Question

A 5 kg object near Earth's surface is released from rest such that it falls a distance of 10 m. After the object falls 10 m, it has a speed of 12 m/s. Which of the following correctly identifies whether the object-Earth system is open or closed and describes the net external force?
A) The system is closed, and the net external force is zero.
B) The system is open, and the net external force is zero.
C) The system is closed, and the net external force is nonzero.
D)The system is open, and the net external force is nonzero.

Answer 1

To solve the problem it is necessary to apply the concepts related to energy conservation. We have a mass of 5Kg that at a distance of 10m has gained 12m / s, with energy losses due to friction.

$KE + W_f = PE$

$\frac{1}{2}mv^2 +W_f = mgh$

$W_f = \frac{1}{2}mv^2 -mgh$

$W_f = \frac{1}{2} 5*12^2-5*9.8*10$

$W_f = 130J$

Since there is a loss of energy due to friction, it means that an external force resulting from friction is interacting on the system, which also makes the system open.

The correct answer is D.