Thanks for your question.
The frames attached to the B and F ports of the Prismatic Joint block
- Must have aligned z-axes
- Must be located along that shared z axis
In your screenshot, the B ports are connected to the same frame, but the F ports are connected to two separate frames. I bet the R and x frames on Cart are located in two different places, and they may have different orientations.
Here are options to fix your model
- You could use a Rigid Transform to ensure the frames for both Prismatic Joints are positioned and oriented correctly.
- You could replace Prismatic Joint1 with a 6-DOF Joint and apply the force along its z axis. You only need one Prismatic Joint to constraint Cart to move along a certain axis, the other joint would just be for applying the force.
- If all you want to do is apply a force, you remove Prismatic Joint1 and just use an External Force and Torque block.