Net Force on Top Block: 115.2 N

[wolves5]

Three blocks are bring accelerated upwards at 3.2m/s2, by a force applied to the bottom block as shown in the figure. The mass of the bottom block is 12kg, the mass of the middle block is 24 kg, and the top block is 36 kg. What is the net force on the top block?

a.44.8 N
b.67.2 N
c.115.2 N
d.240.0 N

I don't know how to start this problem. Do I just do F=(36)(3.2) and get C as my answer?

 

[James98765]

You're correct. The best way to handle this type of problem is to draw a picture (free body diagram) of the top block.

The top block has two forces acting on it.
Force 1: The middle block is forcing the top block upward.
Force 2: Gravity is pulling the top block downward.

The summation of forces in the vertical direction gives:
F_{Second Block} - F_Gravity = F_Net

In many problems, you would be given the force of the Second Block and calculate gravitational force to find net force as shown in the equation above. Because we've been given the mass of the top block along with it's acceleration we can calculate the net force directly and forget the above equation:
F_Net = m a = (36) (3.2 m/s²) = 115.2 N