In order to be successfully in applying Newton's second law to a mechanical system, we must first be able to recognize all the forces acting on the system. This java applet shows the free-body force diagram for a block sits on an inclined plane.

Usage:

1. a block sits on an inclined plane:
• Click the circle near the right edge and drag the mouse up/down to change the angle of inclination theta。
2. Red Arrow represents the gravitational force. (which has two green force components).
• Click near the tip of the red arrow and drag the mouse up/down to change the mass of the block. ( mg)
3. Yellow arrow is the normal force Fn 。
• Under which conditions the normal force becomes zero (vanish)?
4. Click the up-right corner of the block to apply external force F on that block.
• (Drag the mouse to change the vector F, if the mouse is within the circle the force F=0.
5. Click again at the tip of the F vector and drag the mouse button to change the direction of the force F only.
• When will we have a maximum net force?
6. The normal force were determined from external force F and the gravitational force mg.
• Then we will know the friction between the block and the incline plane.
7. The blue arrow is the friction force. Notice: there is a maximum value.
8. All the force are added together to find the net force NF. ( the black arrow)
9. You can change different value of coefficient of static friction from the selection.