Identifying Forces & Free-Body Diagrams
Identifying Forces & Free-Body Diagrams
- Exam questions describe real situations: a diver under water, a box on a rope, a car on a road.
- The first skill is always the same: name every force on the object and draw its direction. Then can do the rest.
The forces you must recognise
- Weight : gravity's pull. Always vertically down, from the centre of gravity.
- : the push from a surface. Always at right angles to the surface. The floor pushes up; a wall pushes sideways.
- Friction : acts along a surface, against the sliding (or against the way the object tends to slide).
- (including air resistance): acts on anything moving through a liquid or gas. It points against the velocity and grows as the speed grows.
- : the pull along a stretched rope, string or wire, pointing away from the object along the rope.
- : an upward push from any fluid. Water pushes up on a boat; air pushes up (very slightly) on a ball. It exists because pressure is larger deeper down.
- Applied push or pull: an engine's driving force, a hand pushing, a motor pulling.
Free-body diagrams
- A free-body diagram shows one object only, with every force on it drawn as a labelled arrow from its centre.
- Forces from the object on other things do not appear. Only forces on the chosen object.
- Choose the one object you care about.
- Check the list one by one: weight; contact with a surface; friction; drag and upthrust if it is in a fluid; tension if there is a rope; any engine force or push.
- Draw each force as an arrow with a name.
- Add the forces (resolve first if any force is at an angle) to get the resultant, then use .
Worked example
Diver under water: three forces
A diver of mass 78 kg is completely under water and moving straight down. At one instant the upthrust on him is 740 N and the viscous drag is 950 N. Find his acceleration. (9702/22/F/M/24 Q2(c)(iii))
- Name the forces. Weight down: . Upthrust up: 740 N. He moves down, so drag acts up: 950 N.
- Resultant, taking up as positive: upwards.
- upwards: he is slowing down fast.
Answer
- The answer direction matters. The diver moves down while the resultant points up, so the resultant slows him: acceleration and velocity can point opposite ways.
Common mistake
Drag opposes the velocity, not gravity. A ball thrown straight up has weight down and air resistance down while it rises. On the way down, air resistance points up.
Your turn— tap to reveal the worked answer (9702-style)
A shuttlecock is hit straight up. Draw the forces on it (a) while it moves up, (b) while it falls back down.
Answer: (a) weight down and air resistance down (both against the upward motion); (b) weight down and air resistance up.
- Falling with air resistance leads to terminal velocity. The full explanation (the three stages and the exam explain-question) is in lesson 2.7, and it is not repeated here.