Question

A 5.5 kg bowling ball initially at rest is dropped from the top of a 12 m building. It hits the ground 1.75 s later. Find the net external force on the falling ball

Answer 1

External Forces- means outside forces acting on an object. We already know that of course gravitational force involved. However, you will realize at the time. If only gravitational force act on it -----> Time= Square root of ((2*height)/9.8)) = 1.55second ( Base on the distance equation with the downward acceleration: g=9.8 m/s^2)
-----> So there must be another the force act on it, WITH AN UPWARD DIRECTION because the Time* ( in the problem ) is longer than the expected time. So it will take longer to land on the ground.
Choose upward direction as positive direction
F(net) = m*a ( Newton Second Law) (1)
Because it is falling down,the net acceleration is downward ( negative sign)
Using the distance equation : h = 0.5*a*(t^2) ( we know that intitial velocity is 0, so I dont have to show in the equation)
h= 12m, t = 1.75s--> t^2=3.0625--> plug in the equation above---> we have the net acceleration: a= -7.84 m/s^2 ( negative sign shown downward direction)
With mass= 5.5kg, plug in mass and a=-7.84 to the (1) equation above
F(net) = 5.5*(-7.84) = -43.1 N