Gravity and Orbits

Question 1

Describe the orbits of moons.

Answer 1

Slightly elliptical

Question 2

Describe the orbits of planets.

Answer 2

Slightly elliptical

Question 3

Describe the orbits of comets.

Answer 3

Very elliptical with the Sun at one focus (near one end of the orbit).

Question 4

Why does a comet travel faster when its nearer the Sun?

Answer 4

Because the increased pull of gravity makes it speed up the closer it gets to the Sun.

Question 5

What is a geostationary satellite?

Answer 5

An artificial satellite with orbital period of exactly one day.

Question 6

How do you calculate speed of an orbit?

Answer 6

speed = distance/time

Question 7

How do you calculate circular orbit speed?

Answer 7

Distance = circumference
so 2 x pi x r (r being the distance from the centre of the object you are orbiting)
So
v = 2 x pi x r /T
T measures in sec
CHECK UNITS in the question and convert before you calculate.

Question 8

Why are geostationary satellites useful?

Answer 8

These are useful in communications because they are always over the same part of the planet.

Question 9

Describe the change in orbital speed of a comet.

Answer 9

A comet travels much faster when it is nearer the Sun than it does in the more distant parts of its orbit.

Question 11

The structure of the Solar System is determined by …

Answer 11

…Orbits - the paths that objects take as they move around each other in space.

Question 12

Describe the path that planets move around the Sun.

Answer 12

Almost circular orbits.

Question 13

If an object is travelling in a circle…..

Answer 13

…. it is constantly changing direction (and so constantly accelerating), which means there is a force acting on it. (F=ma)

Question 14

What is the name of the force causing the object to travel in a circle?

Answer 14

Centripetal force (acting towards the centre of the circle).

Question 15

Why doesn’t the object just fall towards the centre of what it is orbiting?

Answer 15

Because the object is already moving, so it just causes it to change direction.
The object keeps accelerating towards what its orbiting but the instantaneous velocity (which is at right angle to the acceleration) keeps it travelling in a circle.

Question 16

What force keeps the planets and comets in their orbits around the Sun?

Answer 16

Gravitational force. The gravitational attraction of the Sun keeps them in orbits around it.

Question 17

How are satellites kept in their orbits around planets?

Answer 17

By the gravitational attraction of the planet they are orbiting.

Question 18

What happens to the gravitational field strength (g) as the mass increases?

Answer 18

The larger the mass of the body creating the field, the stronger the gravitational field strength.

Question 19

Why does an object weigh more on Earth than the Moon?

Answer 19

The Earth is more massive than the Moon, so an object would weigh more on Earth than it would on the Moon. This is because the gravitational field strength is larger on Earth than the Moon due to the mass of the Earth being larger.

Question 20

What variables affect gravitational field strength?

Answer 20

1. The mass of the body creating the field.
2. The distance from the body creating the field.

Question 21

The closer you get to a star or planet…..

Answer 21

…. the stronger the gravitational force is.

Question 22

How does the strength of the gravitational force affect the instantaneous velocity?

Answer 22

The stronger the force, the larger the instantaneous velocity needed to balance it. So the closer you get to the star or planet, the faster you need to travel to remain in orbit.

Question 23

In a stable orbit, if the speed of the object changes….

Answer 23

… the size (radius) of its orbit must do too.
Faster moving objects will move in a stable orbit with a smaller radius than slower moving ones.

Question 24

Calculate the speed m/s of a satellite that is orbiting above the Earth’s surfaces at an altitude of 600 km. The radius of the Earth is 6400 km and the satellite takes 200 min to orbit the Earth once.

Answer 24