basic physics - why a ball thrown into the air decelerates


I was just confused by the following statement: assuming no air resistance, a ball thrown into the air will declerate at a constant rate (due to gravity)

This confused me because i thought, for the ball to decelerate it must be travelling against a constant negative force (gravity) but i thought how would the ball have even starting moving in the first place if the force of gravity was greater. I presume the flaw in my thinking was that there was an initial force on the ball to get the ball moving, but once thrown the ball has no 'positive' force on it and so decelerates due to graivty.

Is that right?
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B HCommented:
that's exactly correct, the initial force was greater than the force of gravity, but as soon as the source was removed gravity takes over, at a constant rate

if the ball had some self-contained source of propulsion, well then that changes things

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Umm no foolio.

YOU throwing the ball is the force. Eventually it starts to decelerate, due to gravity pulling down on it, and then it accelerates back down at 9.8 m/s before hitting.
Same thing for any projectile....  bullets, rocks, fish, feathers, hot potatoes.

Gravity just drags things down man.
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It all comes down to the question of what is the total net force that is operating on the object at each instant in time.

When the ball is simply being held at a constant height above the ground, the net force on the ball is zero because the downward force of gravity is cancelled out by the upward force of the hand which supports it.

When the ball is then thrown into the air, the force of the hand propelling the ball upward exceeds the downward force of gravity which results in the ball accelerating same direction as the net force (upward).

Once the ball leaves the hand, there is no longer any upward force being imparted to the ball and the only force acting on the ball (excluding air resistance) is the downward force of gravity. Thus the ball accelerates in the direction of the gravitational force.

Since the ball had an upward velocity when it left the hand (based on the frame of reference of the person throwing the ball) we see the ball as decelerating since this is the term that we use when we see a relative velocity decreasing. Deceleration is simply acceleration in a direction that is perceived to be in a direction that is opposite to the velocity within our particular frame of reference.

So, in effect, your perception of the mechanics of this situation is correct.

Does this help?
andiejeAuthor Commented:
hi, thanks tiras for your last comment, that makes perfect sense. However when you said
'Umm no foolio.' are you saying my initial post was wrong because you've just agreed with it and I'm a touch confused
B HCommented:
i thought he's either calling me a fool for my first post, or you a fool for not understanding... either way i thought it was kind of mean :/
Sorry, didn't mean to offend.

Thibault St john Cholmondeley-ffeatherstonehaugh the 2ndCommented:
>declerate at a constant rate
When I saw that I thought your question was going to be about gravity decreasing as you move away from the earth.
What we needs here is some a them there pictures with circles and arrows and everything like that there....

And you can get anything you want.......   at Alice's Restaurant.
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