So I am trying to remember my childhood doing some Physics problems, but seems that I forgot almost everything. But it's not a big thing... that's why I'm training!

My problem is pretty basic although I remember just a little... let me ennounce it:

A $5\ \rm kg$ block of ice falls away from the edge of the roof of a block of flats, at a height of $26\ \rm m$ above the ground. Ignoring air drag, find out:

a) how long it takes for the ice block to hit the ground. b) what is the speed at which the ice block hits the ground. c) how much energy it transfers to the surroundings when it comes to a stop and break into pieces. ($g = 9.8\ \rm m/s^2$)

For a), I tried: $t = \frac{d}{r}$, and gives me $2.653\ \rm s$.

For b), I tried: $9.8 \times 2.653$, and this results in $25.9994\ \rm m/s$

For c) I tried nothing since I don't remember what should I do.

Can someone take a look at this problem and tell me if I am doing it right, or just messing all the things?


closed as too localized by David Z Dec 5 '11 at 22:05

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    $\begingroup$ Hi Peter, and welcome to Physics Stack Exchange! I see that you got some useful answers, which is good, but I wanted to mention that this is not really a site for homework help, it's a site for general conceptual questions. In other words, you should not be asking "am I doing this right?" but rather something like "what does this formula mean?", just to give an example. I've closed this to indicate that it's not really the kind of question for this site. For future reference, remember that you can always edit closed questions to make them more appropriate and get them reopened. $\endgroup$ – David Z Dec 5 '11 at 22:10

Unfortunately, you are messing up quite a bit. d/g gives 2.653 s${}^2$.

You should re-read about free fall in any basic physics book. There, you'll see that when you have constant acceleration (in this case, its $g$), the height as a function of time is $$y(t) = y_0+v_0\,t+1/2\,g\,t^2$$ where $y_0$ and $v_0$ are your starting height and speed, and $y(t)$ is the height at instant $t$. You can solve for the final time knowing that the initial speed is zero, and the distance the block falls is $H=y(t_f)-y_0$. The final expression is also in any book.

The final speed can also be calculated from the equation for speed: $$v(t) = v_0 + g\,t$$

For the energy question, I could give you the formula, but I rather suggest that you first understand clearly the basic concepts of kinematics and dynamics before going to the next level of abstraction that is mechanical energy.


Welcome to physics.stackexhange Peter! The results that you calculated are not correct. What you need are the basic equations of motion $$\begin{align} & v & = &u+at \\ & s & = &ut + \tfrac12 at^2 \\ \end{align}$$ With the initial velocity $u$, the acceleration due to gravity $a=g$. For c) you need the kinetic energy of a moving body or the potential energy of your block before the fall.

  • $\begingroup$ Or You think about potential energy. This is much simpler to calculate. $\endgroup$ – Georg Dec 5 '11 at 14:59

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