Question

Which diagram correctly demonstrates the various forces acting on a ball moving horizontally with some speed?

Answer 1

Please, find the diagrams in the picture attached.

Answer:

• The correct diagram is shown in the separate picture attached.

Explanation:

When you show a model of the forces acting on an object you need to choose a system and show the forces on such system.

In order to do that, you replace what ever is not part of your system by forces, represented by arrows, that show the interactions of your system with the surroundings.

The horizontal line just below the ball represents the surface over which the ball is moving horizontally with some speed.

Then, by including this line in the diagram you cannot add the reaction force from the surface that it represents.

You only can add the reaction force by deleting the line that represents the surface.

Since all the other diagrams include the line and a vertical upward arrow that represents the normal force from the horizontal surface, those diagrams are wrong.

The only correct diagram is the one that does not include the upward vertical reaction (normal force) from the surface. That is the diagram shown in the second picture (the one with one single diagram).