Question

read the scenario and answer question #4 below.
at a busy grocery store, a cashier notices how items move on the checkout conveyor belt and makes the following observations.

• when the belt is off, items placed on it remain stationary.
• when the cashier turns the belt on, items start moving slowly towards the scanner.
• as more items are added, the belt maintains the same speed.
• when a heavy box of detergent is placed on the belt, it briefly slows down but then returns to its normal speed.
• after scanning all items, the cashier turns off the belt, and the items gradually come to a stop.

which of the following best explains the motion of the items throughout this sequence?
a. the items only move when an unbalanced force is acting on them.
b. the items only move when a balanced force is acting on them.
c. unbalanced forces keep the items stationary when the belt is turned off.
d. the belt creates balanced forces, causing the items to speed up continuously.

Explanation:

• Option A: When an unbalanced force acts on an object, its motion changes. In the scenario, when the cashier turns on the belt (applies a force - unbalanced initially as it starts the motion), items move. When more items (extra mass/force consideration in a way as it affects the system's motion) are added (can be seen as an unbalanced force in terms of the belt having to "work" against the added weight to maintain or change speed), the belt's speed might change (like slowing down briefly). When the belt is turned off (removal of the driving force - unbalanced force as the frictional etc. forces act to stop the motion), items gradually stop. This shows how unbalanced forces (starting, adding load, stopping the belt's driving force) affect the motion of items.
• Option B: If items only move when a balanced force acts, but balanced forces result in no change in motion (objects remain at rest or move at constant velocity). But in the scenario, items start moving (change from rest - which requires unbalanced force), slow down (change in speed - unbalanced force), and stop (change from motion to rest - unbalanced force), so balanced force concept does not fit.
• Option C: Unbalanced forces are what cause changes in motion (start, stop, speed change), not balanced forces. Balanced forces keep objects in their state (rest or constant motion), but the items change their state (start moving, slow, stop), so this is incorrect.
• Option D: The belt's operation (turning on/off, adding items) creates unbalanced forces (as explained in Option A). Balanced forces would not cause the observed changes (starting, stopping, speed - changing motion) of the items.

Answer:

A. The items only move when an unbalanced force is acting on them.