Question

An orange of mass 40g falls freely from a tree to the ground through a distance of 2.5m. Calculate the velocity just before it hits the ground.
Velocity = speed/time
I have no speed or time so what do I do.

Answer 1

potential energy (gravitational) = m*g*h
Kinetic energy (motion) = 1/2 * m*v^2

before the orange was dropped from the tree:

KE_inital = 1/2 * m * v_i^2 = 0
(because it's a rest, no velocity)

PE_initial = m*g*h_i
= 40g * 9.81m/s^2 * 2.5 = 981kJ


Before its the ground:

PE_final = m*g*h_f = 0
(because h is just about 0)

KE_final = 1/2 * m * v_f^2

From the conservation of energy law:

KE_inital+PE_inital = KE_final + PE_final

0 + 981 = 1/2*m*v_f^2 + 0

981 = 1/2 * m * v_f^2

solve for v_f

Answer 2

You can also use newton's equations of motion, it's the same concept:


http://www.physicsclassroom.com/calcpad/newtlaws


v_f^2 = v_o^2 +2*a*d

a=acceleration=gravity=9.81 m/s^2
d=distance=2.5 m
v_o = initial velocity = 0 (at rest)