December 10, 2019, 09:02:19 am

### AuthorTopic: Standard Formulas for Projectile Motion?  (Read 62 times) Tweet Share

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##### Standard Formulas for Projectile Motion?
« on: November 17, 2019, 09:59:57 am »
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Hi guys, when using projectile motion do we have to derive the formulas from the SUVAT equations or can we just use the standard formulas? For example, the physics in focus book says to find time of flight the formula is -2uy/g. Do we have to show the derivation or can we just use the formula?

Thank you!

#### fun_jirachi

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##### Re: Standard Formulas for Projectile Motion?
« Reply #1 on: November 17, 2019, 10:24:52 am »
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Hey there!

My physics teacher always told be a good rule of thumb is to always derive things not on the reference sheet (it's just unsafe to outright quote things that aren't). That being said, you should be deriving time of flight every time; for an object launched and landing at the same height, it takes about three lines to derive (shown below!), which in terms of time in exams is basically the same as picking it out of the formula sheet. Essentially, even if you know it, it doesn't take much longer to ensure you get all the marks, instead of losing marks for seemingly pulling stuff out of nowhere.
$0=u+at_{\frac{1}{2}} \\ t_{\frac{1}{2}} = -\frac{u}{a} \\ t=-\frac{2u}{a}$ (note this is only if the object is launched and lands at the same height)

Hope this helps
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