Homework Help: Rocket acceleration problem

1. Jul 21, 2007

MIA6

1. If a ball dropped from a person's hand, and it then undergoes a free fall. The acceleration is 9.81m/s^2, if it undergoes a uniform acceleration, can i use the formula, d=Vt to find the distance it travels? or if it relates to the gravity, i must only use the formula with a, for instance, like d=Vit+at^2/2.
2. If a rocket is launched with a certain speed, does it also have acceleration when it takes off? My teacher said when this rocket goes up, the speed is decelerating due to gravity, so meanwhile when it is still approaching upward, does it still have the acceleration same as the beginning? So the speed is increasing? Then, how come it slows down? Must this acceleration greater than 9.81?
Thanks for explaining.

2. Jul 21, 2007

Dick

You can only use d=Vt if the velocity is constant or it's an average velocity. Otherwise you use the latter form if the acceleration is constant. I can't make a lot of sense out of 2).

3. Jul 22, 2007

Tedjn

2) Gravity does act on the rocket after it is launched, and that would slow the rocket down if it was not being pushed up by its propulsion. The rocket begins to accelerate upward as it is being launched, because it expels its fuel out below it, and that pushes the rocket with an upward acceleration greater than the downward pull of gravity, so the net movement is up.

There's also a distinction between acceleration and velocity (speed). Suppose a rocket is going upward, meaning its velocity is in the up direction. If nothing is pushing it upward (let's say it is no longer using its fuel), gravity will slow it down, but because the upward velocity was so large, the rocket continues moving upward for quite awhile before its velocity will decrease to 0 and then start heading downward.

4. Jul 22, 2007

MIA6

So you mean that if the initial velocity is 0, and it then has a constant acceleration, but we still can't use d=vt?

5. Jul 22, 2007

MIA6

So if the rocket is flying all the way to the top with its acceleration, then it will not be slowed down by gravity, right? IN other words, it would be true as what my teacher told me if the acceleration only happens in takeoff, but then no more acceleration, then it is slowed down by gravity?

6. Jul 23, 2007

MIA6

Can anyone answer my questions above?