work, change in velocity

[stevvo111]

so I had somewhat of an epiphany this morning. I came across a problem yesterday dealing with velocity what causes changes in velocity.

I thought about something I saw about "you need work" to change the velocity. Before I take this and run with it, does it make sense? I was thinking about the W=change(KE), and it makes sense from there.

In addition, every change in velocity is pretty much accompanied by force in the direction... so I figured this statement is true!

For a falling object, work is technically being done since there is a "force" (gravity) acting on the object over a distance (distance to ground), this causing the change in velocity.

Is this logic correct, and if so, how can it be correlated to circular motion?

For constant "speed" object being spun around, there is a force acting on it but it is not in the direction of the object's motion, rather perpendicular to it, so technically the velocity changes due to the force, but not do to work? lol I hope this makes sense.

---

Members don't see this ad.

 

[milski]

That's correct only if the force is not perpendicular to the velocity. A classic counterexample would be a charged particle rotating in uniform magnetic field. In this case the velocity is always perpendicular to the force and there is no work done nor a change in KE. In frictionless environment the rotation will continue forever. The mechanical equivalent would be an object on a string rotating around a pole - again, the tension force here is always perpendicular to the velocity.

To summarize: doing work will lead to a change in KE. Change of KE is the same as change in the magnitude of velocity but it tells you nothing about change of direction of velocity. So you can have a change of direction of velocity and if its magnitude stays the same, there will be no work done.