What's the net force?

Question

A person pushes a block of mass M=6.0 kg with a constant speed of 5.0 m/s straight up a flat surface inclined 30 degrees above the horizontal. The coefficient of kinetic friction between the block and the surface is μ=0.40. What's the net force acting on the block?

 

Answer: 0 N

Answer 1

Apply Newton's second law.

Since a = 0, by Newton's second law,

Fnet = ma = 0 N

Answer 2

Check the question again. I think the following is the intent of the question:

 

Force due to wt of block acting downward along the inclined surface = 6 x 10 sin(30) = 30N

Force due to friction acting downward along the inclined surface = 0.4 x 6 x 10 cos(30) = 20.78N

Net force acting downwards ~ 30 + 20 = 50N

If external force applied = 50N then block will be stationary

In order for the block to move exyternal force must be more than 50N. Once the block begins to move a constant 50N of force will be required to keep the blcok moving with a constant speed.

 

:)