A book slides across a table after Lena shoves it, her hand already clear. How many forces act on the sliding book? Guesses usually range from one to five. There is a routine that gets it right every time.
You now know the whole force inventory: weight, the normal force, tension, friction, and drag. Time to use the full list on one object.
The routine has three steps, and it starts with the only force that needs no contact.
Step 1: weight. The Earth pulls the object straight down, wherever it is near Earth. Write it first, every time.
Step 2: go around everything touching the object, one thing at a time. Each touching thing can exert force only where it touches.
A touching surface gives its perpendicular push, the normal force, and, if the object slides or tends to slide, its parallel friction. No slide and no attempted slide means no friction from that surface: a plate resting on a level counter gets the counter's normal force and nothing parallel.
Wheels change nothing about touching. A rolling cart meets the pavement through its wheels, and the pavement still grips them, so the pavement's normal force and friction belong on the cart's list.
A mug rests on a level desk, nothing else touching it. Which inventory lists every force on the mug?
A stretched rope, string, cable, or chain attached to the object gives a tension force along itself. A touching hand, foot, or bumper gives its own push or pull: name it by its exerter, like 'the push from Lena's hand'.
A pendant hangs still from a necklace chain around a display stand. Which inventory lists every force on the pendant?
Step 3: if the object moves fast through air or water, add the fluid's drag, opposite the motion. For slow movers this course adds drag only when the question says to.
Now the sliding book: nothing touches it but the table, and it is slow. Weight from the Earth, downward. Normal force from the table, upward. Friction from the table, against the slide. Three forces, and Lena's shove is not one of them: her hand no longer touches the book, so its push is over.
A sled is dragged across the snow by a rope. Which object exerts the friction force on the sled?
A camping lantern hangs from a tent pole by one cord, in still air. Inventory the forces on the lantern.
Step 1: weight, from the Earth, downward.
Step 2: the only touching thing is the stretched cord, which gives a tension force up along itself. No surface touches the lantern, so there is no normal force and no friction.
Step 3: the lantern hangs still, so there is no drag.
Two forces: the Earth's downward pull (weight), and the cord's upward pull along itself (tension).
A skydiver falls fast, straight down, before opening the parachute. Inventory the forces on him.
Step 1: weight, from the Earth, downward.
Step 2: nothing touches him: no surface, no rope, no hand.
Step 3: he moves fast through air, so the air's drag acts, upward, opposite his motion.
Two forces: his weight, downward, and the air's drag, upward.
Tobias pushes a shopping cart across a level parking lot, hands on the handle, wheels rolling slowly. Which inventory is complete and correct for the cart?
A cliff diver falls fast, straight down, through the air toward the water. Which inventory is correct while she is still in the air?
A box slides across a gym floor after a shove, the shover's hands already clear of it. A student's inventory reads: weight, normal force, friction, and 'the shove'. Which entry is wrong, and why?