12.2.A.1–1.iiMultiple Choice
A book rests on a table. Which statement correctly identifies the interaction producing the normal force on the book?
Select A, B, C, or D to check your answer.
Directions: Work each problem on your own paper first. For multiple choice, select A, B, C, or D. You will see right away whether you are correct. If you miss, you will see the most likely reason, and you can try again. For short response, type your answer, then compare it with the model solution. Questions mix the essential knowledge threads of Topic 2.2 — forces as interactions between two specific objects (and why nothing can exert a net force on itself), contact forces as the macroscopic effect of interatomic electric forces, the free-body diagram's rule of showing only the environment's forces on a chosen system, drawing every force as its own vector from a single center-of-mass dot, and choosing a coordinate system with one axis parallel to the acceleration.
A book rests on a table. Which statement correctly identifies the interaction producing the normal force on the book?
Select A, B, C, or D to check your answer.
Explain why a person cannot lift themselves off the ground by pulling upward on their own hair, in terms of EK 2.2.A.1.ii.
A magnet is held near a steel paperclip, pulling it across a table without touching it. Which two objects are involved in this force interaction?
Select A, B, C, or D to check your answer.
A problem states only that "a 10 N force acts on a box." Explain what key piece of information is missing from this description, given that forces are interactions between objects, and why that missing piece matters.
At the microscopic level, what actually causes the normal force a table exerts on a book resting on it?
Select A, B, C, or D to check your answer.
Explain why friction and the normal force are both classified as "contact forces," while gravity is not, in terms of what's happening at the atomic scale.
Which of the following is NOT an example of a contact force?
Select A, B, C, or D to check your answer.
A student says, "Friction can't really be a separate force — it's actually just a bunch of tiny electric forces." Evaluate this statement using the distinction between a contact force's microscopic cause and its macroscopic treatment in this course.
A book sits at rest on a table. Which set of forces belongs on the book's free-body diagram?
Select A, B, C, or D to check your answer.
A hand pushes a block across a rough floor at constant speed. List every force that belongs on the block's free-body diagram, and explain why the force the block exerts back on the floor (via friction) is NOT included.
Two blocks, A and B, are in contact and are both being pushed together across the floor by a hand pushing on block A. If the chosen system is block A and block B together, how should the contact force between A and B be treated on this system's free-body diagram?
Select A, B, C, or D to check your answer.
Explain how the answer to the previous kind of question would change if the chosen system were block A alone (not both blocks together), and why.
On a correctly drawn free-body diagram, where should every force vector originate?
Select A, B, C, or D to check your answer.
A free-body diagram shows a box with gravity (straight down) and an applied force (also pointing mostly downward, at a slight angle) drawn overlapping as one combined arrow. Explain what's wrong with this diagram.
A ball is in free fall with only one force acting on it. Which of the following correctly describes how this force should appear on the ball's free-body diagram, according to this course's boundary statement?
Select A, B, C, or D to check your answer.
A system is modeled as a single dot for the purposes of a free-body diagram, even though it might be a large, oddly-shaped object. Explain why this simplification is justified, connecting it to an idea from Topic 2.1.
A block slides down a frictionless incline. Which choice of coordinate axes is most useful for writing this situation's equations algebraically?
Select A, B, C, or D to check your answer.
Explain why standard horizontal and vertical axes work perfectly well for a block being pushed across a flat, horizontal floor, but become awkward for a block on an incline.
A system consists of a hanging mass connected over a pulley to a block on a horizontal table. Which statement about axis choice is correct?
Select A, B, C, or D to check your answer.
Summarize, in your own words, why the choice of coordinate axes has no effect on the physical forces or motion involved in a problem, but can have a large effect on how easy the resulting algebra is.