Question

How do I know when the Fnet in y direction is 0, any hint i have a test tomorrow?

Answer 1

If the vector sum of your forces in the y-direciton is zero, then `F_{"net"}` in the y direction is zero. Also if the acceleration in the y-direction is zero (`a_y =0`), `F_{"net"}` = 0.

Explanation:

If you watch professional physicists solving a problem, they will follow the steps below. They do this for a reason; it makes solving the problem much easier! You may be tempted to skip steps, don't do it!

1. Draw a picture of your object .

2. Write down what you know from the problem , or can figure out from geometry ( `v_x` = 15 m/s `cos(theta)`, x = 5 m, etc. )

3. Figure out what forces are acting on your object. These can be contact forces (things that push or pull on your object directly) or non-contact forces (long-range forces like gravity).

4. Establish a coordinate system . Which way is +x, -x, +y and -y? It doesn't matter what you pick, as long as you are consistent. Usually we pick y to be vertical, +y to be up, x to be horizontal and +x to be towards the right, but it may be easier to use a different system (especially on ramp problems).

5. Draw a free-body diagram of your object, showing all of the forces.

6. Decompose the force vectors into their x and y components.

7. Write out newton's second law for your forces in the y direction and the x direction. Be careful with your signs!

`sum F_{x} = F_{1_x}+ F_{2_x} + F_{3_x} + ... = m a_x`
`sum F_{y} = F_{1_y}+ F_{2_y} + F_{3_y} + ... = m a_y`

After all of that, if you find `sum F_{y} = F_{1_y}+ F_{2_y} + F_{3_y} =0`, then there is no net force in the y-direction. Also, from the above equations, if there is no acceleration in the y-direction, there can be no net force.