AQA GCSE Physics 3-1b

Turning Forces

Circular, Satellite & Planetary MotionGCSE Physics pages 222 to 233

April 10th 2010

AQA GCSE Specification

CIRCULAR MOTION13.2 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 MOTION13.3 What provides the centripetal force forplanets 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.

Centripetal ForceRecap 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

Task

Complete sheet on

Terminal Velocity

Centripetal ForceCENTRIPETAL 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.

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

Factors affecting centripetal force

Centripetal force

INCREASES if:- the object is moved FASTER

- the object’s mass is INCREASED.

- the radius of the circle is DECREASED.

Centripetal Force

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- How?

Acceleration is the change in velocity / time

Remember

velocity is a vector – speed and direction

So we have ……..

What happens if the string breaks?

What direction does the ball move?

Remember Newton 1

String breaking quiz

Draw on the diagram provided

Tasks

Do questions 1- 4 on page 49

Question 12 on page 51

Fill in missing words

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.

centripetal

circulargravitational

increasestowards

greater

WORD SELECTION:

force

centripetal

circular

gravitational

increases

towards

greater

force

Centripetal ForceRecap 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

Circular Motion SimulationsLadybug 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 attraction

• Gravity is an attractive force

• Gravity depends on mass and the distance between objects.

Gravitational field strengthGravitational 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.

Gravitational attractionGravity 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

CompleteAnswersSurface Field Strength

(N/kg)Object mass

(kg)Object weight

(N)

Earth 10 80

Moon 1.6 80

Mars 3.7 740

Jupiter 60 1500

Pluto 0.07 80

800

128

200

25

5.6

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.

decreasesnewtons objectsincreases

mass weight

WORD SELECTION:

masses

decreases

newtons

objects

increases

mass

weight

masses

Gravitational attraction 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.

Gravitational attraction 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.

Gravity SimulationsFree-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

Planetary Orbits

• Gravity cause the planets to orbit the sun

• The time it takes a planet to orbit depend on its distance from the sun

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

Planetary orbits 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.

Planetary orbits 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

Planetary Motion SimulationsMy 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

SatellitesA 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.

Satellites

• 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

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.

QuestionWhat 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:

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.

24lowerlonger slowly

communications

WORD SELECTION:

highermonitoring

24

lower

longer

slowly

communications

higher

monitoring

SatellitesNotes 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.

Satellites 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.

Satellite SimulationsElectromagnetic 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

Turning issuesNotes questions from pages 230 & 231

1. No questions.