Net Force

by Julia Logan

Definition

A Mathematical Model

In order to calculate net force, all EXTERNAL forces acting on a system are added together. The mathematical definition is

[math]\displaystyle{ Fnet = F1 + F2 + F3... }[/math]

Additionally,

[math]\displaystyle{ Fnet = ma }[/math]
where m=mass of the object, and a = acceleration of the object.

This is a result of Newton's Second Law of motion. If there is a nonzero net force acting on an object, that object is accelerating (not traveling at a constant velocity). Interestingly, there is zero net force acting on an object if its velocity is constant. This seems counter-intuitive (surely something is causing the object to move!) but makes sense in the context of Newton's Second Law. The forces are balanced (sum to zero) if there is no acceleration, despite any movement that may be happening.

A Computational Model

Net force is an essential component of the Momentum Principle! We can use the Momentum Principle in vpython to update the position of a moving object. But first, we have to find net force.

Tracing the path of a ball/spring model in vpython using net force and the momentum principle.

  #1 Fspring = -k*s
  #2 Fgravmag = mball * g
  #3 Fgrav = Fgravmag * vector(0,-1,0)
  #4 Fnet = Fspring+Fgrav
  #5 pball = pball + Fnet * deltat
  #6 vball = pball / mball
  #7 ball.pos=ball.pos+vball*deltat

Here, the spring force and the gravitational force are found using formulas (lines 1-3). Then, they are added together to get the net force on the object (in this case a ball, line 4). The net force is then used in the update form of the momentum principle (line 5). In line 6 the velocity is updated, and line 7 the position is updated. Without net force calculations, tracing an object's path would be impossible.

Simple Example

When calculating net force, it is most useful to construct a free body diagram. A free body diagram is a physical representation of the external forces applied to a system. Often, arrows are used to represent forces. In this example, forces are displayed acting on a box.

The first two boxes' net force can be easily calculated by adding the two forces acting on them, respectively. With down being in the negative y direction, for the first box we have:

[math]\displaystyle{ Fnet = (0, 1200, 0) N + (0, -800, 0) N }[/math]
[math]\displaystyle{ Fnet = (0, 400, 0) N }[/math]

And the second box:

[math]\displaystyle{ Fnet = (0, 600, 0) N + (0, -800, 0) N }[/math]
[math]\displaystyle{ Fnet = (0, -200, 0) N }[/math]

The third box has both x-component forces and y-component forces. It is important to separate these in solving for the net force.
y-components:
[math]\displaystyle{ Fnet,y = (0, 50, 0) N + (0, -50, 0) N }[/math]
[math]\displaystyle{ Fnet,y = (0, 0, 0) N }[/math]
x-components:
[math]\displaystyle{ Fnet,x = (-20, 0, 0) N + (0, 0, 0) N }[/math]
Put them together:
[math]\displaystyle{ Fnet = (-20, 0, 0) N }[/math]

Connectedness

Net force is one of the building blocks of Intro Physics, and I would assume all of physics. It's really important for all motion-related topics, specifically Curving Motion. A net force due to the gravitational pull of the Sun in the perpendicular direction is how the Earth revolves around the Sun -- why we have days and nights and years! We can see from the derivative form of the Momentum Principle that any change in momentum is due to a nonzero net force acting on a system. All changes in motion can be attributed to a net force acting in some direction.

[math]\displaystyle{ dP/dt=Fnet }[/math]

History

Some of the earliest records of humans' musings about force come from Aristotle. Aristotle observed the natural world and made assumptions and equations based on what he saw. He described all motion as being either "natural"--circular and infinite OR finite, up and down, in a straight line--or "violent." Aristotle had issues with projectile motion as he could not reconcile the continuing movement of the object with the lack of force being applied. He ended up concluding that the air provides a simultaneous resistant and accelerating force to the object.

These ideas were challenged by Renaissance men such as John Philoponus, John Buridan, and Oresme, with a clear understanding of the conservation of linear momentum not arriving to the world until Descartes. The most famous face we associate with forces in Physics is Sir Isaac Newton, and he certainly played a huge role in the understanding of net force and momentum. However, he initially believed in the idea of impetus, that a projectile has a certain internal force that keeps it moving, and also the idea of transfer, that objects give up parts of their force during a collision. Though he was wrong, these ideas are not at all silly--in fact they seem quite logical--and he later corrected them after a series of experiments. Thus, the second law of motion was established and with it the relationship between force and motion.

See also

These other wiki pages might help:
Momentum Principle
Conservation of Momentum
Newton's Laws and Linear Momentum

External links

Physics Classroom: Net Force Physics Classroom: Drawing Free Body Diagrams

References

Cardenas, Richard. "What is Net Force? - Definition, Magnitude & Equations." Web. 30 Nov 2015. http://study.com/academy/lesson/what-is-net-force-definition-magnitude-equations.html

"Net Force." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc. Web. 30 Nov 2015. https://en.wikipedia.org/wiki/Net_force

The Physics Classroom. "Determining the Net Force." Web. 30 Nov 2015. http://www.physicsclassroom.com/class/newtlaws/Lesson-2/Determining-the-Net-Force

Chabay, Ruth W., and Bruce A. Sherwood. Matter & Interactions. Modern Mechanics. Hoboken, NJ: Wiley, 2011. Print.

Stinner, Arthur. "The story of force: from Aristotle to Einstein." Physics Education. 1994. Web. 30 Nov 2015. http://www.arthurstinner.com/stinner/pdfs/1994-storyofforce.pdf