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 Concepts:
 Center of gravity (center of mass), line of symmetry, single line of
symmetry, multiple lines of symmetry, stability, plumbline,
 Mass, force of gravity, weight, “g”, gravity, gravity on earth, gravity
on the moon, gravity on different planets
 Skills:
 Be able to represent gravity on an object
 Be able to determine the center of gravity on objects with single and
multiple lines of symmetry as well as those of irregular objects
 Be able to determine changes in center of gravity when mass
distribution changes
 Be able to apply the formula w = m ´ g to determine weight of different objects on earth and
different planet given g.

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 Gravity, force which cause falling objects to accelerate, acts on all
parts of an object.
 However it seems that gravity acts at a single point on an object.
 It appears as if to be acting on only a single point on an object.
 This point is called the centre of mass, or centre of gravity.

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 The centre of mass of a person is at their navel .

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 If the person has both arms raised, the centre of gravity will be a
little bit higher.

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 Raising a single arm or leg will move the centre of gravity upwards and
a little to one side.

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 1. Shapes with more than one line of symmetry:

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 Center of gravity is not necessarily a point within the object!

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 2. Shapes with a single line of symmetry:

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 3. Irregular shapes with no symmetry:

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 The experiment is then repeated with the thumbtack in a different
position.
 The point where the two lines intersect will be the position of the
center of gravity

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 The position of center of mass of an object can change if some mass is
added to the object.

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 The center of mass of a person is usually at the navel.
 The center of gravity of this weight lifter moves upwards, because the
top of the weight lifter is heavier than the bottom.

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 The stability of an object depends on two factors.
 The height of the center of gravity, and
 The area of the base

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 Conciously or unconciously we alter our positions to increase our
stability when we take part in almost any sporting activity.

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 This shaded area show sthe base of the boxer, made as wide as possible
by keeping his feet apart

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 A surfer bends his legs and sperates his feet so that he or she is more
stable on the board.
 Bending the legs lowers the centre of gravity; keeping the feet apart
increases the base area.

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 Racing cars have wide tyres and a low center of gravity.
 Why?
 Locate as best as you can the center of gravity of this car.

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 What would most likely happen if the weightlifter did not spread his
feet as he does in the picture while lifting the weight?
 What would most likely happen if the weightlifter did not spread his
feet as he does in the picture when he has lifted the weight?
 What does spreading his feet do?
 If the weightlifter tried to lift the weight standing with his feet
together, would he be able to lift the weight? Why or why not?

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 An object will fall over if its centre of gravity is not above its base.

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 For the same reason, you cannot pick up a coin in front of you by
bending forward while you have your back right up against wall.

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 When you bend over, your center of gravity moves outside your base
—your feet—before your hand reaches the coin, you become unstable and
fall forward.
 And it’s also for the same reason that you can’t stretch one of your
legs out and hold it in air while keeping the whole of the other side
right up against a wall.
 The area of your bases decreases so much that your center of gravity
falls outside of your base and you fall away from the wall.

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 In both the situtations the truck will not topple over because the center
of gravity is above the base.

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 Here it will topple over because the center of gravity is no longer
above the base.

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 Mass: The mass of an object is the amount of matter present in the
object and is measured in kilograms.
 For example, the mass of a carton of chibuku depends only on the amount
of chibku in the carton!
 Weight: The weight of an object is the force of gravity acting on the
object.
 Like other forces, it is meaured in Newtons.
 The force of gravity acting on an object depends on the mass of the
object.

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 The force of gravity acting on a large mass is bigger than the force of
gravity acting on a small mass:

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 On earth, the weight of an object can be found by multiplying the mass
of the object by a factor of 10.
 This factor is called the gravitational field strength.
 It is given a special symbol “g” and has units of N / kg.
 So: Weight = mass ´ g or in
symbols W = Mg
 Weight of a 60 kg man = 60 ´
10 = 600 N
 N.B. It is useful to remember that a mass of 1 kg has a weight of 10
Newtons on earth.
 Gravitational field strength on earth is 10 N/kg.
 A mass of 100 g therefore has a weight of 1 Newton on earth

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 The mass of an object is the same on the earth, on the moon and in deep
space.
 This is because the amount of matter in an object is fixed and cannot
change.
 The weight of an object however is less on the moon than on the earth.
 This is because the force of gravity is weaker on the moon.
 On the moon, g has a value of 1.6 N / kg.
 To find the weight of an object on the moon, multiply the mass of the
object by a factor of 1.6
 The weight of an objetc in deep space is 0 N.
 Ths is because there is no gravity in deep space.

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 If the moon has a weaker gravitational field strength than the earth,
then different planets in the solar system must also have different
strength.
 What does the gravitational field strength depend on?
 Force of gravity depends on two things:
 size (mass) of the planet—the bigger the planet the stronger it’s
gravitational field strength, and
 what it is made up of—if it is made of material that makes it really
dense then it will have a strong force of gravity
 You will notice that in general a planet that has a strong gravitational
field strength has more moons orbiting it.
 What is the order of the gravitational field strength of the nine
planets in our solar system?

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 Weakest to strongest:
 pluto(0.03) < Mercury(3.8)/mars(3.8) < venus(9)/uranus(9.3) <
earth(10) < saturn(12)/neptune(12) < jupiter(27)
