Question

consider the four free body diagrams. free - body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation. do any of the free body diagrams indicate motion? if yes, which one(s). explain.
a no. in each case, the forces are balanced.
b yes. in b the book is slowing down because of the friction force opposing motion.
c you cannot tell if there is any motion. you can only determine how much force is acting on the objects.
d yes. in b and d. in both cases, there is a net force.
in b, there is a net force to the left; in d there is a net force upward.

Explanation:

Step1: Analyze Force Balance

For an object, if net force \( F_{net}=0 \) (balanced forces), it moves at constant velocity or is at rest. If \( F_{net}
eq0 \) (unbalanced forces), it accelerates (changes motion).

• Left diagram: \( F_{norm}=20\,\text{N} \) (up), \( F_{grav}=20\,\text{N} \) (down). \( F_{net}=20 - 20 = 0\,\text{N} \) (balanced).
• Right diagram: \( F_{air}=40\,\text{N} \) (up), \( F_{grav}=25\,\text{N} \) (down). \( F_{net}=40 - 25 = 15\,\text{N} \) (up, unbalanced).

But the question’s options: Option C says force diagrams show forces, not motion (since balanced forces can mean constant velocity or rest; unbalanced means acceleration, but we can’t tell if moving or starting to move from force alone—only that unbalanced forces cause change in motion).

Step2: Evaluate Options

• A: Wrong, right diagram has unbalanced force.
• B: No “book” or friction in given diagrams (diagrams show \( F_{norm}/F_{air} \) and \( F_{grav} \)).
• C: Correct—force diagrams show force magnitude/direction, not current motion (e.g., balanced forces could be rest or constant speed; unbalanced means acceleration, but we can’t tell if moving from force alone).
• D: Diagrams (only two shown) – left is balanced, right is unbalanced (net up), but option D’s description (left/right? Not clear, but key: Force diagrams indicate forces, not motion state (moving vs. not moving) directly.

Answer:

C. You cannot tell if there is any motion. You can only determine how much force is acting on the objects.