AQA GCSE Physics 3-1b
GCSE Physics pages 222 to 233
April 10 th 2010

CIRCULAR MOTION
13.2 What keeps bodies moving in a circle?
Using skills, knowledge and understanding of how science works:
• to identify which force(s) provide(s) the centripetal force in a given situation
• to interpret data on bodies moving in circular paths.
Skills, knowledge and understanding of how science works set in the context of:
• When a body moves in a circle it continuously accelerates towards the centre of the circle. This acceleration changes the direction of motion of the body, not its speed.
• The resultant force causing this acceleration is called the centripetal force.
• The direction of the centripetal force is always towards the centre of the circle.
• The centripetal force needed to make a body perform circular motion increases as:
– the mass of the body increases;
– the speed of the body increases;
– the radius of the circle decreases.
SATELLITE AND PLANETARY MOTION
13.3 What provides the centripetal force for planets and satellites?
Using skills, knowledge and understanding of how science works:
• to interpret data on planets and satellites moving in orbits that approximate to circular paths.
Skills, knowledge and understanding of how science works set in the context of:
• The Earth, Sun, Moon and all other bodies attract each other with a force called gravity.
• The bigger the masses of the bodies the bigger the force of gravity between them.
• As the distance between two bodies increases the force of gravity between them decreases.
• The orbits of the planets are slightly squashed circles (ellipses) with the Sun quite close to the centre.
• Gravitational force provides the centripetal force that allows planets and satellites to maintain their circular orbits.
• The further away an orbiting body is the longer it takes to make a complete orbit.
• To stay in orbit at a particular distance, smaller bodies, including planets and satellites, must move at a particular speed around larger bodies.
• Communications satellites are usually put into a geostationary orbit above the equator.
• Monitoring satellites are usually put into a low polar orbit.

Recap of Forces
The meaning of centripetal force
The three factors on which centripetal force depends
1
2
3
Why an object moving in a circle has an inward acceleration

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.

Situation Centripetal force
Earth orbiting the Sun
Car going around a bend.
GRAVITY of the Sun
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

Centripetal force
INCREASES if:
- the object is moved FASTER
the object’s mass is INCREASED .
- the radius of the circle is
DECREASED .

Why does an object moving in a circle have an inward acceleration ?
When an object moves in a circle, there is unbalanced force on it that acts towards the center of the circle.
Newton 2 what happens if an object has an unbalanced force on it?

Acceleration is the change in velocity / time
Remember velocity is a vector – speed and direction
So we have ……..

What direction does the ball move?
Remember Newton 1
String breaking quiz
Draw on the diagram provided

Choose appropriate words to fill in the gaps below:
WORD SELECTION: gravitational towards force greater centripetal circular increases

Recap of Forces
The meaning of centripetal force
The three factors on which centripetal force depends
1
2
3
Why an object moving in a circle has an inward acceleration

Ladybug Revolution - PhET - Join the ladybug in an exploration of rotational motion. Rotate the merry-go-round to change its angle, or choose a constant angular velocity or angular acceleration. Explore how circular motion relates to the bug's x,y position, velocity, and acceleration using vectors or graphs.
Motion in 2D - PhET - Learn about velocity and acceleration vectors. Move the ball with the mouse or let the simulation move the ball in four types of motion (2 types of linear, simple harmonic, circle). See the velocity and acceleration vectors change as the ball moves.
Motion produced by a force - linear & circular cases - netfirms
Uniform circular motion - Fendt
Carousel - centripetal force - Fendt
Relation between speed and centripetal force - NTNU
Vertical circle & force vectors - NTNU
Circular Motion & Centripetal Force - NTNU
Inertia of a lead brick & Circular motion of a water glass - 'Whys Guy' Video
Clip (3 mins) (2nd of 2 clips)

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 (okay 9.81 N/kg)
Moon’s surface = 1.6 N/kg
Mars’ surface = 3.7 N/kg
Weight is the force of gravity on an object.

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 – it is inversely proportional
P.S These are Sir Isaac Newton’s rules on
Gravity

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

Choose appropriate words to fill in the gaps below: increased.
The Moon’s gravity is about one sixth the strength of the increases
WORD SELECTION: newtons masses objects mass weight decreases

Notes questions from pages 224 & 225
1.
State Newton’s rules on gravity.
2.
Copy and answer questions (a) and (b) on page 224.
3.
Describe how the force of gravity on a space vehicle changes as it travels from the Earth to the Moon.
4.
Copy and answer question (c) on page 224.
5.
Define what is meant by ‘ gravitational field strength ’.
Show that a mass of 200 kg weighs approximately 32 N on the Moon.
6.
Copy and answer question (d) on page 225.
7.
Copy the ‘Key points’ table on page 225.
8.
Answer the summary questions on page 225.

ANSWERS
In text questions:
(a) The force of gravity on it due to the Sun.
(b) Their mass is too small.
(c) The force of gravity on the
Moon is less, so less energy would be needed to escape from the Moon.
(d) The force of gravity due to the Earth.
Summary questions:
1.
(a) Increases
(b) Stays the same
(c) Decreases.
2. (a) The force of gravity is less on the Moon so it is easier for the astronaut to move up and down.
(b) The force of gravity is less on the Moon so the ball can go higher for the same change of gravitational potential energy.

Free-fall Lab - Explore Science
Galileo Time of Fall Demonstration - 'Whys Guy' Video Clip (3 mins) - Time of fall independent of mass - Leads slug and feather with and without air resistance. (1st of 2 clips)
Distance Proportional to Time of Fall Squared Demonstration - 'Whys Guy'
Video Clip (3:30 mins) - Falling distance proportional to the time of fall squared. (2nd of 2 clips some microphone problems)
Lunar Lander - PhET - Can you avoid the boulder field and land safely, just before your fuel runs out, as Neil Armstrong did in 1969? Our version of this classic video game accurately simulates the real motion of the lunar lander with the correct mass, thrust, fuel consumption rate, and lunar gravity. The real lunar lander is very hard to control.
Moonlander Use your thrusters to overcome the effects of gravity and bring the moonlander safely down to earth.
BBC KS3 Bitesize Revision:
Mass and gravity
Weight

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

Notes questions from pages 226 & 227
1.
Copy the table on page 227.
2.
(a) What force is responsible for planetary motion? (b) Why is this force an example of centripetal force?
3.
Explain how orbital speed affects the shape of a planet’s orbit.
4.
State how (a) the speed and (b) the time taken to complete one orbit depends on a planet’s distance from the Sun.
5.
Copy and answer questions (a) and (b) on pages 226 and
227.
6.
Copy the ‘Key points’ table on page 227.
7.
Answer the summary questions on page 227.

ANSWERS
In text questions:
(a) There would probably be a bigger variation of temperature each year.
The tides would be more variable.
(b) Its orbit is about 5 times bigger and it takes about 12 times longer, so it must travel slower than the Earth.
Summary questions:
1. (a) Satellite, Earth.
(b) Earth
(c) Satellite, Earth.
(d) Planet, Sun.
2. (a) (i) Jupiter (ii) Venus
(b) 49 km/s

My Solar System - PhET- Build your own system of heavenly bodies and watch the gravitational ballet. With this orbit simulator, you can set initial positions, velocities, and masses of 2, 3, or 4 bodies, and then see them orbit each other.
Multiple planets - 7stones
Planet orbit info - Fendt
Orrery of Inner Solar System - CUUG
The Solar System - Powerpoint presentation by KT
Solar system quizes - How well do you know the solar system? This resource contains whiteboard activities to order and name the planets corrrectly as well as a palnet database - by eChalk
Hidden Pairs Game on Planet Facts - by KT - Microsoft WORD
Fifty-Fifty Game on Planets with Atmospheres - by KT - Microsoft WORD
Fifty-Fifty Game on Planets that are smaller than the Earth - by KT - Microsoft WORD
Sequential Puzzle on Planet Order - by KT - Microsoft WORD
Sequential Puzzle on Planet Size - by KT - Microsoft WORD
Projectile & Satellite Orbits - NTNU
Kepler Motion - NTNU
Kepler's 2nd Law - Fendt
Two & Three Body Orbits - 7stones
Orbits - Gravitation program
BBC KS3 Bitesize Revision:
Gravitational Forces - includes planet naming applet

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.

• Period of an orbit = time for one orbit of the earth
• Geostationary satellites period of orbit is 24 hours, this means it stays in the same position in the sky
• Polar satellites – orbiting every two or three hours and can scan the whole Earth every day

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 36 000 km
Uses of communication satellites include satellite
TV and some weather satellites.

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

Choose appropriate words to fill in the gaps below:
WORD SELECTION: monitoring higher communications longer lower 24 slowly

Notes questions from pages 228 & 229
1.
With the aid of a diagram explain how a satellite can remain in orbit about the Earth.
2.
How does (a) the speed and (b) the period of a satellite vary with its height above the Earth?
3.
Copy and answer questions (a) and (b) on page 228.
4.
(a) What is meant by a ‘ geostationary orbit ’? (b) Why must satellite TV use geostationary satellites?
5.
What are ‘ monitoring satellites ’? What type of orbit is used for this type of satellite?
6.
Copy and answer question (c) on page 229.
7.
Copy the ‘Key points’ table on page 229.
8.
Answer the summary questions on page 229.

ANSWERS
In text questions:
(a) It can give the location and the height above sea level.
(b) 12
(c) They would be slowed by drag from the atmosphere and would fall back to Earth.
Summary questions:
1. (a) High, equator.
(b) Low, poles.
2. (a) (i) Below (ii) Above
(b) Less energy is needed because the orbit is nearer the ground than a geostationary orbit is.

Electromagnetic Spectrum & Communications - BT
Inside a communication satellite - BT
Projectile & Satellite Orbits - NTNU
Newton's Cannon Demo - to show how orbits occur - by Michael
Fowler
Kepler Motion - NTNU
Kepler's 2nd Law - Fendt
Space craft control - NTNU
How a satellite orbits - BT
Satellite orbits - BT
Inside a communication satellite - BT
BBC KS3 Bitesize Revision:
Satellites & Space Probes

Notes questions from pages 230 & 231
1. No questions.