6.13 Orbits

Question 1

Describe how planets move around the sun (the same goes for moons or artificial satellites orbiting planets)

Answer 1

• A planet that’s moving near to a star will feel a gravitational force towards it.
• This force would cause the planet to just travel towards whatever it is orbiting, but as the planet is already moving, it just causes a change in direction.
• Although its speed remains the same, its direction, and so velocity, is constantly changing.
• The planet keeps accelerating towards what it’s orbiting but the instantaneous velocity (which is at right angle to the acceleration) keeps it travelling in a circle.

Question 2

What happens to an object in orbit when it slows down?

Answer 2

The orbiting object will be pulled into an orbit with a smaller orbit radius - this causes the speed of the object to increase

Question 3

What is the definition of an orbit?

Answer 3

The curved path of one celestial object or spacecraft around another celestial object

Question 4

What does Newton’s First Law state?

Answer 4

That an object travelling at a certain velocity will continue to travel at that velocity, unless acted upon by a resultant force.

Question 5

Why does the velocity of planets in orbit always change?

Answer 5

The speed of the planets stay the same, however they are constantly changing directions. So, as velocity measures both speed and direction, the velocity constantly changes too.

Question 6

What are the two things that result in a steady orbit?

Answer 6

• The instantaneous velocity of the planet/spacecraft in orbit
• The gravitational pull of the bigger planet

Question 7

What would happen if the moon’s orbit around the Earth became too small?

Answer 7

As the distance between the Earth and the Moon is much smaller, the size of the gravitational force will be much greater and so if nothing else changed, the Moon would probably get sucked in towards the Earth. To avoid this, the Moon would have to start travelling much faster so that its instantaneous velocity would increase, which would allow it to maintain a stable orbit and not be pulled in towards the Earth.

Question 8

Velocity is a vector quantity. What does this mean?

Answer 8

It has both magnitude and direction