QCEphysics

Gravitational force and fields

recall Newton's Law of Universal Gravitation

- Newton's Law of Universal Gravitation states that the force of attraction between ach pair of point particle is directly proportional to the product of their masses and inversely proportional to the square of the distance between them

○ Attractive force points inwards, from one point to the other

○ Vector quantity that has magnitude and direction (e.g. __ from A to B)

solve problems involving the magnitude of the gravitational force between two masses

- Remember - the radius between two objects needs to include the radius of the objects (as the measurement is from the centre of one object to the other)

define the term gravitational field

- Gravitational field is the region of space surrounding a body in which another body experiences a force of gravitational attraction

○ Gravitational field strength shows how fast an object accelerates when placed inside a gravitational field]

○ Gravitational field direction is towards the direction of the net gravitational force

solve problems involving the gravitational field strength at a distance from an object.

- Weightlessness occurs when the gravitational field strength between two objects is equal

Syllabus link: Students should be able to consider how gravity keeps planets in orbit around the sun (Unit 2 Topic 1: Linear motion and force).

Circular motion

- Earth pulls the Moon towards it with gravitational force at right angles to the velocity of the Moon around the Earth

○ This is the only force acting on the Moon, and means that the net force is a centripetal force

○ If there was no Earth, there would be no centripetal force, and the Moon would continue to move in a straight line with constant speed (Newton's First Law)

Gravity

- Earth exerts a gravitational force on the Moon, and abiding to Newton's Third Law, the Moon exerts an equal and opposite force on the Earth.

○ Gravitational force is equal for both planets, so using F=ma, it can be determined that the acceleration experienced by the Moon is larger than for Earth

○ This concept is similar for the Earth orbiting around the sun

Orbits

recall Kepler's laws of planetary motion

- First law of planetary motion states that all planets move about the Sun in elliptical orbits, having the Sun as one of their foci

- Second law of planetary motion (law of areas) states that a radius vector joining any planet to the Sun sweeps out equal areas in equal lengths of time

• At the perihelion (closest point) the planet moved faster than at the aphelion (furthest point)

- Third law of planetary motion (law of periods) states that the square of the sidereal period of a planet is directly proportional the cube of its mean distance from the Sun

• Sidereal period is the time it takes for a planet to complete one orbit of another body relative to the stars.

Solve Problems involving Kepler's third law

recall that Kepler's third law can be derived from the relationship between Newton's Law of Universal Gravitation and uniform circular motion.

Syllabus link: Students should be able to recall the Law of Conservation of Energy (Unit 1 Topic 1: Heating processes).

- Law of conservation of energy states that energy cannot be created or destroyed, only transferred or transformed