Presentation on theme: "Drag Force and Terminal Velocity"— Presentation transcript:
1 Drag Force and Terminal Velocity Apparent WeightDrag Force and Terminal Velocity
2 Apparent WeightOur perception of weightlessness is really the absence or reduction of normal (contact) forces acting upon our body.Conversely, we can feel heavier if the normal (contact) forces acting on us are larger than normal.maxair at cedar point – YouTubeDrop Tower at KI – YouTubeME ON DROP ZONE - YouTube
3 Apparent Weight in an Elevator Now suppose you stand on the bathroom scale and ride an elevator up and down. As you are accelerating upwards and downwards, the scale reading is different than when you are at rest and traveling at constant speed.BWhich situation does Anna feel the heaviest? Which situation does she feel the lightest?D
4 Apparent Weight in an Elevator - Concepts There is no change in motion so the upwards push on your feet must be equal and opposite to your weight – you feel normal weightABYour body wants to remain at rest but the elevator starts to move upwards. This imparts more upwards force on your feet than your weight – you feel heavierCA body at rest wants to stay at rest so when the elevator starts to move down there is less force pushing on your feet – you feel lighter.DFree Fall – You feel weightless
5 Apparent Weight in an Elevator Coming DownFNETaaFNETElevator starts accelerating downwardsFNET = m a (down)FN < FgElevator at restZero AccelerationFNET = 0FN = FgElevator at constant speed downwardsZero AccelerationFNET = 0FN = FgElevator is slowing down to a stop while moving downFNET = m a (up)FN > FgThe normal Force (orange) is what she feels
6 Apparent Weight in an Elevator Coming Down - TheoryFNETaaFNETFNET = 0FN = Fg = mgFNET = m a (down)FNET = 0FN = Fg = mgFNET = m a (up)FNET = Fg - FNFNET = FN - FgNormal Weightmg - FN = maFN -mg = maNormal WeightFN = mg - maFN = ma + mgFN = m(g - a)FN = m(g + a)Lower WeightHeavier Weight
7 Apparent Weight in an Elevator Going UpaaFNETFNETElevator at restZero AccelerationFNET = 0FN = FgElevator starts accelerating upwardsFNET = m a (up)FN > FgElevator at constant speed upwardsZero AccelerationFNET = 0FN = FgElevator is slowing down to a stop while moving upFNET = m a (down)FN < Fg
8 Apparent Weight in an Elevator Going UpaaFNETFNETFNET = m a (up)FNET = 0FN = Fg = mgFNET = 0FN = Fg = mgFNET = m a (down)FNET = FN - FgFNET = Fg - FNFN -mg = maNormal Weightmg - FN = maFN = ma + mgNormal WeightFN = mg - maFN = m(g + a)FN = m(g - a)Heavier WeightLower Weight
9 Going up and Down In General: If your apparent weight is lessF = m (g – a)If your apparent weight is moreF = m (g +a)When the contact force is less your apparent weight is less, when the contact force is more your apparent weight is more.
10 Terminal Velocity Consider a skydiver: At the start of his jump the air resistance is small so he accelerates downwards.2) As his speed increases his air resistance will increase.3) Eventually the air resistance will be big enough to equal the skydiver’s weight. At this point the forces are balanced so his speed becomes constant - this is called TERMINAL VELOCITY
11 Terminal Velocity Consider a skydiver: 4) When he opens his parachute the air resistance suddenly increases, causing him to start accelerating upward.5) Because he is slowing down his air resistance will decrease again until it balances his weight. The skydiver has now reached a new, lower terminal velocity.For a typical person, terminal speed is 60 m/s or approximately 135 miles per hour. A person has to fall over 400 yards before you really need to start taking this into account.
12 Velocity-time graph for terminal velocity… Parachute opens – diver slows downVelocitySpeed increases…Terminal velocity reached…On the MoonDiver hits the groundNew, lower terminal velocity reachedTime
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