Jump to content

Math Help


Joe85
 Share

Recommended Posts

Glorious Math guru's, please shower me with your superior intellect.


A ball is thrown down at 20ms^-1 speed from the top of a building. The building is 125 meters tall. The distance travelled before it reaches the ground is as follows:


S=U0t + 1/2gt^2



U0 = 20ms^-1

g = 10ms^-2

t= time


a) Find the time for the ball to drop a fifth of the height of the building


b) Find the time for the ball to reach the ground



Now, i feel fairly confident that i have solved this myself. Having converted this into a quadratic equation i have:


a) t = 1s

B) t = 3.385s


However, when checking my answer against an online caluclator (https://www.calculatorsoup.com/calculators/physics/displacement_v_a_t.php) it states the answer is a complex conjugate. Now, i'm struggling to see how they have come up with a complex number here (i thought for it to be a complex number, the number you find the square root of (#^2- 4ac) had to be negative ie J = -1) but part of the course i am doing does involve complex numbers and i'm slightly worried they are trying to catch you out a bit.


Any help for a dunce would be appreciated. :D

Link to comment
Share on other sites

I may have answered this if it had said Chinese 125 dropped from top of building. But it doesn't, so I don't care.

 

Noted for next time glorious mathematician.

Link to comment
Share on other sites

I think there's an error in the calculator.

If I put the speed and acceleration as negative values, then it gives the right answer

image.png

But, putting your numbers for part a (with t = 1) into the equation s = ut + 1/2gt^2 you get

25 = 20 + 0.5*10

which is correct.


My guess is that the person writing the page confused themselves by writing the equation as ut + 1/2gt^2 - s = 0, which led them to messed up the signs.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

  • Welcome to The Motorbike Forum.

    Sign in or register an account to join in.

×
×
  • Create New...

Important Information

Terms of Use Privacy Policy Guidelines We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.

Please Sign In or Sign Up