hello

I'm new at physics and already stuck ( for quite awhile now ) , so I really need some answers

Object in uniform circular motion is moving around the perimeter of the circle with a constant speed and while the speed of the object is constant , its velocity is changing . The direction is always directed tangent to the circle due to acceleration towards the center


Why is it that if the direction of the force remains perpendicular to the motion will the direction change without the speed changing ?

Is it that since centripetal force only pulls on object for such a brief period of time that it only has time to change velocity's direction and then centripetal force already changes its position and the story again repeats itself ?

So if object A is moving with constant speed along the x axis with velocity vector V ( V for now only has x component ) and something starts pulling it along the y axis with force F1 , then for a brief instant when force F1 is perpendicular to velocity vector V force F1 should change direction of V so that now V also has y component , but still has same magnitude .
And after that ( after it is no longer perpendicular to velocity vector )F1 should also start changing V's magnitude ( by increasing y component )

My point being that if it is true that if direction of force remains perpendicular to the motion will the direction change without the speed changing , then magnitude of x component of an object A shouldn't be equal to the magnitude of velocity object A had prior to pulling on A with force F1 , since F1 already changed the direction of velocity V

I hope someone can clear this up for me

thank you

A better way to think about it is this. In uniform circular motion, the force is always perpendicular to the direction of motion _at_all_times_. Therefore, the force can't do any work on the object, the object's kinetic energy is constant. So the object's speed must be a constant, although its velocity is constantly changing. Does that help?

A better way to think about it is this. In uniform circular motion, the force is always perpendicular to the direction of motion _at_all_times_. Therefore, the force can't do any work on the object

But it does affect object by redirecting its direction



Does that help?

uh , not really :(

My mind understands best if it is explained the way I wrote first post in this thread

Why is it that if the direction of the force remains perpendicular to the motion will the direction change without the speed changing ?

I will use the follwing symbols:
v is the velocity, a vector
|v| is the speed, a scalar
a is the acceleration, a vector
m is the mass, a scalar
F is the force, a vector
* means dot product

What does it mean for the speed to be constant? It means that v*v = |v|^2 is independent of time, right? So we can write this as d(v*v)/dt = 0. Well now you just differentiate to obtain d(v*v)/dt = 2 v*dv/dt = 2 v*a. But this had to be zero for the speed to be constant so canceling the two we find v*a = 0. The force is is given by F = ma so that if v*a = 0 then v*F = 0 (just multiply the first by m). So what have we found? If the force is perpendicular to the velocity, the speed is constant.

... So what have we found? If the force is perpendicular to the velocity, the speed is constant.Very nice. It takes a real smart monkey to answer simple questions so well.

Is it that since centripetal force only pulls on object for such a brief period of time that it only has time to change velocity's direction and then centripetal force already changes its position and the story again repeats itself ?

The centripetal force pulls on the object all the time, but always in a direction at right angles to the velocity of the object.


So if object A is moving with constant speed along the x axis with velocity vector V ( V for now only has x component ) and something starts pulling it along the y axis with force F1 , then for a brief instant when force F1 is perpendicular to velocity vector V force F1 should change direction of V so that now V also has y component , but still has same magnitude.

For a "brief instant", yes.


And after that ( after it is no longer perpendicular to velocity vector )F1 should also start changing V's magnitude ( by increasing y component )

Yes. But this is different from circular motion because here you assume the force doesn't change direction as the velocity changes direction.

So if object A is moving with constant speed along the x axis with velocity vector V ( V for now only has x component ) and something starts pulling it along the y axis with force F1 , then for a brief instant when force F1 is perpendicular to velocity vector V force F1 should change direction of V so that now V also has y component , but still has same magnitude.



For a "brief instant", yes.


But when you see drawings ( on internet or in books ) of forces pulling on objects tossed horizontally , it is portrayed as if the horizontal velocity vector never changes direction , meaning is always horizontal , and only net force keeps changing .

But if what you are saying is true , then at the very moment you toss an object horizontally with velocity say 10km/hour , at that one instant when gravity force is perpendicular to horizontal velocity vector , horizontal velocity vector changes slightly its direction ,and so x component of net force is not 10 km/hour but instead a bit less (say 9.9999... km/hour) ?

Is the difference so negligible between the real magnitude of x component ( 9.9999... km/hour ) and 10 km/hour and thus study books don't even mention it ?

The centripetal force pulls on the object all the time, but always in a direction at right angles to the velocity of the object. I hesitate to correct you here James as you are trying to help and I may just confuse with this post, BUT:

That is not true of a planet in all points of an elliptical orbit except four, apogee & perigee and the two points half way between.

You forgot to say "object moving around a circle."

It'd be great if someone at least made few comments about my last post on the things I got wrong so I can at least...

don't apply this to objects in orbit, leave gravity out. there is no centripetal force for objects in space, it was a fiction of Newton's. that's not what's confusing you is it?...no gravitational attraction for that matter either....

First I need to understand newton's "fiction" and only later will I worry about advanced theories . Now can somebody help me with newton first ?

beginner16:

I wrote a reply to your post earlier, but it was lost when sciforums shifted servers. I'll try to reconstruct what I said...


But when you see drawings ( on internet or in books ) of forces pulling on objects tossed horizontally , it is portrayed as if the horizontal velocity vector never changes direction , meaning is always horizontal , and only net force keeps changing .

You're thinking of objects tossed near the surface of the Earth, where it is a fair approximation to treat gravity as always acting vertically. In that case, the gravitational force only changes the vertical velocity component of the object's motion, and the horizontal is not affected.

If you want better accuracy, once the object has moved a little from its launch point, the gravitational force is not quite vertical any more (since it points towards the centre of the Earth), and it can therefore affect BOTH the horizontal and vertical components of the velocity.


But if what you are saying is true , then at the very moment you toss an object horizontally with velocity say 10km/hour , at that one instant when gravity force is perpendicular to horizontal velocity vector , horizontal velocity vector changes slightly its direction ,and so x component of net force is not 10 km/hour but instead a bit less (say 9.9999... km/hour) ?

Yes.


Is the difference so negligible between the real magnitude of x component ( 9.9999... km/hour ) and 10 km/hour and thus study books don't even mention it ?

Yes. The assumption is that we can treat motion near the Earth's surface as if gravity acts directly downwards at all times, rather than towards the centre of the Earth at all times. Of course, that assumption breaks down if you look at large-scale projectile motion (such as the path followed by an intercontinental ballistic missile, for example).

Thanx for the explanation . The only thing bothering me now is :

Here is my confusion - why is it so important that force is not perpendicular to velocity vector in order to change the magnitude of velocity ?
Why can't instead "another" velocity vector be started independent of already existing velocity vector ? Seems illogical that so little force is needed to start moving an object with mass 1 kg/hour with speed 1km/hour , and yet in order to change magnitude of object's velocity force ( no matter how big this force is ) force must not be perpendicular to velocity vector or it will only change its direction ?

Here is my confusion - why is it so important that force is not perpendicular to velocity vector in order to change the magnitude of velocity ?

There are two explanations for that: a mathematical one, and a hand-waving physical one. The math explanation is obvious, but you need to be familiar with vectors and components. The physical explanation is an appeal to common sense.

To make something speed up or slow down, you must give it a push or pull in the direction you want to speed it up or slow it down. Imagine, for example, pushing a car. If you want it to speed up, you push it at the back. If it is coming towards you and you want to stop it, you push it from the front. But pushing it on the side as it moves past you won't change its speed in the forwards-backwards direction.


Why can't instead "another" velocity vector be started independent of already existing velocity vector ?

If you're asking whether you can create a velocity at right angles to the original motion, yes you can. For example, in the car example, if you push hard enough on the side of the car as it goes past, you can tip the car over, or make it skid sideways across the road. In that case, you are "creating" some sideways velocity.


Seems illogical that so little force is needed to start moving an object with mass 1 kg/hour with speed 1km/hour , and yet in order to change magnitude of object's velocity force ( no matter how big this force is ) force must not be perpendicular to velocity vector or it will only change its direction ?

As I said before, the magnitude of the velocity won't change ONLY in the case of circular motion, where the direction of the FORCE changes as the direction of the object also changes. My car example is different to that, because the "sideways" force is always applied in the same direction - it doesn't change direction as the car's velocity changes.

ok,thank you very much for your help