# Mechanics questions

#### briton

If a ball is thrown in the air and comes momentarily at rest is it at equilibrium?
I thought yes because it's stationary, but then again.. there is a resultant gravitational force.. ?

Also what is "weightlessness" exactly?
Is it when you're in freefall/only gravity acts? so that would be like an astronaut in a satellite orbiting the earth?

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#### whozum

briton said:
If a ball is thrown in the air and comes momentarily at rest is it at equilibrium?
I thought yes because it's stationary, but then again.. there is a resultant gravitational force.. ?
Equilibrium conditions are:

$$\sum F = 0, \sum \tau = 0$$ where [itex] \tau [/tex] is the torque.

Also what is "weightlessness" exactly?
Is it when you're in freefall/only gravity acts? so that would be like an astronaut in a satellite orbiting the earth?
I want to say you feel weightless when your body feels no net force, but there is a net centripetal force during orbit.

#### Dr.Brain

When the ball is at rest at the top , it is being acted upon by the 'g' downwards , so it is not in equilibrium .

Weightlessness is 'free fall under gravity ' This is when your weight downwards (mg) is balanced by pseudo force uppwards when you are freely falling ...

Example: You are in a lift and the rope breaks and then you fall freely , you will lose contact with ground and you are under free fall.

For more:
http://science.howstuffworks.com/weightlessness.htm

#### Doc Al

Mentor
briton said:
Also what is "weightlessness" exactly?
Is it when you're in freefall/only gravity acts? so that would be like an astronaut in a satellite orbiting the earth?
Yes. "Weightlessness" is when your apparent weight is zero. Your apparent weight, the "weight" that you feel, is due to the normal force of whatever surface is holding you up (or other force preventing you from falling). Take away the support, and you feel "weightless". (In physics, the term weight has a specific technical meaning: the force of gravity on an object. So, while you may experience the feeling of "weightlessness", your real weight is not zero!)

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