Force Explained by Dummies

Newton's Third Law

It's been a while (more than three months) since the last time we featured the queen of DotDs to see what she's done to advance the overall stupidification of the internet. Fans of the site will probably recognize the name of Joan Whetzel, whose scientific illiteracy apparently knows no bounds. In her thirty-one (31!) previous appearances, she's managed to spread misinformation about chemistry, physics, geography, mathematics, electricity, and earthquakes; not to mention several forays into home maintenance. Well, Joan's back, this time with a HubPages.com post she called "Calculating Force and Its Effect On Objects at Rest."

This time out, Whetzel was attempting to explain Newton's Laws of motion (all three of them) by explaining how a pickup truck behaves. Right out of the box, we know that Joanie's grasp of statistics is weak, when she tells us that of an "average" pickup,

"...that a it [sic] has a mass of approximately 4,000 to 13,000 pounds (1814 to 5896 kg), giving pickup trucks in general an average mass of 3,900 kg."

Statistically speaking, 3900 kg is the midpoint of the range, not the average. Your ordinary F150 weighs in the neighborhood of 2000 kg; it's not until you get to the (much rarer) F350 that you find curb weights over 3000 kg. That's either bad research of a poor understanding of the meaning of "average" (we suspect the latter). But heck, that's only a quibble. It's when Whetzel starts "explaining" the physics involved that things get really amusing.

Take, for instance, Joan's explanation of Newton's first law applied to a truck:

"...the pickup truck... will remain not moving until the engine is turned, [sic] the engine is put into gear, [sic] and the gas is applied. Once in motion, the pickup truck will remain in that state of motion until the breaks [sic] are applied, until it is no longer being fed gas allowing it to slow down, until the gear box is put into neutral or park, until the engine is turned off, or until the truck crashes."

Joan's vague understanding of auto mechanics notwithstanding, she seems completely unaware of the forces of gravity and friction; not to mention that Newton's first law actually says that once an external force starts the truck moving, only another external force can change its motion – not that it will stop when it runs out of gas!

We'll give Joanie credit for not botching her discussion of Newton's second law, but we can't give that same credit to her rather bizarre explanation of Newton's third law, which she illustrates by saying that,

"...if we are sitting in this pickup truck at rest, and apply force to the gas pedal, thereby causing it to accelerate and move in a forward direction, we will feel the equal and opposite reaction as our bodies are forced backward into the seat... If we bring that pickup to a sudden stop (a state of rest), we feel the opposite and equal reaction as our bodies are forced forward toward the steering wheel and windshield."

Joan's made a common mistake here: she thinks she's explaining Newton's third law, but in reality she is explaining inertia. The fact that a "force to the gas pedal" is not what actually makes the truck move notwithstanding, that's in no way an explanation of the third law, which (more or less) says that for every action, there is an equal and opposite reaction. That's not what Joan describes. 

In fact, using Whetzel's preferred object, a pickup truck, it's fairly hard to give an example of Newton's third law. The best example might be one truck rolling into the bumper of a second, motionless truck. In a perfectly Newtonian world, the impact will start the second truck moving – accelerate it – while reducing the forward motion of the first. If the first truck is much less massive than the second, it may even "bounce" and begin to move backwards. Either way, that change in forward motion is the "equal and opposite reaction" Newton predicts; not the inertial response Whetzel describes.

Scientific illiteracy like this is the reason why Joan is receiving her thirty-second Dumbass of the Day award.

SI - PHYSICS