# This is too confusing

1. May 3, 2006

### allok

hiya

Circular motion is really confusing

Force perpendicular to velocity is said to change only velocity's direction but not its magnitude. But how about object orbiting the earth? If object orbits the earth then force of gravity is perpendicular to it and thus is said it only changes its direction.

Ok, so perpendicular force only can change direction of velocity because it doesn't have any components paralel to velocity. But:

if you break velocity vector of an orbiting object in components ( at any point on its path ) and also break force of gravity into components, then it becomes obvious that force of gravity always has component parallel to some comonent of object's velocity vector and as such it never really is perpendicular to velocity.

I need a little help people

thank you

2. May 3, 2006

### nrqed

If the orbit is circular, the force of gravity will always be perpendicular to the velocity. Pick any point of the orbit. The velocity is tangent to the circle along which the object is moving, right? And the force of gravity is always toward the center of the Earth. You can convince yourself that the force will always be perpendicular to the velocity and therefore the speed (the magnitude of the velocity) does not change.

If the orbit is elliptical, then yes, there are some points where the force is not perpendicular to the velocity and in that case the speed is not constant (in agreement with Kepler's second law of planetary motion)

3. May 3, 2006

### Andrew Mason

ONLY if the orbit is a circle. Most orbits are somewhat elliptical. Force is perpendicular to velocity only at the ends - where the ellipse intersects the major axis.

In an elliptical orbit the central force DOES change the magnitude of velocity because it is not always perpendicular to the velocity.

AM

4. May 3, 2006

### daniel_i_l

Make the X-axis in the direction of the speed and the y-axis in the direction of the gravity. You will see that they don't cancel eachother out. And even if you choose different coords, you'll get the same resault cause all the gravity components in the direction of the velocity will cancel out.

5. May 3, 2006

### arildno

"If object orbits the earth then force of gravity is perpendicular to it and thus is said it only changes its direction."

It is this sentence by you which is flawed; the rest is well thought out.