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Lecture 24 - Midterm II Review

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Newton's Laws

  1. Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it.
  2. $\Sigma\vec{F}=m\vec{a}$
  3. Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.

These laws only apply in inertial reference frames - our reference frame must be stationary or moving with a constant velocity.


The most important quantity related to an object's motion is it's momentum


Momentum can only be changed by a net force acting on the object


So for collisions in which no external force is acting the total momentum is conserved. This is really just another way of expressing Newton's 2nd Law.


The equation of conservation of momentum is a vector equation.


Therefore if we have a collision involving more than one dimension we need to consider conservation of each component of momentum, for example in 2 dimensions



If a collision is perfectly elastic we can add a 3rd equation


Extended objects and sets objects

In general the displacement vector for the center of mass of a system of particles $m_{i}$ can be written as


Differentiating with respect to time gives

$M\frac{d\vec{r}_{CM}}{dt}=\Sigma_{i}m_{i}\frac{d\vec{r}}{dt}$ or $M\vec{v_{CM}}=\Sigma_{i}m_{i}\vec{v}_{i}$

and thus the total momentum of a system is


differentiating once more


$\Sigma_{i}m_{i}\vec{a}_{i}=\Sigma_{i}F_{i}=\Sigma \vec{F}_{ext}$

so we have obtained a new form of Newton's Second Law that works for a system of particles.

$M\vec{a_{CM}}=\Sigma \vec{F}_{ext}$

Angular Momentum

The total momentum of a system is given by


but this does not necessarily imply that every part of the system has $\vec{v}=\vec{v}_{CM}$

There are, however, restrictions on the velocity that each part of the system can take. The motion of the mass around the center of mass in the reference frame of the center of mass needs to maintain the position of the center of mass. Rotation around the center of mass satisfies this requirement.To be able to describe the motion of each point in the system with a single parameter we can consider the angular momentum of each point around the CM.


and to change this we need to apply a torque



Vector cross product

To properly understand angular momentum and torque we will first need to discuss vector cross products.

The cross product of two vectors $\vec{A}$ and $\vec{B}$ is another vector


The magnitude of the vector $\vec{C}$ is $AB\sin\theta$

The direction of $\vec{C}$ is perpendicular to the plane which contains both $\vec{A}$ and $\vec{B}$. Use of the right hand rule can help you determine this direction.

If we consider the vectors in unit vector notation, $\vec{A}=A_{x}\hat{i}+A_{y}\hat{j}+A_{z}\hat{k}$ and $\vec{B}=B_{x}\hat{i}+B_{y}\hat{j}+B_{z}\hat{k}$ the cross product can be expressed as the determinant of a matrix

$\vec{A}\times\vec{B}=\left| \begin{array}{ccc} \hat{i} & \hat{j} & \hat{k}
A_{x} & A_{y} & A_{z}
B_{x} & B_{y} & B_{z} \end{array} \right| = \hat{i}\left| \begin{array}{cc} A_{y} & A_{z}
B_{y} & B_{z} \end{array} \right| - \hat{j}\left| \begin{array}{cc} A_{x} & A_{z}
B_{x} & B_{z} \end{array} \right| + \hat{k}\left| \begin{array}{cc} A_{x} & A_{y}
B_{x} & B_{y} \end{array} \right|$

$=(A_{y}B_{z}-A_{z}B_{y})\hat{i}+(A_{z}B_{x}-A_{x}B_{z})\hat{j}+(A_{x}B_{y}-A_{y}B_{x})\hat{k} $

Calculating Torque

In a case such as that illustrated above we can calculate the magnitude of the torque using


The units of torque are $\mathrm{Nm}$

Conditions for Equilibrium

For an object to have no translational acceleration

$\Sigma \vec{F}=0$

and for this to be true the sum of each component of the forces must be zero.

An object may have no net force on it, but be able to turn, which means for an object to be in equilibrium

$\Sigma \tau = 0 $

Where the torques can be calculated around any axis. Some axes are more sensible than others, a good idea is to choose an axis at place where a force is acting (as the torque due to that force around that axis will be zero.)

Moment of Inertia

In an object with a common angular acceleration which consists of many masses at various distances from the center of rotation, each of which may be subject to a torque we can say that

$\sum\limits_{i}\tau_{i}=(\sum\limits_{i} m_{i}R_{i}^{2})\alpha$

We call the quantity $\sum\limits_{i} m_{i}R_{i}^{2}$ the moment of inertia $I$ and write the rotational equivalent of Newton's Second Law as

$\sum \tau=I\alpha$

Moments of Inertia for extended objects

Usually a rotation axis that passes through the center of mass of an object will be one of the easiest to find the moment of inertia for, because the center of mass usually reflects the symmetry of the object. Moments of inertia on wikipedia.

If we know the moment of inertia of an object around an axis that passes through it's center of mass there is a theorem that can help us find the moment of inertia around a different axis parallel to the axis through the COM.

If the axis of rotation is a distance $h$ from the through the COM then


where M is the total mass of the object.

As an example, a solid sphere rolling around a pole will have moment of inertia

Angular Momentum

For any object which is symmetric around it's axis of rotation


Otherwise the angular momentum along the rotation axis will be of magnitude $L_{\omega}=I\omega$, but there will be other angular momentum components.

For any problem I will ask you you can use $L=I\omega$

Kinetic Energy

Kinetic energy is defined as


but if we want to treat an extended object rotating around a moving center of mass this



Work-energy principle

To change kinetic energy a force must do work on the object,

$W=\int_{a}^{b}\vec{F}\cdot d\vec{l}$

if only conservative forces act the potential energy must change by an equal and opposite amount, which is a statement of the principle of conservation of mechanical energy.

For rotation

$W=\int \vec{F}\cdot\,d\vec{l}=\int F_{\perp}R\,d\theta=\int_{\theta_{1}}^{\theta_{2}}\tau\,d\theta$


For an object to roll without slipping there is a frictional requirement, it is actually not possible for an object to roll on a frictionless surface, it would instead slide. Note that the point in contact with the ground during rolling is actually motionless, so the relevant coefficient of friction is the static coefficient of friction, whereas for sliding the kinetic coefficient of friction is relevant. There is no work done by this force as it is directed perpendicular to the displacement.

The point around which the wheel turns which is also it's center of mass is seen to execute translational motion with velocity $\vec{v}$

If we instead consider the motion of a point on the edge of the wheel we can see it comes to rest, in the reference frame of the ground when it is contact with the ground. We can however also look at it from the perspective of the wheel, in which case the motion of the point of the edge of the wheel is purely rotational, and as the wheel sees the ground moving to the left with velocity the same magnitude as it moves forward in the grounds reference frame the rotational velocity of the wheel must be $\omega=\frac{v}{r}$ or $v=\omega r$.

Rolling without slipping

Rolling Hoop and Disk


$\large K=\frac{1}{2}mv^{2}+\frac{1}{2}I\omega^{2}=\frac{1}{2}mv^{2}+\frac{1}{2}mr^{2}\frac{v^{2}}{r^2}=mv^{2}$




$\large K=\frac{1}{2}mv^{2}+\frac{1}{2}I\omega^{2}=\frac{1}{2}mv^{2}+\frac{1}{2}\frac{1}{2}mr^{2}\frac{v^{2}}{r^2}=\frac{3}{4}mv^{2}$



Recall that for a sliding object (without friction) $v=\sqrt{2gh}$

Solid sphere $I=\frac{2}{5}mr^{2}$
Hollow sphere $I=\frac{2}{3}mr^{2}$



$\Sigma F =ma$

$v= v_{0}+at$

$x= x_{0}+v_{0}t+\frac{1}{2}at^2$



$\Sigma \tau=I\alpha$

$\omega= \omega_{0}+\alpha t$

$\theta= \theta_{0}+\omega_{0}t+\frac{1}{2}\alpha t^2$


phy141/lectures/24.txt · Last modified: 2013/10/25 14:47 by mdawber
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