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## Forces and Torques

### Force and Torque Blocks

You can apply different forces and torques to a model. The table summarizes the different forces and torques that you can represent using SimMechanics™ blocks. For detailed information about these blocks, see the block reference pages.

BlockDescription
External Force and TorqueGeneral force and torque arising outside a model
Internal ForceGeneral force acting between two rigid bodies in a model
Spring and Damper ForceInternal force, acting between two rigid bodies in a model, that accounts for energy storage and dissipation in your model
Inverse Square Law ForceInternal force, acting between two rigid bodies in a model, with a 1/R^2 dependence. Examples include gravity and Coulomb forces.

### Actuating Rigid Bodies

You can actuate a rigid body directly using blocks from the Forces and Torques library. Use the External Force and Torque block to represent an actuation input that arises outside your model. Use the remaining blocks to represent forces that are internal to your model.

The figure illustrates external and internal forces acting on a mechanical system. An external force provides the actuation input to the system. This can be a constant or a general time-dependent input. A spring and damper force acting between the two bodies in the system accounts for energy storage and dissipation. You represent the actuation input using the External Force and Torque block. You represent the internal spring and damper force using the Spring and Damper Force block.

The Forces and Torques blocks contain frame ports. These ports identify the rigid body frames the forces/torques act on. If the block represents an internal force, the block contains two frame ports. Connect these ports to the two rigid bodies the force/torque acts on. If the block represents an external force or torque, the block contains one frame port. Connect this port to the rigid body frame the external force or torque acts on.

The frame origin identifies the point of application for a force or torque. The frame axes identify the directions of the X, Y, and Z force/torque vector components that you specify. Changing the frame position changes also the force/torque application point. Likewise, changing the frame orientation changes also the force/torque direction.

The figure shows three external forces that you can apply to the rocker link of a four-bar mechanism—F1, F2, and F3. Forces F1 and F3 act at the ends of the link, while force F2 acts at its mass center.

To represent one of these forces in a SimMechanics model, you first define the frame to apply that force to. Example Represent Binary Link Frame Tree shows you how to do this. Then, in the block diagram for your model, connect the frame port of an External Force and Torque block to the frame entity that represents that frame—frame port, line, or node. For more information, see Representing Frames.

Finally, in the block dialog box, select the force component(s) that you want to specify. For example, to specify a force acting along the -Y axis of the frame it connects to, select Force > Force (Y). Then, use the physical signal port that the block exposes to input the value of that force component. That value is negative for a force acting along the -Y axis.

The figure shows the modified block diagram of a four-bar model that is present in your SimMechanics installation. You can open the original model by typing sm_four_bar at the MATLAB® command line.

The rectangular frame in the image highlights the blocks that you can use to apply an external force. The frame port that the External Force and Torque block connects to represents the binary link mass center. The block diagram of the binary link subsystem provides this frame. The figure shows the block diagram.

In the External Force and Torque block, physical signal port fy identifies the force component that the block represents—in this case, a force in the Y direction of the frame that the block connects to.

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