Momentum variation
kg m/s
$m$
m
Point Mass
kg
$s$
s
Position
m
$r$
r
Radius
m
$v$
v
Speed
m/s
$\Delta v$
Dv
Speed Diference
m/s
$t_0$
t_0
Start Time
s
$s_0$
s_0
Starting position
m
$t$
t
Time
s
$\Delta t$
Dt
Time elapsed
s
$T$
T
Torque
N m
$\Delta L$
DL
Variation of Angular Momentum
kg m^2/s

Calculations


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Symbol
Equation
Solved
Translated

Calculations

Symbol
Equation
Solved
Translated

 Variable   Given   Calculate   Target :   Equation   To be used



Equations


As the mean Speed ($\bar{v}$) is with the distance traveled in a time ($\Delta s$) and the time elapsed ($\Delta t$), equal to

$ \bar{v} \equiv\displaystyle\frac{ \Delta s }{ \Delta t }$



and with the distance traveled in a time ($\Delta s$) expressed as an arc of a circle, and the radius ($r$) and the angle variation ($\Delta\theta$) are

$ \Delta s=r \Delta\theta $



and the definition of the mean angular velocity ($\bar{\omega}$) is

$ \bar{\omega} \equiv\displaystyle\frac{ \Delta\theta }{ \Delta t }$



then,

$v=\displaystyle\frac{\Delta s}{\Delta t}=r\displaystyle\frac{\Delta\theta}{\Delta t}=r\omega$



Since the relationship is general, it can be applied for instantaneous values, resulting in

$ v = r \omega $

.

(ID 3233)


As the mean Speed ($\bar{v}$) is with the distance traveled in a time ($\Delta s$) and the time elapsed ($\Delta t$), equal to

$ \bar{v} \equiv\displaystyle\frac{ \Delta s }{ \Delta t }$



and with the distance traveled in a time ($\Delta s$) expressed as an arc of a circle, and the radius ($r$) and the angle variation ($\Delta\theta$) are

$ \Delta s=r \Delta\theta $



and the definition of the mean angular velocity ($\bar{\omega}$) is

$ \bar{\omega} \equiv\displaystyle\frac{ \Delta\theta }{ \Delta t }$



then,

$v=\displaystyle\frac{\Delta s}{\Delta t}=r\displaystyle\frac{\Delta\theta}{\Delta t}=r\omega$



Since the relationship is general, it can be applied for instantaneous values, resulting in

$ v = r \omega $

.

(ID 3233)


As the mean Speed ($\bar{v}$) is with the distance traveled in a time ($\Delta s$) and the time elapsed ($\Delta t$), equal to

$ \bar{v} \equiv\displaystyle\frac{ \Delta s }{ \Delta t }$



and with the distance traveled in a time ($\Delta s$) expressed as an arc of a circle, and the radius ($r$) and the angle variation ($\Delta\theta$) are

$ \Delta s=r \Delta\theta $



and the definition of the mean angular velocity ($\bar{\omega}$) is

$ \bar{\omega} \equiv\displaystyle\frac{ \Delta\theta }{ \Delta t }$



then,

$v=\displaystyle\frac{\Delta s}{\Delta t}=r\displaystyle\frac{\Delta\theta}{\Delta t}=r\omega$



Since the relationship is general, it can be applied for instantaneous values, resulting in

$ v = r \omega $

.

(ID 3233)

The definition of average angular acceleration is based on the angle covered

$ \Delta\omega = \omega_2 - \omega_1 $



and the elapsed time

$ \Delta t \equiv t - t_0 $



The relationship between the two is defined as the average angular acceleration

$ \bar{\alpha} \equiv \displaystyle\frac{ \Delta\omega }{ \Delta t }$

within that time interval.

(ID 3234)

Given that the mean Acceleration ($\bar{a}$) equals the speed Diference ($\Delta v$) and the time elapsed ($\Delta t$) according to

$ \bar{a} \equiv\displaystyle\frac{ \Delta v }{ \Delta t }$



and the mean Angular Acceleration ($\bar{\alpha}$) equals the difference in Angular Speeds ($\Delta\omega$) and the time elapsed ($\Delta t$) as per

$ \alpha_0 \equiv \displaystyle\frac{ \Delta\omega }{ \Delta t }$



it follows that

$\bar{a}=\displaystyle\frac{\Delta v}{\Delta t}=r\displaystyle\frac{\Delta\omega}{\Delta t}=\bar{\alpha}$



Assuming that the mean Angular Acceleration ($\bar{\alpha}$) is equal to the constant Angular Acceleration ($\alpha_0$)

$ \bar{\alpha} = \alpha_0 $



and assuming that the mean Acceleration ($\bar{a}$) equals the constant Acceleration ($a_0$)

$ a_0 = \bar{a} $



then the following equation is obtained:

$ a = r \alpha $

(ID 3236)

If we assume that the mean Angular Acceleration ($\bar{\alpha}$) is constant, equivalent to the constant Angular Acceleration ($\alpha_0$), then the following equation applies:

$ \bar{\alpha} = \alpha_0 $



Therefore, considering the difference in Angular Speeds ($\Delta\omega$) along with the angular Speed ($\omega$) and the initial Angular Speed ($\omega_0$):

$ \Delta\omega = \omega_2 - \omega_1 $



and the time elapsed ($\Delta t$) in relation to the time ($t$) and the start Time ($t_0$):

$ \Delta t \equiv t - t_0 $



the equation for the mean Angular Acceleration ($\bar{\alpha}$):

$ \alpha_0 \equiv \displaystyle\frac{ \Delta\omega }{ \Delta t }$



can be expressed as:

$\alpha_0 = \alpha = \displaystyle\frac{\Delta \omega}{\Delta t} = \displaystyle\frac{\omega - \omega_0}{t - t_0}$



Solving this, we obtain:

$ \omega = \omega_0 + \alpha_0 ( t - t_0 )$

(ID 3237)

Since the moment is equal to

$ L = I \omega $



it follows that in the case where the moment of inertia doesn't change with time,

$T=\displaystyle\frac{dL}{dt}=\displaystyle\frac{d}{dt}(I\omega) = I\displaystyle\frac{d\omega}{dt} = I\alpha$



which implies that

$ T = I \alpha $

.

(ID 3253)

The relationship between the angular Momentum ($L$) and the moment ($p$) is expressed as:

$ L = r p $



Using the radius ($r$), this expression can be equated with the moment of Inertia ($I$) and the angular Speed ($\omega$) as follows:

$ L = I \omega $



Then, substituting with the inertial Mass ($m_i$) and the speed ($v$):

$ p = m_i v $



and

$ v = r \omega $



it can be concluded that the moment of inertia of a particle rotating in an orbit is:

$ I = m_i r ^2$

(ID 3602)

In the case of the constant Angular Acceleration ($\alpha_0$), the angular Speed ($\omega$) as a function of the time ($t$) follows a linear relationship with the start Time ($t_0$) and the initial Angular Speed ($\omega_0$) in the form of:

$ \omega = \omega_0 + \alpha_0 ( t - t_0 )$



Given that the angular displacement is equal to the area under the angular velocity-time curve, in this case, one can add the contributions of the rectangle:

$\omega_0(t-t_0)$



and the triangle:

$\displaystyle\frac{1}{2}\alpha_0(t-t_0)^2$



This leads us to the expression for the angle ($\theta$) and the initial Angle ($\theta_0$):

$ \theta = \theta_0 + \omega_0 ( t - t_0 )+\displaystyle\frac{1}{2} \alpha_0 ( t - t_0 )^2$

(ID 3682)

If we start from the starting position ($s_0$) and want to calculate the distance traveled in a time ($\Delta s$), we need to define a value for the position ($s$).

In a one-dimensional system, the distance traveled in a time ($\Delta s$) is simply obtained by subtracting the starting position ($s_0$) from the position ($s$), resulting in:

$ \Delta s = s - s_0 $

(ID 4352)


If we solve for time in the equation of the angular Speed ($\omega$) that includes the variables the initial Angular Speed ($\omega_0$), the time ($t$), the start Time ($t_0$), and the constant Angular Acceleration ($\alpha_0$):

$ \omega = \omega_0 + \alpha_0 ( t - t_0 )$



we obtain the following expression for time:

$t - t_0 = \displaystyle\frac{\omega - \omega_0}{\alpha_0}$



This solution can be substituted into the equation to calculate the angle ($\theta$) using the initial Angle ($\theta_0$) as follows:

$ \theta = \theta_0 + \omega_0 ( t - t_0 )+\displaystyle\frac{1}{2} \alpha_0 ( t - t_0 )^2$



which results in the following equation:

$ \theta = \theta_0 +\displaystyle\frac{ \omega ^2- \omega_0 ^2}{2 \alpha_0 }$

(ID 4386)

Si se deriva en el tiempo la relaci n para el momento angular

$ L = r p $



para el caso de que el radio sea constante

$T=\displaystyle\frac{dL}{dt}=r\displaystyle\frac{dp}{dt}=rF$



por lo que

$ T = r F $

(ID 4431)

Since the moment ($p$) is defined with the inertial Mass ($m_i$) and the speed ($v$),

$ p = m_i v $



If the inertial Mass ($m_i$) is equal to the initial mass ($m_0$), then we can derive the momentum with respect to time and obtain the force with constant mass ($F$):

$F=\displaystyle\frac{d}{dt}p=m_i\displaystyle\frac{d}{dt}v=m_ia$



Therefore, we conclude that

$ F = m_i a $

(ID 10975)


Examples


(ID 15527)


(ID 15530)


ID:(599, 0)