I was working on a lab in class drawing Free Body Diagrams. The problem required we drew an FBD of a ball that is in the motion of being thrown. I drew a slightly diagonal line labelled Applied Force, a vertical line straight down labelled Gravity, and a line opposite of Gravity labelled Normal Force. I was told that there is no normal force at all in this situation, and that the only two forces are gravity and applied force. I was confused because I thought that the ball was being held up by the hand (acting as a surface) which provided a normal force while still being pushed by the hand in the positive direction resulting in an applied force. So in this situation, would the ball have any normal force at all acting on it?


Let's review what a normal force is.

Solid objects are characterized by an inability to occupy the same volume of space as other matter. The microscopic reasons for this behavior are very complicated, but the macroscopic result are simple to describe.

A normal force is the force that enforces the "no occupying the same volume" rule for solids and as such it always acts perpendicularly to the surface of contact and points pushes the two objects away from each other (this is a Newton's 3rd Law pair).

Somethings that are not characteristics of normal forces

  • They do not always point upward.
  • The do not always balance the weight of the object. Or even the vertical component of the weight. You get "balancing" for objects that are in equilibrium.

Now, on to your situation.

  1. There is a contact force in the problem. It exists between the hand (or throwing mechanism whatever it is) and the ball. When you drew it on your FBD you labeled that (quite reasonably, BTW) as "applied force".
  2. The ball is not in contact with a horizontal surface so there is no upward pointing contact force. The "normal force" that you drew was erroneous.
  • $\begingroup$ If I were to throw the ball underhand meaning that the palm of my hand is under the ball, would that qualify as a horizontal surface allowing me to draw the upward pointing contact force? $\endgroup$ – dsowders Nov 10 '16 at 21:59
  • $\begingroup$ The "applied force" is the effect of the hand on the ball. It may include an upward component (it usually does), but there is no reason to break this into more than one force in your FBD. $\endgroup$ – dmckee Nov 10 '16 at 22:39
  • $\begingroup$ So whether you classify the palm as a horizontal surface or not, if I did break the applied force into more than one force, which you said was pointless, would it still be erroneous, or rather correct and just overly-complicated? $\endgroup$ – dsowders Nov 11 '16 at 16:49
  • $\begingroup$ @dsowders Free body diagrams are tools intended to help you apply Newton's Laws correctly and to communicate your understanding to others. Toward those ends the manner of drawing them is closely defined. I would have marked your drawing as incorrect to, because the point isn't to have an argument that "this drawing is defensible", but to learn to draw them in the usual form. There should be one vector associated with the hand because you were told what it was. $\endgroup$ – dmckee Nov 12 '16 at 0:06

Don't forget that normal is a mathematical synonym for perpendicular. There is a normal force in your problem; it is perpendicular to the palm of your hand. That is, the applied force of throwing is the normal force due to your hand.

Calling the force "Applied" doesn't identify the agent of the source, or what kind of force it is (in this case, a compressive contact force). Something similar can be said about centripetal force.

In my opinion, it is better to identify the agent ("rope", "table", "earth", ...) or type of force ("normal", "tension", "compression" ...), perhaps with a notation, something like "the applied force" near the label of the force. Just my opinion for keeping things straight and clear.


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