# Homework Help: Model Rocket question regarding height,time,acceleration

1. Sep 1, 2008

### rsfancy

A model rocket rises with constant acceleration to a height of 4.0 m, at which point its speed is 25.0 m/s.
How long does it take for the rocket to reach this height? Ans: t= 0.32s
What was the magnitude of the rockets acceleration? Ans a= 78 m/s^2

"Find the height of the rocket 0.20 seconds after launch."
I cant seem to get it right, I know its at least close to 1.5, because that was my original answer but the darned online system says "Not quite. Check through your calculations; you may have made a rounding error or used the wrong number of significant figures." the answer is supposed to be to two significant figures.

"Find the speed of the rocket 0.20 seconds after launch."
I havent attempted this part of the question yet because I think I need to use what I figure out from the previous question to work on this.

All this stuff is not hard, but I cant seem to grasp the simple concepts that Im missing in figuring it out, any help and/or guidance would be really appreciated.

2. Sep 2, 2008

### rl.bhat

h = 1/2*a*t^2. Put the values of a and t and see how much you get.

3. Sep 2, 2008

### LowlyPion

The thing to keep in mind is what forces (hence accelerations) are acting on the object at different points in time. Since they are asking you for the V and x at a time during which the object was undergoing uniform acceleration starting from rest (V=0) then the equation for finding distance x (or h), is going to be simply
x=1/2 a t2 because the other terms will be 0 because it started at 0 from rest.
Likewise for finding speed you know starting from rest that V = a t

Basically you can use the formulas that relate x, V and t according to what you know and what you need to figure out. Here is a link to various kinematic equations that you might find useful.

https://www.physicsforums.com/showpost.php?p=905663&postcount=2