<< Chapter < Page Chapter >> Page >

Water in a bucket

If you don't mind taking a chance on getting wet, fill a small bucket about half full of water. Then swing the bucket rapidly in a circle in a vertical plane.

If you can swing the bucket fast enough, the water will stay in the bucket even when the bucket is upside down. Why is that? The water molecules want tomove in a straight line. However, the inside surface of the bucket exerts a centripetal force on the water molecules causing them to accelerate toward thecenter of the circle. The centripetal force increases with the speed of the bucket. As long as the centripetal force is greater than the weight of thewater, the water won't fall out of the bucket onto your head. However, if you allow the speed of the bucket to decrease, you will reach the point wheregravity will overcome, and you will probably get wet.

The moon and the Earth

Don't ask me how it got started in the first place, but somehow the moon got started circling the Earth.

The speed of the moon and the radius of its orbit is just exactly right so that the gravitational force that the Earth exerts on the moon causes the moonto accelerate towards the Earth. The amount of acceleration toward the earth, when combined with the speed of the moon, causes the moon to move in a uniformcircular orbit around the Earth instead of either flying off into space or crashing into the Earth.

The work-energy explanation

It is probably time to start explaining this phenomena in a more technical and less anecdotal manner.

The application of a centripetal force for uniform circular motion causes the direction of the object to be changed without changing its speed. Let's see if we can explain this froma work-energy viewpoint.


Recall from an earlier module that work is a force acting upon an object to cause a displacement . Also recall that the amount of work done on an object, expressed in Joules, is given by

Work = F * D * cosine(theta)


  • F represents the force that causes the displacement expressed in Newtons.
  • D represents the amount of the displacement expressed in meters.
  • theta is the angle between the direction of the displacement and the line of action of the force.

Centripetal force points toward center of circle

As we showed (or claimed to show) earlier, the centripetal force for an object in uniform circular motion always points in the direction of the center of the circle. At the sametime, the velocity vector, which represents the direction of the displacement is tangential to the circle. Therefore, the angle between the centripetal force andthe direction of displacement is 90 degrees. This means that the centripetal force does no work on the object, because the cosine of 90 degrees is zero.

No work is done

When no work is done upon an object by external forces, the total mechanical energy, consisting of potential energy plus kinetic energy, of the objectremains constant. If an object is moving in a circle in a plane that is parallel to the surface of the Earth, the effect of gravity may pull the entire planetoward the surface of the Earth, but it won't effect the circular motion unequally with respect to the position of the object in the circle.

Questions & Answers

What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Accessible physics concepts for blind students. OpenStax CNX. Oct 02, 2015 Download for free at https://legacy.cnx.org/content/col11294/1.36
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Accessible physics concepts for blind students' conversation and receive update notifications?