Friction, buoyancy, tension and the rest
The everyday forces you name on a free-body diagram, what resists motion, and a skydiver with two terminal velocities.
A stationary drop of water on a level table feels balanced forces. Tilt the table and the forces become unbalanced — it slides. What would the SAME experiment look like with a drop of honey, or glue, or oil?
The buoyant force on an object equals…
An object floats when…
A 0.20 m radius sphere of density 500 kg m⁻³ hangs fully underwater from a cable at 60° to the horizontal, tension 120 N. What is the upthrust on it?
A string can…
At the angle where a block just starts to slide, μ equals…
Static friction compared with dynamic (kinetic) friction on the same surfaces is usually…
A ball falling through a fluid reaches terminal velocity when…
The instant a skydiver’s parachute opens, drag is…
Why is the terminal velocity under an open parachute so much lower than in freefall?
Sketch (in words) what would happen to a drop of honey, a drop of glue, and a drop of oil on the SAME tilted-table experiment. Which would take longest to start visibly moving, and why?
Everyday contact forces (friction, tension) are all really…
F = kx describes a force that changes an object’s…
Key points
- • Named forces: weight, normal force, tension, buoyant force, spring force, friction, drag.
- • F_B = ρ_fluid V g = weight of fluid displaced. F_B > W floats, F_B < W sinks.
- • A string only pulls. Tension is found from F_net = ma, not a standalone formula.
- • f = μN, with μ_static > μ_dynamic; at the point of sliding on an incline, μ = tan θ.
- • Friction, drag, viscosity and electrical resistance are one family: things that oppose motion.
- • A falling object reaches terminal velocity when the resistive force balances its weight.
- • A skydiver has TWO terminal velocities: fast in freefall, much slower under an open parachute, because drag depends on k as well as v.
Formulas
- f = μN
- μ = tan θ (at the point of sliding)
- F_B = ρ_fluid V g
- F_B = W + T sin θ (tethered underwater, cable at θ to the horizontal)
- F_drag = kv² (fast motion through air)
Key terms
- Tension :
- the pulling force transmitted along a string, rope or cable
- Buoyant force :
- the upward force on an object in a fluid, equal to the weight of fluid displaced
- Terminal velocity :
- the constant speed reached when the resistive force balances the weight
Common errors
- • Giving tension a fixed formula instead of finding it from F_net = ma.
- • Forgetting μ_static > μ_dynamic.
- • Thinking a denser/more viscous fluid gives a HIGHER terminal velocity — it is the opposite.
- • Using cos θ instead of sin θ for a cable’s vertical pull, when θ is measured from the horizontal.
- • Thinking a parachute reduces speed by reducing weight, rather than by increasing drag.