Final answer:
In the scientific sense, work is not being done when a student carries a lunch tray towards a table, assuming it is held at a constant height, because while the force is applied upwards to counteract gravity, there is no displacement in the direction of the force.
Therefore, the correct answer is: option 'A student carries a lunch tray towards a table.'
Step-by-step explanation:
Work in the scientific context occurs when a force causes displacement in the direction of the force. With this definition in mind, let's consider the scenarios presented:
- A flight attendant lifts luggage to place in an overhead bin - This is work since the luggage is moving in the direction of the applied force (upwards).
- A kid pulls a wagon towards his garage - Work is done here as the wagon is displaced in the direction of the pull.
- A student carries a lunch tray towards a table - No work is done in this case if the tray is carried at a constant height since the force applied (upwards to counteract gravity) does not cause horizontal displacement in the direction of the force.
- A person pushes a grocery cart to the checkout line - Assuming the cart is being pushed horizontally and moves in that direction, then work is done.
Recall that carrying or holding an object at a constant velocity and height involves applying forces, but because there is no displacement in the direction of the force (gravity being counteracted by an upward force), no work is done on the object.
This scenario is illustrated with the person holding a briefcase and doing no work, since there's no displacement even though energy is expended within the muscles.
The key is that there must be displacement and a component of force in the same direction as the movement for work to occur scientifically.