Torque, equilibrium, energy, power, circular motion, SHM, Hooke's Law · 35 terms
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A rigid body free to rotate about a fixed point called a fulcrum.
A see-saw is a lever — the pivot is the fulcrum. Longer arm = less force needed.
The product of the force and the perpendicular distance from the axis of rotation to the line of action of the force.
Opening a door: push near the hinge (small distance) = hard; push at the far edge = easy.
Moment = F × d. Must use perpendicular distance. Also called torque.
A pair of equal, parallel forces acting in opposite directions, separated by a distance.
Turning a steering wheel — two hands apply equal forces in opposite directions.
Torque of a couple = F × d (d = distance between the forces)
1. The sum of clockwise moments equals the sum of anticlockwise moments. 2. The sum of upward forces equals the sum of downward forces.
A balanced seesaw: heavier person closer to the middle so the moments match.
Both conditions must be stated for full marks.
The point through which the weight of an object appears to act.
A uniform ruler's centre of gravity is in the middle. Irregular shapes: find it by balancing.
Energy transferred to a body by a force moving it through a displacement in the direction of the force.
Pushing a trolley across a room transfers energy to it — that's work.
W = F × d × cosθ. Work is only done if there is displacement in the direction of the force.
The ability to do work.
A battery stores energy. A moving car has energy. Energy is always conserved — just changes form.
Energy stored due to position or state (gravitational) or conformation (elastic).
A book on a high shelf has more gravitational PE than on the floor.
Gravitational PE = mgh
Energy possessed by a body due to its motion.
A faster-moving car has more kinetic energy — it's harder to stop.
KE = ½mv²
The rate at which work is done, or the rate of energy conversion.
A powerful engine does the same job in less time. Two people climbing stairs: same work, but faster person has more power.
P = W/t = Fv
The ratio of useful energy output to total energy input, expressed as a percentage.
A light bulb that converts 20 J of 100 J input to light is 20% efficient — 80 J wasted as heat.
Efficiency = (useful output / total input) × 100%
Energy cannot be created or destroyed — it can only be converted from one form to another.
A rollercoaster: PE at the top converts to KE at the bottom; total energy stays the same (ignoring friction).
1 joule is the work done when a force of 1 newton moves an object 1 metre in the direction of the force.
1 watt is doing 1 joule of work per second.
Energy from a source that is naturally replenished and will not run out, e.g. wind, solar, tidal.
Energy from a source that will eventually run out, e.g. oil, coal, natural gas.
Energy output as a percentage of energy input for a machine.
The unit of angle defined as the angle subtended at the centre of a circle by an arc equal in length to the radius.
If you walk along an arc the same length as the radius, you've turned through 1 radian ≈ 57°.
2π radians = 360°
The rate of change of angle with respect to time.
A record spinning at 33 rpm has a certain angular velocity — every point on the disc turns through the same angle per second.
ω = θ/t = 2πf = 2π/T
The distance travelled along the arc per unit time.
v = rω
Linear speed directed tangentially to the circular path.
At the edge of a roundabout you move faster than someone near the centre — same angular velocity, bigger radius = bigger tangential speed.
The time taken to complete one full revolution.
T = 1/f = 2π/ω
The force directed towards the centre of the circular path that is necessary to keep a body moving in a circle.
Swinging a ball on a string in a circle: the tension in the string is the centripetal force pulling the ball inward.
F = mv²/r = mω²r. This is not a separate force — it is provided by tension, gravity, friction etc.
The acceleration directed towards the centre of the circle experienced by a body moving in a circular path.
a = v²/r = ω²r
An orbit above the equator where the satellite's orbital period equals Earth's rotation period (24 h), so it appears stationary from Earth.
TV satellites are geostationary — your dish always points in the same direction.
Only one altitude allows this — about 36,000 km above Earth.
The force between two masses is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
F = Gm₁m₂/r²
The square of the period of a satellite's orbit is directly proportional to the cube of its orbital radius.
T² ∝ r³
Motion where the acceleration is always directed towards the equilibrium position and is directly proportional to the displacement from the equilibrium.
A pendulum swings back and forth — the further from centre, the stronger the pull back to the middle.
a ∝ -x (negative because it opposes displacement). Must state both: proportional to displacement AND directed towards equilibrium.
Moving back and forth repeatedly with a steady rhythm about a fixed point.
One complete back-and-forth movement from one extreme, through equilibrium, to the other extreme, and back again.
The number of complete cycles per second.
f = 1/T
The time taken to complete one full cycle of oscillation.
T = 1/f
The maximum displacement of the oscillating body from the equilibrium position.
A guitar string plucked hard has a large amplitude — it vibrates widely and sounds loud.
The progressive reduction in amplitude of oscillations due to energy loss (usually to friction or air resistance).
A pendulum in air gradually swings less and less — energy is lost to air resistance.
Distinguish: light damping (gradual decay), heavy damping (returns slowly without oscillating), critical damping (returns to equilibrium as fast as possible without oscillating).
The restoring force in a spring is directly proportional to the extension, provided the elastic limit is not exceeded. F = -kx.
Stretch a spring — the further you stretch, the harder it pulls back, until it's stretched too far and deforms permanently.
Always add: "provided the elastic limit is not exceeded."