When a rubber ball bounces, it first accelerates in the positive direction (downward) then it bounces (hits the floor) .The velocity then becomes negative as the ball declerates while travelling upward - accelerates in the negative direction. After some time, the velocity is finally cancelled out by the downward force of gravity on the ball.
At this point the same amount of force pushing the ball upward is the same pushing it downward - it is at zero velocity.
My question is, how long does this period of zero velocity (motionlessness) last.
Answer
You're confusing weightlessness with motionlessness - they are two very different things.
You feel weightless when you are in free-fall. That is, you are accelerating downward at $9.8\ \mathrm{m/s^2}$. This is always true at the very apex of the motion of an object, but it is generally true for most of the rest of its path, too. The only caveat is that air resistance will add to or subtract from your acceleration. For instance, if you fall for long enough, you will reach terminal velocity, at which point you feel like you have just as much weight as standing still on the ground, but you are supported by a blast of air rather than your legs.
Motionlessness occurs for just a single instant1 for an object thrown into the air. Because there is no air resistance if there is no motion, this is also a moment of weightlessness.
To feel (near-) weightlessness for an extended period of time while moving, I recommend going sky diving, or even riding a roller coaster with a vertical drop.
1 To address the edit: This is just a single instant, lasting for a duration of precisely $0$ in time. In the classical case of a projectile in a uniform gravitational field with no air resistance, the velocity as a function of time is $$ v(t) = v_0 - gt, $$ and there is exactly one time $t$ for which $v(t) = 0$.
No comments:
Post a Comment