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Physics 101

Physics 101. Achievement Standard AS90183 Demonstrate understanding of mechanics in one dimension 5 Credits. Significant figures. Significant figures are important in physics and we need to understand what role 0 plays in this. Consider the following: 21000 240 0.00045

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Physics 101

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  1. Physics 101 Achievement Standard AS90183 Demonstrate understanding of mechanics in one dimension 5 Credits

  2. Significant figures Significant figures are important in physics and we need to understand what role 0 plays in this. Consider the following: 21000 240 0.00045 all these have the same number of sig figs: 2

  3. When zero is not a place holder then it becomes significant e.g. 23.00 0.330 22045 4 sig figs 3 sig figs 5 sig figs

  4. Rounding When rounding we need to be able to consider what is important and note when the zero is important or not: try these 25.34 round to 2 sf 65.68 round to 3 sf 0.4997 round to 3 sf

  5. If we have to find a solution based on some rounded figures, then this will affect our final answer. Consider: The answer can be no more accurate than the input data, thus 2 sf is the best we can do, giving 5.8 2 sf 3 sf

  6. Motion The motion of an object, such as a runner or a car, is described using quantities like the distance covered, the time taken, speed reached and the acceleration required to achieve such a speed. To define time and distance we generally use two well known and universal units.

  7. Motion • The Second (s), this is the S.I. unit for time.Time is given the symbol t in formulas. • The Metre (m), this is the S.I. unit for distance. Distance is given the symbol d in formulas

  8. Other units for distance include Km, mm, µm Other units for time include Min, hours and days

  9. Motion When we look at distance travelled in a given time we are investigating the speed of something.

  10. Motion We may further qualify speed Average speed:This is the speed calculated over a defined distance. Instantaneous speed:This is the speed at a particular point during a journey and can be described as the actual speed at a particular point in time. When the speed during a journey does not change then it can be described as a steady, uniform or constant speed.

  11. Motion By the very definition of speed as distance covered in a given period of time we define the formula for speed as:

  12. Motion The units used to measure speed are related to the way in which the speed value is calculated. For example if we measure the distance in kilometres (km) and time in hours (h) then speed will be defined in km per hour. Often in physics and science since we measure in metres and seconds, speed is quoted in metres per second. We write these units in a particular way: • km per hour = kmh -1 • metres per second = ms-1

  13. Motion Examples: Determine the average speed of a car traveling 128km between 2:15 pm and 4:15 pm one day. Distance = 128 km Time = 2 h

  14. Motion Example: Determine the average speed of a bike wheeling down a slope of length of 525m in 25 s. Distance = 525 m Time = 25 s

  15. Vectors • In physics we often deal with specific vector values. • Vectors have a size and a direction • We consider initially two vectors: • Displacement (distance travelled in a specific direction) • Velocity (speed in a specific direction)

  16. To calculate displacement we just need to pay attention to the distance travelled and the direction. Opposite directions have negative effects To calculate velocity we divide the displacement achieved during a period of time, by the time taken. The maths is the same as for speed.

  17. Distance Time (dt) graphs Distance time graphs display information regarding the distance movement of an object over a period of time. The gradient (slope) of the line of the graph describes the speed of the object.

  18. The slope of any graph is determined as: On a distance time graph, this becomes:

  19. Distance (m) Distance (m) Distance (m) Time (s) Time (s) Time (s) Object has a constant speed, faster than previous Object stationary, distance does not change over time Object has a constant speed

  20. Distance (m) Distance (m) Time (s) Time (s) When a d-t graph does not have a straight line then the object concerned is changing speed. Speeding up accelerating Slowing down decelerating

  21. Speed time graphs Speed time graphs are used to give a more detailed and an accurate picture of the changing speeds of an object during a journey The gradient (or slope) of the line of the graph gives the acceleration of the object.

  22. Spped (ms-1) Spped (ms-1) Spped (ms-1) Time (s) Time (s) Time (s) Constant speed Change speedAccelerating Changing speedDeceleration

  23. Area under S-T Graph We can determine the distance travelled by an object from the area under a speed time graph. Divide the area of the graph up into easily calculated areas (e.g. rectangles and triangles) Find the area of each subdivision and then sum all the areas together

  24. 6 Spped (ms-1) 4 2 A B C 0 0 5 10 15 20 Time (s) Area under S-T Graph • Example: Find areas of sections A, B and C

  25. Acceleration • Acceleration is the rate of change of speed of an object. • For example if the acceleration of an object is stated as 2 ms-2, then its speed is increasing by 2ms-1 every second.

  26. The formula to determine acceleration is defined by: Where v1 is the starting speed and v2 the finishing speed t1 is the start time and t2 the finish time If the value for acceleration is negative then the object is slowing down or decelerating

  27. Examples: • Find the acceleration of a car starting from rest at zero seconds and reaching a speed of 15ms-1 after 5 seconds. V1=0, v2=15, t1=0, t2=5

  28. Examples: • Find the acceleration of a skier going downhill. They start at 7ms-1 and increase their speed to 13ms-1 between 28s and 31s after starting down the slopes. V1=7, v2=13, t1=28, t2=31

  29. Force Diagrams We can represent the forces being applied to an object using what is called a force diagram. Arrows are used to represent the direction of a force and a value is written. Forces in opposing directions subtract from each other Force at right angles do not affect each other.

  30. Force Diagram Example 600 N 2000 N 40 N 600 N 2500 N 40 N Applied force represented with an arrow pointing in the direction of the force. We identify the value with a number of Newtons (N). Equal forces from opposite directions are said to be balanced forces.

  31. Illustrations

  32. 400N 400N 5000N 5000N Balanced forces • When the two forces applying in opposite directions are equal in size we say that the forces are balanced. • If we consider the situation below, what will be the resulting motion of the object?

  33. Newton’s laws • We can state that “an object will remain stationary or move at a steady speed in a straight line unless acted on by an unbalanced force” • This is known as Newton’s 1st law of motion. • We have already met his 2nd law, f=ma.

  34. Exploding trolley experiments • Place the trolley so that the plunger is facing into open space. Trigger the plunger and observe what happens. • Now place the trolley so that the plunger will strike a solid object, such as a wall. Trigger the plunger and observe what happens this time • Now place two trolleys so that the plunger will strike one of the trolleys while both trolley are free to move. Trigger the plunger and observe what happens this time.

  35. 700N 500N Mass of car 275Kg Direction of forces • Forces are applied in a direction and can result in movement of an object. • The direction of that movement will be in the direction of the resulting force • Thus find the resulting force value direction and associated acceleration for the diagram below.

  36. Newton’s laws Newton’s third law states: “for every action force there is an equal and opposite reaction force” This means that when we apply a force against something, like pushing against a wall then the wall pushes back against us.

  37. Identify the action and reaction forces in the following

  38. Pressure? What is pressure? The application of a force over a specific area. If the area is very small then the pressure will be very large. The formula to connect these is: Where P= pressure, F=force (N) and A=area (m2)

  39. Consider an elephant weighs a lot, but has a large foot pad. The force of its weight is spread out and the animal can stand easily on its feet all day. • A lady sometimes wears a stiletto heel shoe. Her weight will be significantly less than an elephant, but over the small area of the stiletto heel the pressure will be enormous. • Moral your foot will hurt if stood on by an elephant, but a stiletto will pierce your foot

  40. example What is the pressure experienced by a 15 Kg sledgehammer resting on the top of a tent peg size 2 cm x 1 cm. P=F/A, F = 15x 10 = 150 N A = 0.02 x 0.01 =0.0002m2 P = 150/0.0002 = 750000 Nm-2 ( or Pa)

  41. The pressure of the atmosphere around us is 100 000 Pa. How many Kg of atmosphere must be pressing down on each square metre of Earth? P=F/A, F=P.A= 100000Pa .1 m2 F= 100 000 N, F=mg, thus m = F/g = 100 000/10 = 10 000 kg (10 tonne)

  42. Gravitational force Gravity is a non–contact force that exists between two objects with a mass. The mass of the Earth is so big we state that an object is attracted to the Earth. An object with a big mass is attracted to the Earth by a bigger force. Thus weightlifters get a higher score for lifting a greater mass above their heads, it is more difficult.

  43. Mass and Weight Mass describes the amount of matter in an object and is measured in Kilograms (kg) Weight is a measure of the force due to gravity acting on an object. We define: W= m x g W=weight (N) m = mass (kg) g= gravitational pull in N/Kg

  44. Weight on different places On Earth the g= 10 N/kg, thus the weight of any object is 10 times the mass. Different planets have gravitational pulls. On Mars, Venus and the Moon it is less, on Jupiter, Saturn and the Sun it is greater.

  45. Weight on different places

  46. Weight on different places

  47. W d f Force and distance When a force is applied to an item we do work. Depending on how far the item is push and by what amount of force will determine the amount of work done. We summarise this by the following equation: W= f d Where W = work done in Joules (J) f = force applied in Newtons (N)d = distance travelled in metre (m)

  48. Examples • How much work is done when pulling a 25 Kg bag of scoria stones with 200N 15m. W=fxd, f= 200N, d=15mW=200 x 15 =3000J • A 100Kg car is pushed by a force of 145N, find: a) the acceleration of the car f=ma, f=145N, m=100kg145=100.a, a=145/100=1.45 ms-2 b) The work done to move the car 0.5 km W=fd, f =145N, d=500mW=145 x 500 = 72500J

  49. Watt is Power Power is a measure of how much work is done in a given period of time. As usual time is measured in seconds and work in Joules. Thus power is defined as the number of joules used per second. P = W/t The unit for power is the Watt (W) after James Watt and his work on the steam engine in 1770’s. Now do: Page 27 of blue book

  50. Potential energy There are various types of potential energy and we have discussed these previously: • Elastic • Chemical • Nuclear • Gravitational

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