Catch This Embarrassing Moment on Camera

Image source: Barefoot Friends

A teenager accidentally falls off a 10-meter-high platform diving board. His friend whips out his camera and takes a picture when the teenager is halfway to the water below to commemorate the embarrassing event and posts it on Facebook. How many seconds after the teenager fell off did his friend take the picture?

Details and assumptions

  • The acceleration of gravity is 9.8 m/s 2 -9.8~\mbox{m/s}^2 .
  • Neglect the size of the teenager.


The answer is 1.01.

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13 solutions

Daniel Ferreira
Oct 7, 2013

{ α = 9.8 m / s 2 V o = 0 m / s S o = 10 m S = 5 m t = S = S o + V o t + α t 2 2 5 = 10 + 0 + 9.8 t 2 2 9.8 t 2 2 = 5 9.8 t 2 = 10 t 2 = 1 , 020 t = 1.01 s \begin{cases} \alpha = - 9.8 \; m/s^2 \\ V_o = 0 \; m/s \\ S_o = 10 \; m \\ S = 5 \; m \\ t = \end{cases} \\\\ S = S_o + V_o \cdot t + \frac{\alpha \cdot t^2}{2} \\\\ 5 = 10 + 0 + \frac{- 9.8t^2}{2} \\\\ \frac{9.8t^2}{2} = 5 \\\\ 9.8t^2 = 10 \\\\ t^2 = 1,020 \\\\ \boxed{t = 1.01 \; s}

Something is wrong, is it 1,020 or 1.020

Rindell Mabunga - 7 years, 8 months ago

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Aqui no Brasil não utilizamos tal notação; por isso, considere um erro de digitação. Se quiseres, podes comprovar analisando as respostas abaixo - Samuel e Pedro!

Daniel Ferreira - 7 years, 8 months ago

People in different countries have different notation for decimal places.

Daniel Liu - 7 years, 8 months ago

O que você fez com o 10 após o 5 = 10 + 0 + ...

Anderson Clemente - 7 years, 8 months ago

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Ora, subtraí 5. Faça passo a passo e notará que economizei algumas linhas, talvez por essa razão não tenha percebido! Passe a variável para o lado esquerdo da igualdade e o termo independente para a direita...

Daniel Ferreira - 7 years, 8 months ago
Andi M
Mar 6, 2014

t = 2 h g t = \sqrt{\frac{2h}{g}} which h = 10/2 = 5 meters

H = (g.t^2)/2

5 = 4,9t^2 t = 1,01

Sean Elliott
Dec 16, 2013

To solve this problem, we use the equation d = d 0 + v t + 1 2 a t 2 d=d_0+vt+\frac{1}{2}at^2 . Note that in this particular problem, we have that v = 0 , a = 9.8 v=0,a=9.8 . We set d 0 = 0 d_0=0 for simplicity. We are trying to solve for the time t t when the teenager has gone 5 5 meters. Thus the equation is 5 = 1 2 9.8 t 2 5=\frac{1}{2}9.8t^2 or t = 10 9.8 1.010 t=\sqrt{\frac{10}{9.8}}\approx\boxed{1.010}

Ali Ahmed
Jan 29, 2014

=h/g =10/9.8 =1.020

Janaky Murthy
Jan 10, 2014

teenafer felt half way that is s2-s1=10/2=5m s2-s1=ut+1/2at^2 u=0m/s a=g

solve 5=0+1/2 10 t^2 t^2=1 t=1s(negative value not possible)

Mridul Gupta
Jan 8, 2014

Use II Eq. of Motion i.e. s=u t+1/2 g*t^{2} u=0 and s=5m(half way down) then after solving t=1.01 sec

Swapnil Rajawat
Jan 6, 2014

Given, h=10 m (halfway height= 5 m) u=0 m/s (The boy was initially at rest) g=9.8 m/s ^2

we know, by the second equation of motion that, h=u t+1/2 at^2

put the values and you will get the correct answer. (Put h=5 m)

It is a girl apparently, not a boy.

Samuel Wong - 7 years, 5 months ago

Hello,y axis motion...

halfway down=5m,as initial velocity=0 m/s, so by applying v^2=u^2-2gs,substitute u=0 m/s and g= -9.8m/s/s, V^2=0^2-(2x-9.8x5)=98 therefore,v=(98)^0.5,use v=u-gt to find the time,so t=v-u/-g=(98)^0.5 - 0 / (-(-9.8))=~1.01s..

thanks...

Half of 10m = 5m, so : 5 = 10 + 0.t - 4.9.t.t => 0=5 - 4.9 t^2 => t^2 = 1.001 => t = 1.01s

Pradeep Gb
Dec 17, 2013

5=0-0.5 9.8 t^2 solve it. you will get t=1

Raphael Konishi
Oct 13, 2013

Using S=So +vot + at²/2:

5 = 10t²/2

t = 1s

Pedro Junqueira
Oct 7, 2013

H = g . t^2 /2 5 = 9,8 . t^2 /2 5 = 4,9 . t^2 t^2 = 1,02 t = 1,01

why H = 5

Le Hau - 7 years, 8 months ago

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The halfway of 10 is 5...

Pedro Junqueira - 7 years, 7 months ago

Whoa, dude. Whoa.

Manssa G - 7 years, 8 months ago

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