Question

A cube of mass m1=7.6 kg is sitting on top of a second cube of the same size and mass m2=1.3 kg while both are in free fall. Ignoring any air resistance, what is the magnitude of the normal force with which the bottom cube is acting on the top cube?

Answer 1

Answer:

Zero

Explanation:

To solve the problem, we just need to analyze the forces acting on the top cube.

There are two forces acting on it:

- The force of gravity (the weight of the cube), downward, which is

$W=m_1 g$

where

$m_1 = 7.6 kg$ is the mass of the cube

$g=9.8 m/s^2$ is the acceleration of gravity

- The normal reaction exerted by the bottom cube on the top cube, upward, which we indicate with R

So, the equation of the forces for the top cube is

$m_1 g - R = m_1 a$

where

$a=9.8 m/s^2$ is the acceleration of the cube, since it is in free fall

Solving for R, we find:

$R=m_1 g - m_1 a = m_1 (g-a)=(7.6)(9.8-9.8)=0$