A force perpendicular to velocity does not change speed?

AI Thread Summary
A force applied perpendicular to the direction of an object's velocity does not change its speed, but it alters the direction of the velocity vector. To maintain a constant speed while changing direction, the force must remain perpendicular to the velocity throughout the motion. An instantaneous force, such as a quick push, can change speed because it is not continuously adjusted to remain perpendicular. The distinction between speed and velocity is crucial; while speed remains constant, the velocity vector can change due to the perpendicular force. Misunderstandings in the application of these concepts can lead to incorrect conclusions about motion.
mackygood
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Homework Statement



There's a ball moving at a constant velocity V and I apply a force perpendicular to the direction of the velocity. Why is it that there's no change in speed?

If I add a perpendicular force, there would be a new vector thus resulting in a new velocity with a different direction and speed right?

http://img22.imageshack.us/img22/8261/forceh.jpg \[/URL]

Thanks for any answers :D
 
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In order to keep the speed unchanged and only change the direction of motion, the force must be kept perpendicular to the velocity as the velocity changes. Your picture shows an initial upward velocity and a sideways force. As soon as the velocity starts changing direction, so must the force. This is not illustrated in your diagram.
 
thanks for your reply!

What if the force was instantaneous? Would it still not affect the speed?
 
mackygood said:
What if the force was instantaneous? Would it still not affect the speed?
I'm not sure what you mean by "instantaneous". If you mean something like an impulsive force (a quick push to the side that lasts a short time), then that could certainly change the speed since it would not be continually adjusted to be perpendicular to the velocity.
 
I think you are confusing velocity and speed. A perpendicular force will not increase velocity but it will increase speed.
 
skeptic2 said:
A perpendicular force will not increase velocity but it will increase speed.
A perpendicular force will not change the speed but will change the velocity.
 
If an object is traveling upwards at velocity v and at point p it is influenced by force f in the horizontal direction, its velocity [in the upwards direction] is unchanged but its speed has increased to √(v^2 + f^2).
 
skeptic2 said:
If an object is traveling upwards at velocity v and at point p it is influenced by force f in the horizontal direction, its velocity [in the upwards direction] is unchanged but its speed has increased to √(v^2 + f^2).
Among your many clues that this cannot possibly be correct is that you're adding two quantities with different units.
 
Hurkyl said:
Among your many clues that this cannot possibly be correct is that you're adding two quantities with different units.

Yes, that was a pretty stupid mistake. I wasn't thinking about what I was writing. What were the others?
 

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