Forces and motion

Forces and motion

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  MOTION What does ismean for an object to be stationary? Is there any such thing? If an object is in motion, we can describe its position, and how fast it is changing its position We can also describe the direction it is moving All motion is described in relative, not absolute terms Usually relative to the earth‟s surface in the local area, but not for rockets, planets, stars
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  CONCEPTS - MOTION Speed– rate of change of position (distance unit per time unit, e.g. metre per second) Velocity – same as speed but with a direction specified (e.g. 30 m/s North) Distance – total distance travelled relative to the ground (distance unit, e.g. metre) Displacement – current distance from point of origin, including direction (e.g. 25 m West)
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  CONCEPTS - ACCELERATION Acceleration– rate of change of velocity (not speed) Unit of distance per unit of time per unit of time (e.g. metre per second per second, m/s/s, ms-2) Can involve a change of speed or a change of direction (or both) Can be in the same direction or the opposite direction compared to motion, or at an angle to it
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  CONCEPT - FORCES Apush or pull that acts on an object Measured in an SI unit called the newton (N) which is a kilogram metre per second per second, kgms-2) How we arrive at this unit will become obvious Can involve contact (e.g. a tug pulling a ship, a bulldozer pushing dirt) or not (e.g. earth pulling on the moon)
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  EXAMPLES OF FORCES Agolf club hitting a ball People pushing a car Magnets attracting or repelling A book pushing down on a desk and the desk pushing upward on the book Gravity pulling downward on a flying seagull Positively charged protons in the nucleus attracting negatively charged electrons
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  GRAVITY Gravity is aparticular kind of force It is a result of the nature of matter in this universe (related to the Higgs field and the Higgs boson) It causes all masses to be attracted to all other masses It is a very weak force (compared to electrostatic force, for example), which means it becomes important when masses are very large, but it exists for all masses It decreases with the square of separation distance
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  CONCEPTS - MASSAND WEIGHT Mass – measures the amount of matter in something in mass units (e.g. kg) Weight – the force that gravity, at a particular place, exerts on a particular mass Mass of an object is the same everywhere in the universe Weight of an object depends on the local gravitational field Objects have the same mass but about 1/6 the weight on the moon
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  NEWTON’S FIRST LAW(OF MOTION) First Law – “An object remains at rest, or in motion in a straight line at constant speed, unless acted on by an unbalanced external force” (since motion is relative, „rest‟ and „uniform straight line motion‟ only depend on your point of view!) This is not intuitive to us, since things tend to slow down – but this is because friction is acting (see below) Think about the situation in outer space…
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  NEWTON’S SECOND LAW SecondLaw – “The acceleration of an object is directly proportional to the (external, unbalanced) force acting on it and inversely proportional to its mass” Usually written as an equation for calculations: a = F/m Alternatively, F = ma It makes sense that the bigger the force the bigger the acceleration „Inertia‟ is the name we give to the idea that bigger mass will lead to smaller acceleration with the same force
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  NEWTON’S THIRD LAW ThirdLaw – “For every force there exists another force of the same magnitude (size), that acts in the opposite direction, on a different object” People talk about „action‟ and „reaction‟ forces, but that can be confusing – it‟s only a point of view Earth pulls the moon, and the moon pulls back with the same force: but because the earth has more mass, the moon accelerates more (i.e. changes its direction of motion, moving in (nearly) a circle) There are no exceptions to the Third Law
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  FRICTION Friction is akind of force that occurs when objects are in contact with each other, either stationary or moving relative to each other Frictional forces always oppose the motion that causes them Energy (which is a topic for another day) is lost to frictional forces, causing heat Different kinds of frictional: static (stationary relative to each other), kinetic (sliding past each other), rolling
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  AIR RESISTANCE When anobject is in „free fall‟ in an atmosphere, as it gains speed the air will oppose the motion – this is a form of friction The gravitational force remains constant and the air resistance increases with velocity – „terminal velocity‟ is reached when the two forces balance
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  BUOYANCY When an objectis placed in water it displaces water The weight force due to that displaced water acts upward on the body – the force of buoyancy If buoyancy balances the weight of the object, it will float, if not, it will sink