1 mark question on kinematics, or a stone fallin through air.

[mutineer123]

Homework Statement

A stone is dropped from the top of a tower of height 40 m. The stone falls from rest and air
resistance is negligible.
What time is taken for the stone to fall the last 10m to the ground?

A 0.38s B 1.4s C 2.5s D 2.9s

Homework Equations

The Attempt at a Solution

I first found the final velocity for the first 30m. V^2= 2x9.8x30 (ignore directions)

then i used that final velocity as my initial velocity in V=u+at where V=0. My answer is C. But its wrong.

 

[gneill]

Why not find the time to fall 30 m, then the time to fall 40 m, and take the difference?

 

[mutineer123]

Why not find the time to fall 30 m, then the time to fall 40 m, and take the difference?

How do i find the time t0 fall 40m? Both the final and initial velocity will be zero then won' it.

 

[gneill]

How do i find the time t0 fall 40m? Both the final and initial velocity will be zero then won' it.

Find a kinematic equation that expresses the distance as a function of acceleration and time.

 

[Emilyjoint]

Applying your method V is not = 0.The final speed is just before it hits the ground and you have taken the 'final' speed to be when it hits the ground and stops.
The velocity you calculated at 30m is u for the last 10m

 

[mutineer123]

Applying your method V is not = 0.The final speed is just before it hits the ground and you have taken the 'final' speed to be when it hits the ground and stops.
The velocity you calculated at 30m is u for the last 10m

Thanks, yeah forgot about that. Plugged it in and got .383, close enough. Thank you