Physics Quiz

Free Body Diagrams – The following note in italics was told to 2013 students

It’s about definitions, not math

Quiz: (A picture was given of a mass on a trianglewith triangle on a platform.)

Vacuum environment. Definitely friction between the rectangle and the triangle. At the moment, no friction between the triangle and the anchored platform. The triangle could conceivably slip on the anchored platform.

  1. Are any of the forces shown, 1, 2, 3, or 4, unreal or improperly done?

No (and you can see them numbered below in the FBD below)

  1. Give meaningful symbolic labels to the forces 1, 2, 3, and 4.

Whatever labels student decides, label for forces 2 and 4 must be identical. Meanwhile the labels for forces 1 and 3 must NOT be identical. Typical label for forces 2 and 4 would be f for friction. Typical labels for forces 3 and 1 would be n and n2 respectively.

  1. Does either of the two diagrams have a missing force? If so, put it in as a vector and give it a symbolic label.

For full credit, the added vector must:

  • Act on M
  • Be in the proper direction (see it in the diagram below)
  • Be labeled with whatever symbol was used to name force 3 in student’s #2 response. For what I wrote, my missing force would be labeled as n.

The responses of questions 1 through#3 can be communicated by the following perfect FBD’s:

n2 (#1)

n (#3) f (#2)

f (#4) n(student had to add this.)

mg

Mg

  1. You need no trigonometry to know about writing the following:

Fx = Max

( -fx ) + ( +nx ) = M(0) → fx = nx for 1 of 2 possible points.

Later, when you get to the trig, you’ll end up writing“fcos(θ) = nsin(θ)”

  1. 2nd Law. Saying 3rd Law here signals careless reading. The 3rd Law is entirely irrelevant in Question 4’s mathematics.
  1. Consider the force labeled that is force 3. Clearly identify the force that is equal to it, because they form an action/reaction pair.

“Clearly identify” means that the only acceptable answer for credit is to briefly communicate that this answer is the same as whatever thing you added in question 3.

  1. Name the Newton Law that predicts the equality that you stated in #6.

3rd Law. Applying anything but 3rd Law here signals careless reading.

Making FBD’s perfect is impossible without a complete knowledge of Newton’s Third Law. Newton’s Third Law is applied in the making of the FBD, and therefore, Newton’s Third Law lies at the very heart of understanding what forces actually are. Once the FBD’s are perfect, any math that is done with them never applies Newton’s Third Law. Newton’s Third Law doesn’t lead to any useful algebra. It’s more important than that.

Newton’s 2nd Law always leads to powerful algebra. But one can’t get that far if the FBD had never been perfect. And for the FBD to have been perfect, the 3rd Law had to have been in one’s thinking at the very outset of making the FBD.

That’s how these laws are prioritized. I don’t think standard textbooks do a great job making these priorities clear.