Lecture 3 net_force
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This document discusses forces, friction, and coefficients of friction. It explains that friction opposes motion and causes objects to slow down without a continuous application of force. It also discusses the normal force being perpendicular to surfaces and defining the coefficient of friction as the ratio of the friction force to the normal force. Examples are provided of coefficients of friction for different surface combinations and how friction allows objects to stay on roads when rounding curves at certain speeds due to the maximum static friction force.
Lecture 3 net_force
- 1. Net Forces, Friction, Coefficients of Friction Newton’s 3rd Law
- 2. Sliding Book Example 2 Why do things not continue to move at constant velocity? 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? Friction Friction A net force of zero No, Inertia keeps the object moving!
- 3. Forces are Vectors, Directions are Important 3 Force #1 Force #2 Force #1 Force #2 Total Force Total Force = 0
- 4. Newton’s 3rd Law 4 For every action , there is an equal and opposite reaction. Force on Newton by Einstein = Force on Einstein by Newton Newton tries to outsmart Einstein by loading his own cart with lead bricks)
- 5. Friction opposes Motion 5 Force on person by box Force on floor by box Force on box by floor It’s the sum of all forces that determines the type of motion. Force on box by person Acceleration What kind of motion is created by Unbalanced Net Force?
- 6. Friction due to the Surface 6 Corrugations in the surfaces grind when things slide. Lubricants fill in the gaps and let things slide more easily. How does the corrugated surface change Friction?
- 7. Why Doesn’t Gravity Make the Box Fall? 7 Force of Earth acting on Box (weight) Force of Floor acting on Box Force from floor on box and gravity = Net Force of zero. If the floor vanished, the box would begin to fall.
- 8. What forces are not shown? 8 Force on person by box Force on floor by box Force on box by floor Force on box by person The Normal Force and the Force of Gravity
- 9. 9 When we drew the box and floor, with the “normal” force and the force of gravity, these weren’t strictly force pairs forces on the box that result in a Net zero acceleration of the box The real pairs have to involve the earth: box floor earth satellite • Force Pairs: • earth-box (grav) • box-floor (contact) • earth-satellite (grav)
- 10. Questions 10 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?
- 11. Static and Sliding (Dynamic) Friction 11 Static frictional force: when nothing is sliding Sliding frictional force: when surfaces are sliding Static frictional forces > sliding frictional forces
- 12. Earth Quakes and Friction 12 (USGS)
- 13. “Normal” Forces and Frictional Forces 13 Weight of block Decompose Vector Normal Force Friction Force Weight of block Reaction Force From Ramp “Normal” means perpendicular Friction Force = Normal Force (coefficient of friction) Ffriction = Fnormal
- 14. Coefficient of Friction Material on Material s = static friction k = kinetic friction steel / steel 0.6 0.4 add grease to steel 0.1 0.05 metal / ice 0.022 0.02 brake lining / iron 0.4 0.3 tire / dry pavement 0.9 0.8 tire / wet pavement 0.8 0.7 14
- 15. Stay on the road! 15 What does it take to stay on the road around a curve? using s = 0.8 as average for tires on road, Ffriction = 0.8 mg (Normal force is just mg on level surface) Fcurve = macurve = mv2/r where r is radius of curve, say 50 m (e.g. cloverleaf exit ramp) Enough friction if Fcurve < Ffriction happens if v2 < 0.8gr, or v < 20 m/s = 44 m.p.h.
- 16. Air Resistance 16 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
- 17. Summary 17 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).