Height, time, gravity acceleration

  • #1

Homework Statement


"A stone is thrown vertically upwards from the top of a building 29 metres high at an initial speed of 11 metres/sec. How long (in seconds) will it take the stone to hit the ground? The height at any time is given by

s = 29 + 11*t – 4.9*t2.

Give your answer correct to one rounded off decimal place (but don't round off as you work the problem), eg., 4.37 becomes 4.8."


Homework Equations



-29 = 11t - 4.9t^2
t(11 - 4.9t) = -29 <<< Do I do this? Do I need it?

The Attempt at a Solution


The answer is 3.801, but how do I get this?
Please help me
 

Answers and Replies

  • #2
LowlyPion
Homework Helper
3,097
5

Homework Statement


"A stone is thrown vertically upwards from the top of a building 29 metres high at an initial speed of 11 metres/sec. How long (in seconds) will it take the stone to hit the ground? The height at any time is given by

s = 29 + 11*t – 4.9*t2.

Give your answer correct to one rounded off decimal place (but don't round off as you work the problem), eg., 4.37 becomes 4.8."


Homework Equations



-29 = 11t - 4.9t^2

That's fine as far as it goes.

But what you have is a quadratic equation.

You can use the quadratic formula to solve the general equation:
http://en.wikipedia.org/wiki/Quadratic_equation#Quadratic_formula

One of the answers will give you your solution. The other root of the equation is likely a (-) number, indicating another solution of different construction that would also satisfy the same quadratic relationship.
 

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