Consider an object thrown upward with a force which is much greater than its weight. Obviously the object will stop after a certain interval of time due to air resistance. But what if there is no air resistance?
closed as unclear what you're asking by Bill N, AccidentalFourierTransform, ACuriousMind♦, Qmechanic♦ Apr 30 '16 at 15:28
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If there was no air resistance, the object would still slow down, stop momentarily, then return to the ground - because it is pulled downwards (towards the centre of the Earth) by the force of gravity (ie the object's weight).
More than one force can act on an object at the same time. Here, both gravity and air resistance act on the object. Gravity always acts downwards, and it is always approximately the same for the object. (It decreases slightly as you go up, but you would have to go up 30km before it decreased by 1%.) Gravity is always there - when the object is in the thrower's hand, when it leaves the thrower's hand, even when it is up in the air and stops moving. Air resistance only acts when the object moves through the air, always opposes motion, and increases as speed through the air increases.
If there were only air resistance acting on the object and no gravity, the object would slow down and stop completely. It would not come back. But that is an imaginary situation which doesn't happen for most objects.
When both these forces act on the object it slows down more quickly than if only gravity acts, and it does not reach quite as high as when there is no air resistance.
The force from the thrower's hand also acts on the object, but it stops as soon as it the object leaves contact with the hand, and starts again when the object is caught.
Another force acting on the object is the buoyancy force caused by the atmosphere. But this is also usually much smaller than the other forces unless the object has a large volume and a small weight, like a balloon.