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Mechanics

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Mechanics

  1. 1. Unit types Physical Quantities – is one that can be measured or calculated and expressed in numbers
  2. 2. Scalar Quantities Scalar Quantities – is one with magnitude bit has no direction. (Think of a length of string, it has length but it has no direction) Examples of Scalar Quantities – length, mass, time, electric current, area, density, energy, electric charge/current, power and temperature
  3. 3. Vector Quantities Vector Quantities – one which has both magnitude and direction. (Think of acceleration of a car, it has magnitude but it has direction) Examples of Vector quantities – displacement, velocity, acceleration, force and momentum
  4. 4. Displacement Displacement – (s) is distance in a given direction. This is a vector quantity with a SI unit of meter (m). Think of your commute to school. Do you commute in a straight line from your home to school? Most of us must travel around roads that lead off track. Displacement is a straight line from point A to point B.
  5. 5. Velocity Velocity – (v or u) is the rate of displacement with respect to time. This is a vector quantity with SI unit of meter per second (ms-1). This is the speed in which an object can get from A to B in a straight in relation to time. Velocity = displacement time Average Velocity = initial velocity + final velocity time
  6. 6. Acceleration Acceleration – (a) is the rate of change of velocity with respect to time This is a vector quantity with an SI unit of meter per second squared (ms-2). Think of a fast sports car, when it starts of it does not immediately reach its maximum speed. It must climb or accelerate to this speed.
  7. 7. v = u + at s = ut + ½at² v² = u² + 2 ad u is the initial velocity, v is the final velocity, a is the acceleration,d is the ‘distance’ travelled, t is the time
  8. 8. Question 1 A body has an initial velocity of 8 ms-1 due west. Find its velocity after one minute if its acceleration is 1.5ms-2 W. 8
  9. 9. Question 2 A body has an initial velocity of 12.2 ms-1. Find its velocity after 3 minutes if its acceleration is 1.25 ms-2 W. 9
  10. 10. Question 3 A body accelerates from rest at a rate of 2.2 ms-2 for 9 seconds. Find(i) the initial velocity velocity of the body(ii) the distance travelled by the body after 9seconds. 10
  11. 11. Question 4 A runner accelerates from rest at the beginning of a race at a rate of 1.8 ms-2 for 5 seconds. Find (i) the initial velocity (ii) the final velocity after 5 seconds (iii) the distance ran in the time given. 11
  12. 12. Question 5 A body moves from rest for 4 seconds at an acceleration 6.4 ms-2. If its velocity remains constant for the next 8 seconds, and the body comes to rest after a further 10 seconds. Find(i) the initial velocity after the initial 4seconds.(ii) the distance travelled after the initial 4seconds 12
  13. 13. Question 6 A body begins to move from rest for 4 seconds, at an acceleration of 5ms-2. Its velocity remains constant for the next 8 seconds, and comes to rest after a further 7 seconds. Find(i) the velocity after the initial 4 seconds(ii) the distance after the initial 4 seconds.(iii) the distance travelled after thesubsequent 8 seconds(iv) the total distance travelled by the body. 13
  14. 14. Gravity Gravity: The force of attraction between any two bodies is directly proportional to the product of their masses, and inversely proportional to the square of the distance between them. Acceleration due to Gravity , ( g ) (i) Acceleration due to gravity , (g) means that free-falling bodies experience an acceleration towards the centre of the earth. g = 9.8 ms– 2 ( for all bodies, irrespective of their mass).
  15. 15. Gravity N.B.!!!! (a) If a body is moving from rest, its initial velocity is zero. (b) If a body falls vertically, its initial velocity is zero, and its acceleration is the acceleration due to gravity, g. => g = 9.8 ms– 2 . (c) If a body is travelling vertically upwards, its acceleration is – 9.8 ms– 2. (d) In calculations, use a formula with only one unknown.

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