 What happens if you do the following? › Take your Physics textbook and put it on a scale in the following orientations:  On its front?  On its back?  On its spine?  On its side?  On its top?  On its bottom? 2

 What happens if you do the following? › Take your Physics textbook and put it on your hand in the following orientations:  On its front?  On its back?  On its spine?  On its side?  On its top?  On its bottom? 3

 Why is this the case?  Has the force changed?  What has changed? 4

 This is pressure – the application of a force over a given amount of area. 5

 The units of pressure in the SI System are:  Videodisc Side 1 Chap. 28 6

 Galileo showed that falling objects accelerate equally regardless of mass, as long as air resistance is negligible.  For example a 15 kg rock will fall at essentially the same rate as a 4 kg shot put.  Galileo demonstrated this at the Leaning Tower of Pisa, destroying Aristotle’s idea that the rock should fall about 4 times faster. 7

 Remember mass ⍺ weight (W=mg)  So:  Or the acceleration of the rock and shot put are equal. 8

 Where W is the weight (force) of the rock and m is the mass of the rock.  W is the weight of the shot put and m is the mass of the shot put.  The ratio of the weight to the mass is the same for both objects, therefore their acceleration will be the same. 9

 All objects undergo the same acceleration at the same place on Earth. 10

 So, if there is no air resistance, a feather and a kg of lead will fall at the same rate, because their ratio of weight to mass is the same.  Videodisc – Side 1 Chapter 25 11

 The feather and the lead will not fall at the same rate when you take into account air resistance.  The feather has significantly higher air resistance than does lead.  So the net force acting on the lead is changed slightly by air resistance.  The net force acting on the feather is changed significantly by air resistance. 12

 The feather quickly reaches the point where the air resistance will equal the weight.  At this point the net force is 0 and there is no further acceleration.  When there is no additional acceleration on the feather, it has reached terminal speed or terminal velocity. 13

 There is little effect of air resistance on the lead.  The acceleration is only slightly diminished by air resistance.  It will take much longer for the air resistance on the lead to build up enough to be the same as its weight.  Again at this point which will be a significant velocity, lead has reached its terminal velocity. 14

 The terminal velocity of a sky diver will range from 150 km/hr to 200 km/hr dependent on the diver’s weight and body orientation.  What orientation(s) would give the highest terminal velocity?  The slowest?  Will a heavier diver reach a higher terminal velocity than a lighter diver in the same orientation? 15

 With the increased air resistance of a parachute, the diver will slow to between 15 km/hr and 20 km/hr.  The higher the speed the more that air resistance increases. 16

 When Galileo dropped objects off the Leaning Tower of Pisa, the really did not hit at the same time, but the difference was to small for him to detect.  Newton finally explained this effect with the concept of air resistance.  Videodisc – Side 1 Chapter 27 17