A 1.6 kg rock is dropped from a height of 15 m. With what speed will it strike the ground. Ignore air resistance. Solve....

Question

A 1.6 kg rock is dropped from a height of 15 m. With what speed will it strike the ground. Ignore air resistance. Solve using conservation of energy and NOT kinematics. Answer: 17m/s

Answer 1

POTENTIAL ENERGY = MGH

KINETIC ENERGY     = MV²/2

G = 9.8 M/S²   (Gravity accel at earth surface)

 

 

THE SUM OF POT ENERGY AND KINETIC ENERGY

SHOULD BE EQUAL AT POINTS (FROM H=0 - 15M) IN THE FALL. 

 

          AT H = 15M                AT H=0

SO   (MGH + MV²/2)_A  =    (MGH + MV²/2)_{B            A,B not part of eqn, denotes location}

 

AT H=15  V= 0           AND     AT H=0   MGH=0    SO,

 

          (MGH + 0)_A     =     (0 + MV²/2)_{B                  A,B not part of eqn, denotes location}

 

WE CAN DIVIDE OUT THE M

   AT H = 15M       AT H=0

         GH          = V²/2          OR   V² = 2GH

 

 V = SQRT(2GH) = SQRT(2*9.8*15)  =  17.1 M/S