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Unformatted text preview: UCSD Physics 10 Net Forces, Friction, Air Resistance
Let's get real UCSD Physics 10 Recall the Sliding Book Example Why do things not continue to move at constant velocity? Don't things strive to be at rest (I know I do)? If the sliding book slows down, what's the force responsible? How could I keep it moving at a constant velocity? Do I need to apply a force to keep it moving? Why? Spring 2008 2 UCSD Physics 10 Forces are Vectors so Directions are Important
Total Force Force #2 Force #1 Force #1 Total Force = 0 Force #2 Spring 2008 3 UCSD Physics 10 Newton's 3rd Law
For every action (force), there is an equal and opposite reaction (opposing force). Force on Newton by Einstein = Force on Einstein by Newton (But their accelerations need not be the same: Newton tries to outsmart Einstein by loading his own cart with lead bricks) Spring 2008 4 UCSD Physics 10 Friction is a Force Force on box by person Force on person by box Force on floor by box Force on box by floor It's the sum of all the forces that determines the acceleration. Every force has an equal & opposite partner. Spring 2008 5 UCSD Physics 10 Friction Mechanism Corrugations in the surfaces grind when things slide. Lubricants fill in the gaps and let things slide more easily. Spring 2008 6 UCSD Physics 10 Why Doesn't Gravity Make the Box Fall?
Force of Floor acting on Box Force from floor on box cancels gravity. If the floor vanished, the box would begin to fall. Force of Earth acting on Box (weight) Spring 2008 7 UCSD Physics 10 What's missing in this picture? Force on box by person Force on person by box Force on floor by box Force on box by floor A pair of forces acting between person and floor. Spring 2008 8 UCSD Physics 10 Wait: We cheated two slides back... When we drew the box and floor, with the "normal" force from the floor canceling the force of gravity, these weren't strictly force pairs but these are the two canceling forces on the box that result in zero acceleration of the box The real pairs have to involve the earth:
box floor Force Pairs: earth-box (grav) box-floor (contact) earth-satellite (grav)
earth satellite Spring 2008 9 UCSD Physics 10 Don't all forces then cancel? How does anything ever move (accelerate) if every force has an opposing pair? The important thing is the net force on the object of interest Force on box by person Net Force on box Force on box by floor Spring 2008 10 UCSD Physics 10 Some Questions Does friction always exert a force that tends to bring things to a halt? What does this say about the direction of the frictional force, relative to the velocity vector? What do you think would happen if we loaded lead bricks into the box? Would it become harder to slide? What are some ways to reduce frictional forces?
Spring 2008 11 UCSD Physics 10 Static and Sliding (Dynamic) Friction Static frictional force: when nothing is sliding Sliding frictional force: when surfaces are sliding Static frictional forces always greater than sliding ones Lubrication provides microscopic rollers between surfaces Spring 2008 12 UCSD Physics 10 Big Consequences! Spring 2008 (USGS) 13 UCSD Physics 10 "Normal" Forces and Frictional Forces
Reaction Force From Ramp Friction Force
Decompose Vector "Normal" means perpendicular Normal Force Weight of block Weight of block Friction Force = Normal Force (coefficient of friction) Ffriction = Fnormal Spring 2008 14 UCSD Physics 10 Coefficient of Friction
Material on Material s = static friction 0.6 0.1 0.022 0.4 0.9 0.8 k = kinetic friction 0.4 0.05 0.02 0.3 0.8 0.7 15 steel / steel add grease to steel metal / ice brake lining / iron tire / dry pavement tire / wet pavement Spring 2008 UCSD Physics 10 Stay on the road! What does it take to stay on the road around a curve? using s = 0.8 as average for tires on road, Ffriction = 0.8mg (Normal force is just mg on level surface) Fcurve = macurve = m v2/r where r is radius of curve, say 50 m (e.g., cloverleaf exit ramp) Got enough friction if Fcurve < Ffriction happens if v2 < 0.8gr, or v < 20 m/s = 44 m.p.h. Spring 2008 16 UCSD Physics 10 Air Resistance We're always "neglecting air resistance" in physics Can be difficult to deal with Affects projectile motion Friction force opposes velocity through medium Imposes horizontal force, additional vertical forces Terminal velocity for falling objects Dominant energy drain on cars, bicyclists, planes
Spring 2008 17 UCSD Physics 10 Drag Force Quantified With a cross sectional area, A (in m2), coefficient of drag of 1.0 (most objects), sea-level density of air, and velocity, v (m/s), the drag force is: Fdrag = 0.65Av2 Newtons Example: Bicycling at 10 m/s (22 m.p.h.), with projected area of 0.5 m2 exerts 32.5 Newtons requires Fv of power 325 Watts to maintain speed Spring 2008 18 UCSD Physics 10 "Free" Fall Terminal velocity reached when Fdrag = Fgrav (= mg) For 75 kg person subtending 0.5 m2,
vterm 50 m/s, or 110 m.p.h. which is reached in about 5 seconds, over 125 m of fall actually takes slightly longer, because acceleration is reduced from the nominal 9.8 m/s2 as you begin to encounter drag Free fall only lasts a few seconds, even for skydivers Spring 2008 19 UCSD Physics 10 Trajectories with Air Resistance Baseball launched at 45 with v = 50 m/s: Without air resistance, reaches about 63 m high, 254 m range With air resistance, about 31 m high, 122 m range Vacuum trajectory vs. air trajectory for 45 launch angle. Spring 2008 20 UCSD Physics 10 Air Trajectories, cont. Now v = 40 m/s, optimal angle (45 for vacuum, 40 for air): Optimal angle for 40 m/s in air gets 97 m range vs. 164 m for vacuum trajectory. Golf balls actually have optimal launch of 25-30, helped by lift forces associated with spin of the ball. Spring 2008 21 UCSD Physics 10 Summary and Assignments Every force has an equal, opposing force Friction opposes motion, requiring continued application of force to maintain constant velocity Air resistance produces terminal velocity, alters trajectories of projectiles (for the worse). Assignments Read Chapters 2,3,4,5 (see page selections on web) Read Chapter 7 HW 3: due Friday (4/25): Hewitt 2.E.22, 2.E.29, 2.E.33, 3.E.27, 3.P.3, 3.P.4, 3.P.10, 4.E.1, 4.E.6, 4.E.10, 4.E.30, 4.E.44, 4.P.1, 5.E.17, 5.P.2, 7.R. (4&7) (count as one), 7.R.16, 7.E.40, 7.P.2, 7.P.4 Spring 2008 22 ...
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