# Circular Orbits and weightlessness

## Homework Statement

A small projectile is launched parallel to the ground at height h = 1 m with sufficient speed to orbit a completely smooth, airless planet. A bug rides in a small hole inside the projectile. Is the bug weightless? Explain.

## The Attempt at a Solution

I didnt understand the concept and the question so i dont know a way to start :(

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Well you need to start by looking at what forces (or more appropriately, accelerations) the bug will be experiencing. Do you know what they are?

Well you need to start by looking at what forces (or more appropriately, accelerations) the bug will be experiencing. Do you know what they are?
there certainly is Force of gravity acting downward. no air resistance coz its airless.
and nothing more i can think of

Really? So astronauts only experience gravity? Would they still be weightless if this was the case?

Really? So astronauts only experience gravity? Would they still be weightless if this was the case?
yes coz they dont have any normal force i guess

Gravity is an acceleration towards the earth. When gravity is the only force, it pulls you straight down towards the planet at Xm/s2. To be in a stable orbit (and subsequently experience weightlessness), you need to provide a specific velocity.

The projectile is neither ascending nor descending. Which means the acceleration downwards, towards the planet is equal to the centripetal acceleration.

It is in freefall.

Now, you know there's gravity acting downwards, but is anything acting upwards?

If the forces on the spacecraft are balanced so that it remains at a constant altitude, is the bug also experiencing those forces? If not, why not?

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yea i know forces always comes in pair

and there is no Tension force hmmm is is the gravitational force by the projectile on the Earth (although i doubt it)

OR is it true that if there is no air in the planet then there is no gravity and if there is no gravity the thing keeps on moving in a constant velocity?

sorry man i am all over the place not just good with physics

Forces come in pairs? You're discussing the wrong thing.

The presence of air on a planet has nothing to do with gravity (at least it's negligable as far as you're concerned).

Forget all that stuff you just wrote, start below:

I'm going to point you to some sources for you to read as you don't appear to understand the mechanics of orbit and weightlessness.

These two are important for you to read, the second has a question for you to try:
http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/hump.html
http://www.physicsclassroom.com/class/circles/u6l4d.cfm

This is a bit detailed, but if you feel like expanding it's a good read:
http://en.wikipedia.org/wiki/Weightlessness

So is the bug weightless?

yes it is weightless but i am still kinda confused as u point out earlier "When gravity is the only force, it pulls you straight down towards the planet " so why isnt the astronaut orbitting earth come straight to Earth.
Is gravity centripetal force then?

Sorry, error there (it's late I'm thinking of the wrong thing).

Gravity is the only force acting on the projectile / craft.

The orbital velocity gives centripetal acceleration towards the centre of the planet.

The projectile is constantly falling towards the planet, giving you the sensation of weightlessness.

Imagine, when you jump off something and you're falling. It's like that, but because of your orbital velocity you never hit the ground.

Read through that link I gave you, it explains it far better than I could.

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D H
Staff Emeritus
Is the bug weightless? Explain.

In particular, it depends completely on what you mean by "weightless". What do you mean by "weightless"? (Or even more important, what does your teacher / your textbook mean by the term?)

Imagine, when you jump off something and you're falling. It's like that, but because of your orbital velocity you never hit the ground.

Read through that link I gave you, it explains it far better than I could.
just like a ball moving in a uniform circular velcoity