Free-body diagrams – inclines, lifts and pulleys
PhysicsForces & DynamicsAges 15–16Virtual lab
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Six situations: a block on a rough incline (at rest or sliding down), a block sliding up a slope, a mass pulled up on a rope, a floating block, a person on a bathroom scale in a lift and two blocks joined over a pulley. Drag force arrows (weight, normal force, friction, tension, upthrust) onto each body, turn them to the right direction and press Check; when the diagram is right, the sim works out every force and the acceleration from Newton's second law. In the lift the scale reads N = m(g + a), zero in free fall; in the lift's frame an inertial force −ma is added.
Lesson: Free-body diagrams and Newton's second law
What it shows
A free-body diagram shows one body on its own with every force that acts on it, drawn from its centre as in the particle model. Each force comes from something touching or pulling the body: the Earth (weight), a surface (normal force and friction), a rope (tension) or a fluid (upthrust). Adding the forces gives the resultant, and Newton's second law ΣF = ma gives the acceleration. In an accelerating lift the scale reads N = m(g + a); in the lift's own frame an inertial force −ma balances the forces. The model uses g = 9.81 m/s², one coefficient μ for static and kinetic friction and a light rope and pulley.
How to use
Choose a Situation. Drag a force from the tray onto the body and drop it where the force points; drag the round tip to turn it, or back to the tray to remove it. Press Check and follow the hints. Run moves the bodies. In the lift, set Lift acceleration a, press Record scale reading and compare the Reference frame options. Show the answer draws the forces to scale.
Parameters you can change
- Situation Block on a rough incline, Block sliding up an incline, Mass pulled up on a rope, Floating block, Person on a scale in a lift, Two blocks over a pulley
- Mass m (block A on the pulley) 0.5–10 kg
- Mass of block B 0.1–10 kg
- Slope angle θ 5–60 °
- Coefficient of friction μ 0–1
- Rope tension T 0–60 N
- Density of the block ρ 100–950 kg/m³
- Person's mass m 20–120 kg
- Lift acceleration a (up +) -9.81–5 m/s²
- Reference frame Ground, Lift
- Hide the person's mass
- Show the answer (forces to scale)
Questions to explore
- Why does friction point down the slope when a block slides up it?
- What does a bathroom scale read in a lift whose cable has snapped, and why?
- Which force must you add to the diagram when you describe the person from inside an accelerating lift?
