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Since r is constant, the term θ × d r d t = 0 . Since v = d s d t is the tangential velocity and ω = d θ d t is the angular velocity, we have

v = ω × r .

That is, the tangential velocity is the cross product of the angular velocity and the position vector, as shown in [link] . From part (a) of this figure, we see that with the angular velocity in the positive z -direction, the rotation in the xy -plane is counterclockwise. In part (b), the angular velocity is in the negative z- direction, giving a clockwise rotation in the xy- plane.

Figure A is an XYZ coordinate system that shows three vectors. Vector Omega points in the positive Z direction. Vector v is in the XY plane. Vector r is directed from the origin of the coordinate system to the beginning of the vector v. Figure B is an XYZ coordinate system that shows three vectors. Vector Omega points in the negative Z direction. Vector v is in the XY plane. Vector r is directed from the origin of the coordinate system to the beginning of the vector v.
The vectors shown are the angular velocity, position, and tangential velocity. (a) The angular velocity points in the positive z- direction, giving a counterclockwise rotation in the xy- plane. (b) The angular velocity points in the negative z -direction, giving a clockwise rotation.

Rotation of a flywheel

A flywheel rotates such that it sweeps out an angle at the rate of θ = ω t = ( 45.0 rad / s) t radians. The wheel rotates counterclockwise when viewed in the plane of the page. (a) What is the angular velocity of the flywheel? (b) What direction is the angular velocity? (c) How many radians does the flywheel rotate through in 30 s? (d) What is the tangential speed of a point on the flywheel 10 cm from the axis of rotation?


The functional form of the angular position of the flywheel is given in the problem as θ ( t ) = ω t , so by taking the derivative with respect to time, we can find the angular velocity. We use the right-hand rule to find the angular velocity. To find the angular displacement of the flywheel during 30 s, we seek the angular displacement Δ θ , where the change in angular position is between 0 and 30 s. To find the tangential speed of a point at a distance from the axis of rotation, we multiply its distance times the angular velocity of the flywheel.


  1. ω = d θ d t = 45 rad / s . We see that the angular velocity is a constant.
  2. By the right-hand rule, we curl the fingers in the direction of rotation, which is counterclockwise in the plane of the page, and the thumb points in the direction of the angular velocity, which is out of the page.
  3. Δ θ = θ ( 30 s ) θ ( 0 s ) = 45.0 ( 30.0 s ) 45.0 ( 0 s ) = 1350.0 rad .
  4. v t = r ω = ( 0.1 m ) ( 45.0 rad / s ) = 4.5 m/s .


In 30 s, the flywheel has rotated through quite a number of revolutions, about 215 if we divide the angular displacement by 2 π . A massive flywheel can be used to store energy in this way, if the losses due to friction are minimal. Recent research has considered superconducting bearings on which the flywheel rests, with zero energy loss due to friction.

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Angular acceleration

We have just discussed angular velocity for uniform circular motion, but not all motion is uniform. Envision an ice skater spinning with his arms outstretched—when he pulls his arms inward, his angular velocity increases. Or think about a computer’s hard disk slowing to a halt as the angular velocity decreases. We will explore these situations later, but we can already see a need to define an angular acceleration    for describing situations where ω changes. The faster the change in ω , the greater the angular acceleration. We define the instantaneous angular acceleration     α as the derivative of angular velocity with respect to time:

Questions & Answers

what is electromagnetism
David Reply
It is the study of the electromagnetic force, one of the four fundamental forces of nature. ... It includes the electric force, which pushes all charged particles, and the magnetic force, which only pushes moving charges.
what is units?
Subhajit Reply
units as in how
What is th formular for force
Joseph Reply
F = m x a
State newton's second law of motion
Seth Reply
can u tell me I cant remember
force is equal to mass times acceleration
The acceleration of a system is directly proportional to the and in the same direction as the external force acting on the system and inversely proportional to its mass that is f=ma
The uniform seesaw shown below is balanced on a fulcrum located 3.0 m from the left end. The smaller boy on the right has a mass of 40 kg and the bigger boy on the left has a mass 80 kg. What is the mass of the board?
Asad Reply
Consider a wave produced on a stretched spring by holding one end and shaking it up and down. Does the wavelength depend on the distance you move your hand up and down?
Sohail Reply
how can one calculate the value of a given quantity
Helen Reply
To determine the exact value of a percent of a given quantity we need to express the given percent as fraction and multiply it by the given number.
briefly discuss rocket in physics
Ibrahim Reply
ok let's discuss
What is physics
Nura Reply
physics is the study of natural phenomena with concern with matter and energy and relationships between them
a potential difference of 10.0v is connected across a 1.0AuF in an LC circuit. calculate the inductance of the inductor that should be connected to the capacitor for the circuit to oscillate at 1125Hza potential difference of 10.0v is connected across a 1.0AuF in an LC circuit. calculate the inducta
Royalty Reply
L= 0.002H
how did you get it?
is the magnetic field of earth changing
tibebeab Reply
what is thought to be the energy density of multiverse and is the space between universes really space
can you explain it
Energy can not either created nor destroyed .therefore who created? and how did it come to existence?
Suzana Reply
this greatly depend on the kind of energy. for gravitational energy, it is result of the shattering effect violent collision of two black holes on the space-time which caused space time to be disturbed. this is according to recent study on gravitons and gravitational ripple. and many other studies
and not every thing have to pop into existence. and it could have always been there . and some scientists think that energy might have been the only entity in the euclidean(imaginary time T=it) which is time undergone wick rotation.
What is projectile?
Nana Reply
An object that is launched from a device
2 dimensional motion under constant acceleration due to gravity
Not always 2D Awais
no comments
why not? a bullet is a projectile, so is a rock I throw
bullet travel in x and y comment same as rock which is 2 dimensional
no all pf you are wrong. projectile is any object propelled through space by excretion of a force which cease after launch
for awais, there is no such thing as constant acceleration due to gravity, because gravity change from place to place and from different height
it is the object not the motion or its components
where are body center of mass on present.
Balwant Reply
on the mid point
is the magnetic field of the earth changing?
does shock waves come to effect when in earth's inner atmosphere or can it have an effect on the thermosphere or ionosphere?
and for the question from bal want do you mean human body or just any object in space
A stone is dropped into a well of 19.6m deep and the impact of sound heared after 2.056 second ,find the velocity of sound in air.
Sisco Reply
9.53 m/s ?
In this case, the velocity of sound is 350 m/s.
some calculations is need. then you will get exact result.
i mean how? isn't it just a d over t?
calculate the time it takes the stone to hit the ground then minus the stone's time to the total time... then divide the total distance by the difference of the time
awit lenard. Hahahah ari ga to!
Practice Key Terms 5

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Source:  OpenStax, University physics volume 1. OpenStax CNX. Sep 19, 2016 Download for free at http://cnx.org/content/col12031/1.5
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