# Short Version : 5. Newton's Laws Applications

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Short Version : 5. Newton's Laws Applications

Example 5.3. Restraining a Ski Racer
A starting gate acts horizontally to restrain a 60 kg ski racer on a frictionless 30 slope. What horizontal force does the gate apply to the skier? since y n x : y : x Fh Fg

Alternative Approach Net force along slope (x-direction) : y n   Fh
Fg

5.2. Multiple Objects  Example 5.4. Rescuing a Climber
A 70 kg climber dangles over the edge of a frictionless ice cliff. He’s roped to a 940 kg rock 51 m from the edge. What’s his acceleration? How much time does he have before the rock goes over the edge? Neglect mass of the rope.

Tension T = 1N throughout

5.3. Circular Motion Uniform circular motion 2nd law: centripetal

At what angle should a road with 200 m curve radius be banked for travel at 90 km/h (25 m/s)? y x : y : n x a Fg

Example 5.7. Looping the Loop
Radius at top is 6.3 m. What’s the minimum speed for a roller-coaster car to stay on track there? Minimum speed  n = 0

Conceptual Example 5.1. Bad Hair Day
What’s wrong with this cartoon showing riders of a loop-the-loop roller coaster? From Eg. 5.7: n  m g =  m a =  m v2 / r ( a  g ) Consider hair as mass point connected to head by massless string. Then T  m g =  m a where T is tension on string. Thus, T = m ( g  a )  ( downward ) This means hair points upward ( opposite to that shown in cartoon).

Frictional Forces Pushing a trunk:
Nothing happens unless force is great enough. Force can be reduced once trunk is going. Static friction s = coefficient of static friction Kinetic friction k = coefficient of kinetic friction k : < 0.01 (smooth), > 1.5 (rough) Rubber on dry concrete : k = 0.8, s = 1.0 Waxed ski on dry snow: k = 0.04 Body-joint fluid: k = 0.003

Example 5.11. Dragging a Trunk
Mass of trunk is m. Rope is massless. Kinetic friction coefficient is k. What rope tension is required to move trunk at constant speed? y y : x : n T fs x Fg

Rolling wheel:

Skidding wheel 滑動的輪子 : kinetic friction 動摩擦 k  0.8 Rolling wheel 滾動的輪子 : static friction 靜摩擦 s  1 Rolling friction 滾動摩擦 r  0.01

Dynamics of Wheels F fr fs

Example Stopping a Car k & s of a tire on dry road are 0.61 & 0.89, respectively. If the car is travelling at 90 km/h (25 m/s), determine the minimum stopping distance. the stopping distance with the wheels fully locked (car skidding). (a)  = s : (b)  = k :

Steering Car turning to the left. Bicycle turning to the left.
More details

Example 5.9. Steering A level road makes a 90 turn with radius 73 m.
What’s the maximum speed for a car to negotiate this turn when the road is (a) dry ( s = 0.88 ). (b) covered with snow ( s = 0.21 ). (a) (b)

5.5. Drag Forces Drag force: frictional force on moving objects in fluid. Depends on fluid density, object’s cross section area, & speed. Terminal speed: max speed of free falling object in fluid. Parachute: vT ~ 5 m/s. Ping-pong ball: vT ~ 10 m/s. Golf ball: vT ~ 50 m/s. Sky-diver varies falling speed by changing his cross-section. Drag & Projectile Motion

Simple Machines