# PHYSICS UNIT 3: CIRCULAR & ROTATIONAL MOTION

## Presentation on theme: "PHYSICS UNIT 3: CIRCULAR & ROTATIONAL MOTION"— Presentation transcript:

PHYSICS UNIT 3: CIRCULAR & ROTATIONAL MOTION

CIRCULAR MOTION Uniform Circular Motion
period, T: time for one complete revolution, unit: s speed is constant v = 2pr/T (r: radius) velocity is constantly changing (because direction is changing)

CIRCULAR MOTION all turning objects have centripetal (center- seeking) acceleration

CIRCULAR MOTION centripetal acceleration, ac = v2/r
the greater the speed, the greater the centripetal acceleration the smaller the radius of the turn, the greater the centripetal acceleration a centripetal acceleration requires a centripetal force

CIRCULAR MOTION no centripetal force = no turning (linear motion)

CIRCULAR MOTION centripetal force, Fc = mv2/r
any force can be a centripetal force: gravity (planets & moons), friction (car turning a corner), tension (ball on a string), etc.

CIRCULAR MOTION Frames of Reference - inside a turning object, there seems to be a centrifugal (center- fleeing) force pulling on objects

CIRCULAR MOTION Frames of Reference - outside the turning object, we see objects inside move in a straight line (inertia), until they get pulled into the turn by centripetal force

CIRCULAR MOTION centrifugal force only exists within the turning object’s frame of reference - it is a fictitious force

CIRCULAR MOTION Frames of Reference - things moving on a rotating object seem to be made to turn by the “coriolis force”

UNIT 3: CIRCULAR & ROTATIONAL MOTION
PHYSICS UNIT 3: CIRCULAR & ROTATIONAL MOTION

QUIZ 3.1 A 50 kg child on a merry-go-round sits 2.0 m
from the center and makes 2.0 complete revolution every second. (a) Find the child's period. (b) Find the child's tangential speed. (c) Find the child's centripetal acceleration. (d) Find the centripetal force on the child.    v = 2pr/T ac = v2/r Fc = mv2/r

UNIT 3: CIRCULAR & ROTATIONAL MOTION
PHYSICS UNIT 3: CIRCULAR & ROTATIONAL MOTION

UNIVERSAL GRAVITATION
Newton's Law of Universal Gravitation: masses attract gravity force is proportional to each mass twice the mass = twice the force gravity force is inversely proportional to the square of the distance between the masses twice the distance = ¼ the force distance measured from center of mass: point on a body around which mass is balanced

UNIVERSAL GRAVITATION
Newton's Law of Universal Gravitation: Fg = Gm1m2/r2 FG: force of gravity, m: mass, r: distance between masses G: universal gravitational constant, 6.67×10-11 Nm2/kg2 gravity is only significant for very large masses

UNIVERSAL GRAVITATION
acceleration due to gravity, g = Gme/r2 Also known as the gravitational field strength me: earth's mass (5.97×1024 kg) r: distance from earth’s center (6.38×106 m + altitude) g is only 9.80 m/s2 at sea level – it decreases as altitude increases g is different on other planets & moons (it depends on the planet’s mass and radius)

UNIVERSAL GRAVITATION
Orbits: gravity provides the centripetal force stable orbit: Fc = Fg orbit speed v = √Gme/r orbit period T = 2pr/v geosynchronous orbit: T = 24.0 hrs, satellite stays over same position on earth

UNIVERSAL GRAVITATION
Orbits always falling but never reaching the ground "Weightlessness" is NOT gravity-less no gravity = no orbit weightless is no normal force

UNIT 3: CIRCULAR & ROTATIONAL MOTION
PHYSICS UNIT 3: CIRCULAR & ROTATIONAL MOTION

ROTATIONAL MOTION Rotational Motion: rotation around an internal axis
angle, q: how much an object has rotated, unit: radian (rad) 2p rad = 360º = 1 revolution (rev) angular velocity, w = Dq/t : rate of rotation, unit: rad/s frequency: revolutions per second, unit: Hertz, Hz 1 Hz = 1 rev/s = 2p rad/s

ROTATIONAL MOTION Rotational Motion & Circular Motion
angular acceleration, a= Dw/t: rate of change in rotation, unit: rad/s2 Rotational Motion & Circular Motion for any point on a spinning object: v = rw ac = rw2

ROTATIONAL MOTION torque, t: rotating effect of a force, unit: Nm
t = Fdsinq d: "torque arm" or "lever arm“ q: angle between F and d torque direction: clockwise (c) or counterclockwise (cc)

ROTATIONAL MOTION Torque is zero when q = 0º or 180º
Torque is maximum when q = 90º

ROTATIONAL MOTION Newton's Laws for Rotary Motion
A spinning object keeps spinning with constant angular velocity unless a net torque acts on it A net torque causes an angular acceleration For every action torque, there is an equal and opposite reaction torque

ROTATIONAL MOTION Rotational Equilibrium: object is balanced, or moves with constant angular velocity, due to no net torque Stc = Stcc F1 F2 d1 d2 F2d2 = F1d1

UNIT 3: CIRCULAR & ROTATIONAL MOTION
PHYSICS UNIT 3: CIRCULAR & ROTATIONAL MOTION

UNIT 3 REVIEW v = 2pr/T ac = v2/r Fc = mv2/r
Fg = Gm1m2/r2 G = 6.67×10-11 Nm2/kg2 g = Gmp/r2 v = √Gmp/r me = 5.97×1024 kg re = 6.38×106 m w = Dq/t a= Dw/t v = rw ac = rw2 t = Fdsinq Stc = Stcc