Circular Motion and Gravity Physics 201 Lecture 5

We now focus on forces that bend motion – an example is in projectile motion Constant Force Central Force

Uniform circular motion requires a specific magnitude of net force This represents the period of the motion

Two applications: Banked curves and the mass spectrometer

Rotating frames introduce two inertial forces Centrifugal Force Coriolis Force Movement toward the rotation will push you into the rotation “Stationary” objects will be pushed away from the rotation

Gravity is universal, but usually is applied to one big object The gravitational constant is the least accurately measured physical constant Astronomy cannot measure mass directly, only GM ObjectGM in SI units Sun Earth Moon

Weight is gravitational Weight is a force not due to contact – which is unusual (easy to consider it “intrinsic”) Using Newton’s formula, we can calculate the acceleration due to gravity Identification of gravity and weight is historically significant – first “unification” moment in physics

Kepler’s third law Consider a planet in uniform circular motion – gravity provides the centripetal force: This allows us to relate orbital period to mass: If we know the distance and the period, we can estimate mass – but measuring distance is surprisingly difficult

Applications of Kepler’s third law: Saturn’s rings and evidence for dark matter Rings of Saturn Dark Matter Kepler’s law will tend to tear objects apart Kepler’s law is not valid in galactic core Observed speed of stars in galaxies are too high