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SI base units

SI base units. There are only 7 ‘base units’ in the International System (SI). All other units can be derived from 2 or more of these units. Physical Quantity Unit and its abbreviation Distance metre (m) Mass kilogram (kg) Time second (s) Temperature kelvin (K) Current ampere (A)

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SI base units

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  1. SI base units There are only 7 ‘base units’ in the International System (SI). All other units can be derived from 2 or more of these units. Physical QuantityUnit and its abbreviation Distancemetre (m) Mass kilogram (kg) Time second (s) Temperature kelvin (K) Current ampere (A) Quantity of matter mole (mol) Brightness of light candela (cd)

  2. Prefixes Science often deals with very large and small numbers and we have therefore introduced special prefixes to indicate these values.

  3. Forces Since force has a direction, it is also a vector quantity. Force measured in newton (N). • Effects of a force: • A force can: • Make an object go faster • Make an object slow down • Change the direction in which the object is moving • Compress an object • Stretch an object • Prevent the movement of an object by balancing other forces that are acting on the object.

  4. Adding forces There are often 1, 2 or more forces acting on an object. When we add these forces, we find the resultant force acting on the object. All objects on or near the earth have a weight (the force of gravity that the earth exerts upon them.) The symbol for weight - or the force due to gravity is Fg – measured in newton (N). The combined effect of 2 or more forces acting on an object is called the resultant force of the other forces. Definition: Resultant vector: The resultant of any number of vectors is the single vector that has the same effect as all the vectors acting together.

  5. Adding forces Consider a book lying on a table: Force of table on book book Weight Fg Since the book is not moving, the forces are equal and thus balanced. The resultant of the 2 forces on the book is thus 0 N.

  6. Adding forces A 3 N 2 N R = 1 N B B 3 N R = 5 N C C 2 N 3 N 3 N R = 2 N D D 2 N Consider the horizontal forces acting on the box in the sketches below. (Remember the weight of the box is cancelled by the upward force of the floor on the box.) Forces on object: Resultant force on object 2 N 2 N R = 0 N A

  7. Newton’s first law. Definition:An object will remain at rest or, if it is moving, will carry on moving in a straight line unless acted upon by an resultant force. This means: If object has no forces acting on it, or if forces acting on it are balanced (zero resultant) the object will remain at rest forever. If object is moving and no resultant force acts on it, it will continue to move in a straight line forever. If object moves at constant speed, the resultant forces are zero. If a resultant force acts on the object, it will accelerate in the direction of the force.

  8. Newton’s first law. Stationary: 2 important forces – name them. 4 important forces – name them. Moving with constant velocity: Which forces are balanced?

  9. Newton’s first law. • The shuttle ‘Atlantis’ is orbiting the Earth. • Discuss and explain the following questions: • Are the engines running? • What forces are there on the shuttle? • What happens when the shuttle re-enters the atmosphere?

  10. Balanced & resultant forces Force of table on box Force of table on box Force of table on box Force of table on box Fg weight Fg weight Fg weight Fg weight 2N friction 2 N applied 3 N friction 3 N applied 3 N friction 4 N applied Stationary: Stationary: Constant velocity: Already moving Resultant force – acceleration:

  11. Inertia A force is required to change the motion of an object. This means that objects resist a change to their motion. This ‘resistance to the change in motion’ of an object is known as its inertia. The bigger the mass of an object, the bigger its inertia. The greater the inertia, the more difficult it is to accelerate the object and the harder it is to stop it moving or change its direction of motion. If the baby and the baby seat are not properly strapped in, the baby will continue to move forward – as a result of its inertia – when the brakes are applied sharply by the driver.

  12. Inertia & mass Mass is the amount of matter contained by an object and depends upon the number of atoms and their size. Inertia is the tendency to resist change in an object’s motion. Both mass and inertia are scalar quantities and are measured in kg. They are equal to one another. A box having a mass of 5kg also has an inertia of 5kg. This equal arm balance would be used to measure the mass of an object. However, the inertia would be measured by establishing the acceleration an object would have when a certain force is applied to the object.

  13. Newton’s Third Law Consider this couple on ice skates. If the man pushes on the lady with a force of 10 N, she pushes back on him with a force of 10 N. Newton stated: To every action there is an equal but opposite reaction. It is easier to explain and understand as follows: N.B. Definition Newton’s Third Law: If a body A exerts a force on body B, then body B exerts a force on A of equal magnitude but opposite in direction.

  14. Newton’s Third Law w The parachutist is in ‘free fall’. His weight is 1000 N (mass = 100 kg) and the earth attracts him with a force of 1000 N. The parachutist also attracts the earth with a force of 1000 N, but since the earth is huge compared to the mass of the man, the man is seen to fall towards the earth. Ignoring air friction, the parachutist falls towards the earth at a rate of 10 m.s-2.

  15. Newton’s Third Law w Earth pulls orbiting shuttle down. Shuttle pulls earth upwards. If the weight of the shuttle is say 100 000 N, this means that the earth pulls the shuttle downwards with a force of 100 000 N. At the same time the shuttle also exerts a force of 100 000 N upwards on the earth. Whether the objects move relative to one another, depends upon the relative masses of the two objects. Here the mass of the earth is huge relative to the shuttle – so the earth is not seen to move up to the shuttle – rather the shuttle moves down to the earth.

  16. Newton’s Third Law A man pushes his vintage car with a force of 400 N. If this is the ‘action’ force, give the size and direction of the ‘reaction’ force. On what object would the reaction force act? If the maximum friction of the car is 300 N, will it accelerate? Explain. What prevents the man from being pushed backwards? Explain.

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