Hi Jule!
Let's first define our axis of movement. In the picture, it looks like x and y are angled such that the x axis is parallel with the slope. We'll call this local x and local y (x and y).
We do need another set of axes, the global x and y (let's denote it as x* and y*), which we will state as y being vertically down to the bottom of your paper/screen, and x being the perpendicular axis to the vertical y.
So here are some rules that we know:
- Gravity always pulls down
- Normal force is opposite gravity and always perpendicular to the surface a body is resting on
- Friction force is always parallel and opposite the direction of motion
Knowing this, we can draw:
- Gravity is straight down, using the global y axis (y*)
- Normal force is upwards, but since the body is on a slope, the vector points up in the local y axis (y)
- Friction force is resisting the upward motion, therefore it is parallel to the slope but downwards in the local x axis (x). Depending on if the box is moving or not, you would draw Fk (kinetic friction is for bodies in motion) or Fs (static friction is for bodies not in motion).
- We know that the applied force will always be in the direction that the force is applied in. If that is applied at an angle, that angle will be conserved.
A cool way to check your answer is to draw the vectors and put them tip to tail. If your body isn't moving, you should end up with a closed loop of vectors. In this case, draw the gravity vector, the resultant normal force vector (due to gravity), the applied force vector, the resultant normal force vector (due to applied force that isn't parallel to the slope), and the friction vector.
