# A kid falls off the top bunk of a bed, how long till he wakes up?

1. Sep 25, 2010

### ADH

2 kids have a sleepover, one kid falls out of the top bunk. The bunk is 2 meters high. How long till wakes up?

I was told "This is fancy way of asking how long it will take an object dropped from two meters with zero initial velocity will take to reach the floor."

Thank you.

2. Sep 25, 2010

### rock.freak667

Yes it is, so what equation do you know involves these terms? (Remember, acceleration due to gravity is 9.81 m/s2)

3. Sep 25, 2010

### ADH

Well I know the distance = 2 m. The acceleration is 9.8 m/s^2. So we have to find when he hits the ground. I'm not sure how to do this. That's why I posted it.
Thank you.

4. Sep 25, 2010

### bjd40@hotmail.com

the accln may defined as the change (final - initial, i.e the change here) in velocity devided by the time. construct the eqn. and put initial velocity as zero, get an eqn with time, t and final velocity, v, as two unknown quantities. now get another eqn. equating the initial energy of the boy, potential energy here, and final energy( kinetic energy). it follows from conservation of energy concept. thus u can get an eqn with only one unknown, v. the rest is easy.

5. Sep 26, 2010

### rock.freak667

your relevant equation should be s=ut+1/2at2 where u = initial velocity.

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