Here is my working

Can someone please tell me what I have done wrong here? I can't see what is wrong and have asked many people far smarter than me who also can't spot the error.


closed as off-topic by David Z Sep 15 at 10:42

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Homework-like questions should ask about a specific physics concept and show some effort to work through the problem. We want our questions to be useful to the broader community, and to future users. See our meta site for more guidance on how to edit your question to make it better" – David Z
If this question can be reworded to fit the rules in the help center, please edit the question.

  • 1
    $\begingroup$ For next time, it is convenient to introduce. You could briefly describe your two initial formulas and why you are equating then together, although this time people have been able to infer the problem. $\endgroup$ – jinawee Sep 15 at 6:44
  • $\begingroup$ Duplicate Free fall - equation - what am I doing wrong? $\endgroup$ – Farcher Sep 15 at 8:03

You're using $v$ in two different ways. In the first few equations, $v$ means the final velocity after a period of uniform acceleration, assuming one starts from rest. In the equation $v = s/t$, $v$ means the average velocity instead. That's half as much, which is why you're off by $2$.

This is a general warning for learning physics. In other high school courses, if you just take the formulas you're given and mash them together, without worrying what the variables mean, you can get the right answer. In physics, you really have to understand what you're doing. Many formulas will have the letter $v$ in them, but the meaning of that letter will be different, because there can be many different important speeds in most problems.


In the last step $v^2=2v^2$ means $ v=0$. So you cannot divide both sides by $v^2$ because that is division by 0. That is why you get nonsense.


Not the answer you're looking for? Browse other questions tagged or ask your own question.