tell me about free fall

[sciman]

A question from gravity.If I take two things say gold and aluminium of same shape and size and fall it from a buliding , which will fall first or they will fall at the same type?And if I repeat the same activity in vaccum , what will happen?What matters in free fall?

[swansont]

Same shape and size should be identical behavior, since the additional forces will then be identical. In a vacuum it's only gravity, and then all things accelerate at g.

[insane_alien]

well if the building is tall enough then the gold one will hit first as friction will cause the aluminium one to stop accelerating at a slower velocity than the gold one.

 

in a vacuum they would hit at the same time.

[sciman]

well if the building is tall enough then the gold one will hit first as friction will cause the aluminium one to stop accelerating at a slower velocity than the gold one.

This was your answer.

But the friction should be same for both as they are of same shape and size.Then they should fall at the same time.

[insane_alien]

yes the friction force will be the same at the same velocity but the mass is different, so the weight is different.

 

terminal velocity occurs when the forces balance. aluminum is less dense than gold, so the gold sphere will be heavier. (pulls some numbers out of thin air)

 

say the gold sphere weighs 10kg and the aluminium sphere ways 5 kg(these are not accurate values) and g=10 m·s^-2 (its early, i'm tired, i hate maths) the gold sphere will have a force of 100 newtons and the aluminium on will have a force of 50N.

 

the velocity required for 50N of drag is slower than the velocity required for 100N of drag. ergo the the gold sphere will fall faster as it has the higher terminal velocity.

 

in a vacuum there is no drag so both hit the ground at the same time.

[swansont]

Ah, right. I didn't account for the possibility of terminal velocity being reached.

[insane_alien]

well, you don't have to reach terminal velocity, just enough so it significantly affects the acceleration of the lighter.

[Sisyphus]

There's also the factor of buoyancy when falling through a fluid such as air. If the mass of the displaced air is significant compared to the falling body, that will make it fall slower, as well. The "practical" weight of the falling body is the weight of the body - the weight of the air it displaces. As mentioned above, a lower weight has a harder time overcoming air resistance. Sometimes this value is even negative, as in the case of a helium balloon, and the object "falls" up.

 

But again, to reiterate, neither air resistance nor buoyancy play any role in a vacuum.

[insane_alien]

i think we can consider the buoyant forces negligble if the spheres are solid. but if you want to be pedantic...

[swansont]

I was actually thinking of bouyancy when I said "forces." If you are using balloons instead of lumps of different metals, it will certainly be noticable.

[insane_alien]

well in a low density difference situation like that then they do become quite dominant. i wasn't considering buoyancy her because, well, the density of gold is a few orders of magnitude bigger than airs.