Physics: Falling Objects?

Question

A ball is thrown directly downward, with an initial speed of 7.40 m/s, from a height of 30.2 m. After what time interval does the ball strike the ground?

Answer 1

First state all of your givens. (Approach EVERY problem like this! Never think its too simple or short!!)

Vi=7.4 m/s

df=30.2m

di=0m

t=?

Use a Kinematics equation to find it's final velocity.

Vf^2=Vi^2+2a(df-di)

Vf^2=7.4^2+2(9.81)(30.2)

*Remember the acceleration due to gravity is 9.81 m/s^2!!!!!*

Vf^2=647.284

Vf=sqrt 647.284

Vf=25.44177667

Now that we have the final velocity. Lets use another kinematics equation to find the time.

Vf=Vi+at

*isolate t*

Vf-Vi=at

t=(Vf-Vi)/a

t=(25.44177667-7.4)/(9.81)

t=1.8 seconds.

So it takes it 1.8 seconds to hit the ground.  This makes sense because the ball accelerates as it is falling decreasing the time it take to hit the ground.

Answer 2

19 seconds

Answer 3

s = (1/2)g t^2 + v0 t

s = 4.9 t^2 + t

s = 30.2;  t = ?