Gravitational fields -CH18

Question 1

Gravitational field strength definition

Answer 1

The gravitational field strength, g, is defined as the gravitational force experienced per unit mass by an object at that point in a gravitational field. It is a vector quantity, with the units Nkg-1
or ms-2

Question 2

What is newton’s law of gravititation

Answer 2

Newton’s law of gravitation states two point masses attract each other with a force that is directly proportional to the product of their masses, and inversely proportional to the square of
their separation.

Question 3

Why is gravitational force negative

Answer 3

The negative sign is used to show the
attractive nature of the force.

Question 4

what does the formula g=GM/r^2 show

Answer 4

The gravitational field strength for a point
mass. This shows that the field strength for an object, such as a planet, does not depend on the mass of the object in orbit around the planet, only on the mass of the planet and the distance between
them.

Question 5

What is Kepler’s first law

Answer 5

The orbit of a planet is an ellipse, with the sun at one focus.
– The eccentricity of the ellipse is very low, so the motion can be modelled as circular.

Question 6

What is Kepler’s second law

Answer 6

A line segment joining a planet and the sun sweeps out
equal areas during intervals of equal time.
Why? This is because the speed of the
planet is not constant – the planet moves
faster when it is closer to the sun.

Question 7

What is Kepler’s third law

Answer 7

The square of the orbital period T is proportional to the cube of the average distance r from the sun

Question 8

How can Kepler’s third law be proved

Answer 8

This can be proved by considering the forces acting on the planet. Centripetal force is required to keep the planet in orbit, and this force is provided by the gravitational field of the sun.

Question 9

Geostationary satellites

Answer 9

Have an orbital period of one day
They travel in the same direction as the rotation of the earth
This means that they remain above the
same point on the Earth’s surface

Question 10

What is gravitational potential

Answer 10

Gravitational potential, Vg, is defined as the work done per unit mass to move an object to that point from infinity. - Jkg^-1
Vg is negative, which represents how energy is required to move the object out of the field.

Question 11

What is gravitational potential energy

Answer 11

Gravitational potential energy is the work done to move an object with mass m from infinity to a point in that field.