1. Turning Forces Circular, Satellite & Planetary Motion
April 2012

2. Circular Motion
An object requires a force for it to move along a circular path.
If this force is removed the object will continue to move along a straight line tangentially to the circle.

3. Centripetal Force
CENTRIPETAL FORCE is the general name given to a centrally directed force that causes circular motion.
Tension provides the CENTRIPETAL FORCE required by the hammer thrower.

4. Other examples of centripetal force
Situation
Centripetal force
Earth orbiting the Sun
GRAVITY of the Sun
Car going around a bend.
FRICTION on the car’s tyres
Airplane banking (turning)
PUSH of air on the airplane’s wings
Electron orbiting a nucleus
ELECTROSTATIC attraction due to opposite charges

5. Factors affecting centripetal force
INCREASES if:
- the object is moved FASTER
- the object’s mass is INCREASED.
- the radius of the circle is DECREASED.

6. Choose appropriate words to fill in the gaps below:
An object will only move along a __________ path if it is constantly acted on by a centripetal _________. The force is always directed __________ the centre of the circular path.
Centripetal force ___________ if the object moves in a smaller radius path or at a __________ speed.
An example of a _________ force is the Moon orbiting the Earth due to the Earth’s _____________ pull on the Moon.
circular
force
towards
increases
greater
centripetal
gravitational
WORD SELECTION:
gravitational
force
greater
circular
towards
centripetal
increases

7. Gravitational attraction
Gravity is a force exerted by all objects on each other.
Gravitational force:
- is always attractive
- increases if the mass of the objects is increased
- decreases if the distance between the objects is increased

8. Gravitational field strength
Gravitational field strength is equal to the force exerted on an object of mass 1kg.
On the Earth’s surface the gravitational field strength is about 10 N/kg
Moon’s surface = 1.6 N/kg
Mars’ surface = 3.7 N/kg
Weight is the force of gravity on an object.

9. Complete Answers Earth 10 80 Moon 1.6 Mars 3.7 740 Jupiter 60 1500
Surface
Field Strength (N/kg)
Object mass (kg)
Object weight (N)
Earth 10 80
Moon
1.6
Mars
3.7
740
Jupiter 60
1500
Pluto
0.07
800
128
200 25
5.6

10. Choose appropriate words to fill in the gaps below:
Gravity is a force exerted by all ________ on each other because of their ________.
Gravitational force __________ if the distance between the objects is increased but __________ if their masses are increased.
_________ is the force of gravity on an object. On the Earth’s surface an object of mass 1kg has a weight of 10 __________.
The Moon’s gravity is about one sixth the strength of the Earth’s because its _________ is much lower.
objects
masses
decreases
increases
weight
newtons
mass
WORD SELECTION:
increases
newtons
objects
decreases
masses
mass
weight

11. Planetary orbits
The orbits of the planets are slightly squashed circles (ellipses) with the Sun quite close to the centre.
The Sun lies at a ‘focus’ of the ellipse

12. Planets move more quickly when they are closer to the Sun.
slower
faster
The above diagram is exaggerated!

13. The time taken for a planet to complete one orbit increases with its distance from the Sun.
Mercury
88 days
Venus
225 days
Earth
1 year
Mars
2 years
Jupiter
12 years
Saturn
29 years
Uranus
84 years
Neptune
165 years

14. Satellites
A satellite is a lower mass body that orbits around a higher mass body.
- The Moon is a natural satellite of the Earth.
- The Hubble Space Telescope is an artificial (man-made) satellite of the Earth.
- The Earth is a satellite of the Sun.

15. How a satellite orbits
To stay in orbit at a satellite must move at a particular speed.
too fast
too slow
correct speed

16. Communication satellites
These are usually placed in geostationary orbits so that they always stay above the same place on the Earth’s surface.
VIEW FROM ABOVE THE NORTH POLE

17. Geostationary satellites must have orbits that:
- take 24 hours to complete
- circle in the same direction as the Earth’s spin
- are above the equator
- orbit at a height of about km
Uses of communication satellites include satellite TV and some weather satellites.

18. Monitoring satellites
They are used for weather, military, and environmental monitoring.
They have relatively low orbital heights (eg 500 km).
They take typically 2 hours to complete one orbit.
They are considered to be in polar orbits even though their orbits do not always pass over the poles.

19. Question
What are the advantages / disadvantages of using a polar orbiting rather than a geostationary satellite for monitoring?
ADVANTAGES
- it is nearer to the Earth allowing more detail to be seen and
- it is easier to place into orbit
- it eventually passes over all of the Earth’s surface
DISADVANTAGE
- unlike a geostationary satellite it is not always above the same point on the Earth’s surface so continuous monitoring is not possible

20. GPS / SatNav
The satellites used for the Global Positioning System (GPS), as used in SatNav, are in ‘polar’ orbits.
GPS makes use of about 30 polar orbiting satellites.

21. Choose appropriate words to fill in the gaps below:
A satellite is a ________ mass object orbiting around a ________ mass body.
The larger the orbit of a satellite the more ________ it moves and the ________ it takes to complete one orbit.
Geostationary satellites are used for _____________ and have an orbital period of _____ hours.
_____________ satellites normally use polar orbits.
lower
higher
slowly
longer
communications 24
monitoring
WORD SELECTION:
monitoring
higher
communications
longer
lower 24
slowly