# Orbits - Physicslocker

1 of 34 Boardworks Ltd 2009 2 of 34 Boardworks Ltd 2009 Forces and acceleration An object will remain stationary or will move in the same direction at a constant speed, unless the forces acting on it are not balanced.

This will cause an acceleration in the direction of the stronger force. This can make an object slow down or speed up, or it can cause it to change direction. 3 of 34 Boardworks Ltd 2009 Acceleration in a circle A motorbike drives round a corner at a constant speed. Its direction

changes as it goes round the corner, so even though its speed is constant, it must be accelerating. This acceleration must be at right angles (perpendicular) to the direction of movement as it turns the corner, otherwise its speed could not be constant. Which way do you think the motorbike is accelerating, towards the inside of the bend, or away from it? 4 of 34

Boardworks Ltd 2009 Forces causing circular motion Any object that moves in a circle must be accelerating towards the centre of that circle. What causes this? What equation do you know that links force and acceleration? F = ma Force and acceleration are both vector quantities, unlike mass, so according to this equation, their directions must be equal. All circular motion must therefore be caused by a force

acting towards the centre of the circle. This type of force is known as a centripetal force. 5 of 34 Boardworks Ltd 2009 Centrifugal force or centripetal force? Swing a mass around in a circle on the end of a string. Do you feel a force pulling your hand outwards? This is often called a centrifugal force. You might have heard that centrifugal forces cause circular motion, but this is not good physics! Consider what is happening in this case. The mass on the end

of the string is the object that is performing circular motion, so it is the forces on this object that are important: centripetal force The force on your hand is a reaction force, which can be ignored when studying the motion of the mass. 6 of 34 Boardworks Ltd 2009 Thinking about circular motion It is important to think of circular motion as an object being

continuously prevented from moving in a straight line, rather than as if the object is being flung outwards from the centre. A washing machine dries clothes by spinning them round very fast: The sides of the drum provide the centripetal force that keeps the clothes moving in a circle, but water is free to escape in straight trajectories through the holes in the sides.

7 of 34 Boardworks Ltd 2009 Examples of centripetal forces Here are two more examples of circular motion caused by centripetal forces: Can you work out the direction of the force in each case, and describe the type of force involved? 8 of 34

Boardworks Ltd 2009 Factors affecting centripetal forces 9 of 34 Boardworks Ltd 2009 Factors affecting centripetal forces How does the centripetal force depend on mass? F = ma, so force is proportional to mass. The greater the mass, the larger the centripetal

force needed to maintain circular motion. How does the centripetal force depend on speed and radius? F = ma, so force is proportional to acceleration. If the truck is going faster, or if its radius is smaller, then it is changing direction more quickly, so its acceleration is greater. The greater the speed, and the smaller the radius, the larger the centripetal force needed to maintain circular motion. 10 of 34 Boardworks Ltd 2009

Understanding centripetal forces 11 of 34 Boardworks Ltd 2009 12 of 34 Boardworks Ltd 2009 What is gravity?

If a skydiver steps out of a plane, which way does he move? What causes this effect? Gravity is a universal force which attracts any mass to every other mass in the Universe. Every mass has its own gravitational field, like the one surrounding Earth, but it takes two objects to make a gravitational force. Gravity is a very weak force, so small objects dont stick together, but if at least one mass is very large, the effect of gravity is easy to see. Skydivers always fall back to Earth!

13 of 34 Boardworks Ltd 2009 Factors affecting gravity The bigger the mass, the stronger its gravitational field, so the Sun has a much stronger gravitational field than Earth. But the further apart two objects are, the weaker the gravitational forces between them. So when a skydiver jumps out of a plane, he falls to Earth, not towards the Sun! Gravitational fields are stronger: around larger masses

at shorter distances. The gravitational force between two objects can be increased: by increasing the size of either or both of the masses by decreasing the distance between them. 14 of 34 Boardworks Ltd 2009 Gravitational chaos! Every mass in the universe attracts every other. Thats a lot of forces to keep track of! But gravity is a very weak

force, so most gravitational forces at the Earths surface can be ignored. The gravitational field of a pen, a person or even a large mountain is too weak to have a noticeable effect, so the only gravitational field you need to consider is Earths. 15 of 34

Boardworks Ltd 2009 Gravity at the Earths surface Gravitational fields get weaker with increasing distance. Do you feel any lighter on the top floor of your house than on the ground floor? The Earth is so large that small changes in height dont affect weight, so gravitational field strength is effectively constant: weight = mass gravitational field strength = mass 10 N/kg This applies to objects at the Earths surface, at the top of a mountain, or even in an aeroplane at 30000 feet

but be careful! This does not apply to satellites in orbit, or to the forces between planets and stars. 16 of 34 Boardworks Ltd 2009 Understanding gravity 17 of 34 Boardworks Ltd 2009

Gravity as a centripetal force Examples of centripetal forces can be found in many everyday contexts, but what about circular motion on a large scale? What is the centripetal force that makes orbits possible? Unlike a mass on a string, stars and planets are not physically connected to each other, but they are attracted to each other by gravity. How does circular motion under gravity compare to the types of

circular motion we are used to? 18 of 34 Boardworks Ltd 2009 Circular motion under gravity The centripetal force required to keep a planet in circular motion depends on mass, radius and speed. But the gravitational force that a star actually provides only depends on mass and radius. This means that for any specific radius, a planet must move at one specific speed to stay in orbit. When a mass on a string is swung at an increasing speed,

the tension increases, while the radius remains constant: If a planet orbits a star at an increasing speed, the force between them does not increase, so it moves out of that orbit: 19 of 34 Boardworks Ltd 2009 Circular motion under gravity

20 of 34 Boardworks Ltd 2009 Elliptical orbits In 1605 Johannes Kepler used his observations of the orbit of Mars to predict that, rather than moving in perfectly circular orbits, all the planets follow elliptical orbits around the Sun: focus Each orbit forms an ellipse with the Sun at one focus.

The two focuses of an ellipse are similar to the single centre of a circle. 21 of 34 Boardworks Ltd 2009 Comets Most of the planets travel around the Sun in near-circular orbits. Comets also travel around the Sun but in highly elliptical orbits. gas tail dust tail

The head of the comet is a lump of ice and dust a few kilometres across. The tail only appears when the comet is near the Sun. It consists of gas and dust which are released by the heat of the Sun. 22 of 34

Boardworks Ltd 2009 Data analysis 23 of 34 Boardworks Ltd 2009 24 of 34 Boardworks Ltd 2009

What is a satellite? A satellite is an object that orbits a planet. Satellites can be natural or they can be artificial. The largest satellite orbiting Earth is the Moon. This is Earths only natural satellite. Artificial satellites are put into orbit for a range of purposes, such as mapping and surveillance. The same physics applies to satellites orbiting the Earth as to planets orbiting the Sun.

25 of 34 Boardworks Ltd 2009 Types of orbit 26 of 34 Boardworks Ltd 2009 Uses for geostationary satellites Geostationary satellites are particularly useful because they

stay fixed above a single point on Earth. This makes them useful for communications and satellite TV broadcasting, because the satellite never goes out of range. Satellite dishes can be fixed to face in the correct direction, without the need to track the movement of the satellite. Geostationary satellites are also used for weather forecasting.

27 of 34 Boardworks Ltd 2009 Problems with geostationary satellites There are some disadvantages to geostationary satellites. All geostationary satellites must orbit over the equator at a specific altitude of 36000 km. There are limited slots in this orbit, which can lead to disputes when different countries want a certain slot. A geostationary satellite can only see a certain area of the Earths surface the rest is hidden from view.

All geostationary satellites are a long way from Earth, which causes delays in signals. This can be a disadvantage during commercial or military communications. 28 of 34 Boardworks Ltd 2009 Uses for polar orbit satellites Polar satellites are particularly useful because they orbit at a low altitude and high speed.

This makes them useful for mapping, as they can image the Earths surface in higher resolution than more distant satellites. It also makes them useful for observation purposes, such as military surveillance, or weather monitoring, as they can view the whole of the Earths surface in one day. However, polar satellites must be tracked from the ground, and will be out of range for much of the time, causing delays in data retrieval. 29 of 34

Boardworks Ltd 2009 Which type of satellite? 30 of 34 Boardworks Ltd 2009 31 of 34 Boardworks Ltd 2009

Glossary 32 of 34 Boardworks Ltd 2009 Anagrams 33 of 34 Boardworks Ltd 2009

Multiple-choice quiz 34 of 34 Boardworks Ltd 2009

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