Drag Force. Drag is a mechanical force generated when a solid moves through a fluid. Is Air fluid?

Transcription

1 Feline Pesematology

2 Drag Force Drag is a mechanical force generated when a solid moves through a fluid. Is Air fluid?

3 Drag factors Does drag increase/decrease with 1. Density of fluid? 2. Velocity of the solid? 3. Area of the solid?

4 where A is your cross-sectional area, v is your speed, ρ is the density of air (1.29 kg/m 3 ) and the drag coefficient C describes how slippery (aerodynamic) an object is.

5 The net force on you as you fall is Velocity increases due to gravity, F net = ma => acceleration depends of v => NOT a FREE- FALL anymore.

6 This means velocity can possibly take a value such that F net =0 This velocity is called terminal velocity which is a constant

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8 (0.5)

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10 How does the Cat flip itself to land right way?

11 Determine the velocity of a falling body as a function of time when 1. drag is directly proportional to speed

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13 Uniform Circular Motion

14 Condition for circular motion A body needs a Centripetal force to make a curved motion or move in a circle If you identify the nature of this Force, the calculations become easy.

15 Centripetal acceleration a c = v 2 /r Þ Centripetal Force F c = ma c = mv 2 /r

16 V V F V V

17 Examples

18 Examples String tied to a string What is the centripetal Force? Centripetal Force provided by the Tension Force

19 What happens to tension in this case?

20 Examples Moon going around earth What is the centripetal Force? Centripetal Force provided by the Gravitational Forces between earth and the moon

21 Examples Ball rolling through a hole What is the centripetal Force? Centripetal Force provided by the Normal Force

22 Examples Car going along a curve What is the centripetal Force?

23 Equations for a Car on curve The centripetal force is provided by the Frictional force specifically by Static Friction F s

24 Car having a tendency to slide down because of lower than required speed

25 Car moving faster than required and going up

26 - When a car turns to the left why do you slide to the right?

27 If you were an astronaut on a spaceship going around earth, would you slide outwards and cling on to the wall of the spaceship? Why/ Why not?

28 Problem - UCM Curved portions of highways are always banked (tilted) to prevent cars from sliding off the highway. When a highway is dry, the frictional force between the tires and the road surface may be enough to prevent sliding. When the highway is wet, however, the frictional force may be negligible, and banking is then essential. Figure represents a car of mass m as it moves at a constant speed v of 20 m/s around a banked circular track of radius R = 190 m. (It is a normal car, rather than a race car, which means any vertical force from the passing air is negligible.) If the frictional force from the track is negligible, what bank angle θ prevents sliding? (Ans - 12deg)

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31 Problems - Drag A raindrop with radius R = 1.5 mm falls from a cloud that is at height h = 1200 m above the ground. The drag coefficient C for the drop is Assume that the drop is spherical throughout its fall. The density of water ρw is 1000 kg/m 3, and the density of air ρa is 1.2 kg/m 3. 1) The raindrop reaches terminal speed after falling just a few meters. What is the terminal speed? (27 Km/hr) 2) What would be the drop's speed just before impact if there were no drag force? ( 550 km/hr)