P5a - Satellites, Gravity and Circular Motion

Question 1

What is a satellite and how are they held in orbit? Give an example of a natural satellite.

Answer 1

• a satellite is an object which orbits another in space.
• held in orbit by a gravitational force.
• the moon is a natural satellite.

Question 2

Fill in the blanks.

Gravity is a force of __________ between 2 ________. The greater the mass the _________ the gravitational pull (doubling the mass _________ the pull). The greater the distance the _________ the gravitational pull (doubling the distance _________ the pull by a ___________.

Answer 2

• attraction.
• masses.
• greater.
• doubles.
• smaller.
• reduces.
• factor of 4.

Question 3

What two things keep planets to stay in orbit in our solar system?

Answer 3

• gravitational pull of the sun.
• their own motion.

Question 4

Why do comets have elliptical orbits?

Answer 4

They speed up when they are closer to the sun as the gravitational pull is stronger.

Question 5

What are the differences between polar orbits and geostationary orbits?

Answer 5

• polar orbits orbit over the poles and orbit lower in the atmosphere. These orbits travel faster because the pull of gravity is greater.
• geostationary orbits stay above the same point on the equator, and orbit the earth once every 24 hours, so it is slower than polar orbits.

Question 6

Explain centripetal force. Give an example, and explain why.

Answer 6

• a force of an object that travels in a circle, where the force is focused in the centre of the circle.
• e.g. gravity because it is continually making the object accelerate towards the Earth but its motion keeps it moving in a circle. The object is accelerating because its direction of motion is changing.

Question 7

Explain how satellites are used for communication. What type of orbit do they have?

Answer 7

• satellites often act as relay stations to send tv signals to homes, as signals travel in straight line, so they relay signals as the Earth curves.
• signals are sent from a tv station and are receieved by the satellite.
• the satellites filter the signals, changes and amplifies the frequency before sending them back to earth.
• often have a geostationary orbit.

Question 8

Explain how satellites are used for GPS. What kind of orbit do they have?

Answer 8

• network of approx. 30 satellites orbit the Earth, so that a min. of 4 satellites are ‘visible’ at any point on Earth.
• each satellite transmits information on its position and the current time at regular intervals.
• these signals are intercepted by any GPS receiver, which works out how far away the satellite is based on how long it took for the messages to arrive.
• when it has this info from at least 3 satellites, it can pinpoint your location.