Can you me understand how to calculate net force

To start off.. add any and all the vectors acting on the same axis (x and y), making sure to pay attention to the director of the vectors. you are often able to ignore many of the forces that cancel each other out. make sure to focus on the forces that are actualy involved in how the object will move. Then you calculate the net force acting on the object in the situation
Since nothing is happening along the y-axis, you can ignore the Fg (300N)and FN (20N) forces.

FNET = Fa + Ff

= 80N + -70N

FNET = 10N

you have to make the friction a negative force
because it is pointing in the direction opposite to the applied force.

When you want to calculate the acceleration of an object, always use the net force acting on it.

you aren’t subtracting the two forces.
Instead, it’s up to you to remember that if one of the forces is pointing in one direction (like to the right), and the other force is pointing in the other direction (like to the left), you will need to make one positive and the other negative.

i don’t really know how to explain this super well. i hope this helps!!

The rocks expand when there hot/warm  and contract when they are cold.