ECE 105 - Physics of Electrical Engineering 1
Forces and Motion
Force is a vector, and therefore includes direction. For any vector a, ▯a has the same
magnitude but opposite direction.
Given AB we can ▯nd A or B’s position based on the position of the other one
~ ~ ~
OB= O +AAB
for any Oxis the location of x relative to the origin.
We can break any vector into components by ▯nding the angle between it and the plane
we want to model it o▯ of.
Example: for A = [email protected]
, we can ▯nd it’s components with relation to the standard x-y
~ ~ ▯
A y Acos20
A = Asin20 ▯
1 Constant Acceleration
a▯t = v ~ ▯ v~
~f= v ~i+~a▯t
For the position vector d;d =fd + vi ~i▯t + ~1a(▯t) 2
2 2 ▯ ▯
vf = v~i+ 2~ a ▯d ~
For any three objects a;b; and c
v~ = v~ + v~
ca cb ba
read "the velocity of c with respect to a is equal to the velocity of c with respect to b plus
the velocity of b with respect to a.
▯ ▯ r for very close points.
For circle with center O and radius r 0r 1:: connected to object on circumferance with velocity
~0;v~1::: tangent to circumferance, a = v1 ▯0. For arc length between object (at di▯erent times
t0;t1:::) s, ▯ =r. For small ▯ ▯ 1;jv ~1▯ v ~0j = ▯v:
jaj = v▯
a is perpendicular to ~v
v = t = t
) ~a = r ▯ = r
2 s = r▯
dt = v = rdt
a = r
Types of Forces
A force is a push or pull interaction between two objects, reponsible for changing motion.
When unstretched, no spring forces exist. When a string is pushed from equilibrium, its
spring force pushes back toward equilibrium.
F = ▯k▯x
The tension force pulls an object toward a rope and a rope toward an object. Ropes can
The normal force "pushes back" against other objects via molecular electromagnetism. It
is always perpendicular to the surface for any surface-to-surface contact. Technically, it is a
type of spring force.
Friction is the interaction between an object and a surface. It is a real force which acts
opposite the direction of sliding, and is always tangent to surface.
f / N is an experimental fact. f = ▯N, where ▯ is the coe▯cient of friction. ▯ is dependant
on the type of objects and must be determined experimentally.
Kinetic friction is when objects are sliding relative to each other and static friction is
when objects are not yet sliding
fs▯ ▯ s
Example: A 50kg person is in a 1000kg elevator at rest. When the elevator begins to rise,
the person notices her weight is 600N. How far does the elevator move in 3s?
▯F = m~ a
F ▯ mg
~a = n
600 ▯ 50g
d = i t + at
= 0 + 22:2)9
An object can be said to have a total energy equal to the sum of the various forms of energy
it may posess.
The kinetic energy of an object is determined by its mass and velocity
K = mv
4 For any object with a changing velocity
vf = vi+ 2ad
1 mv = 1 mv + mad
2 f 2 i
K f K + iFd
▯K = ▯Fd
Potential Gravitational Energy
Potential gravitational energy is a measure of stored energy of an object based on its
height. It is essentially non-sensical to determine an object’s "absolute" potential gravita-
tional energy, thus we often simply solve for the di▯erence in energy.
For a distance