Page 87 - Schaum's Outline of Theory and Problems of Applied Physics

72 FRICTION [CHAP. 6

(a) 500 N (c) 750 N
(b) 600 N (d ) 1500 N

6.8. The coefficient of static friction between a car’s tires and a level road is 0.80. If the car is to be stopped in a maximum
time of 3.0 s, its speed cannot exceed
(a) 2.4 m/s (c) 7.8 m/s
(b) 2.6 m/s (d ) 23.5 m/s

6.9. A force applied to a 50-kg box on a level floor is just enough to start it moving. The coefficients of static and kinetic
friction are, respectively, 0.5 and 0.3. If the same force continues to be applied to the box, it will have an acceleration
of approximately

(a) 2 m/s 2 (c) 4 m/s 2
(b) 3 m/s 2 (d ) 5 m/s 2

6.10. After descending a slope, a skier coasts on level snow for 20 m before coming to a stop. If the coefficient of friction
between skis and snow is 0.05, the skier’s speed at the foot of the slope was
(a) 3.1 m/s (c) 6.3 m/s
(b) 4.4 m/s (d ) 19.6 m/s

Supplementary Problems

6.1. Ships are often built on ways that slope down to a nearby body of water. Often a ship is launched before most of its
interior and superstructure have been installed, and is completed when afloat. Is this done because the added weight
would cause the ship to slide down the ways prematurely?

6.2. A box is being pushed across a floor. Can the frictional force on the box ever exceed its own weight? Can the frictional
force ever exceed the applied force?

6.3. An 80-lb wooden crate rests on a horizontal wooden floor. If the coefficient of static friction is 0.5, how much force
is needed to set the crate in motion?

6.4. A force of 300 N is sufficient to keep a 100-kg wooden crate moving at constant velocity across a wooden floor. What
is the coefficient of kinetic friction?

6.5. A force of 1000 N is applied to a 1200-kg car. If the coefficient of rolling friction is 0.04, what is the car’s acceleration?

6.6. The coefficients of static and kinetic friction for stone on wood are, respectively, 0.5 and 0.4. If a 150-kg stone statue is
pushed with just enough force to start it moving across a wooden floor and the same force continues to act afterward,
find the statue’s acceleration.
6.7. A car whose brakes are locked skids to a stop 70 m from an initial velocity of 80 km/h. Find the coefficient of kinetic
friction.
6.8. A driver sees a horse on the road and applies the brakes so hard that they lock and the car skids to a stop in 24 m. The
road is level, and the coefficient of kinetic friction between tires and road is 0.7. How fast was the car going when the
brakes were applied?

6.9. A truck moving at 100 km/h carries a steel girder that rests on its wooden floor. What is the minimum time in which
the truck can come to a stop without the girder moving forward? The coefficient of static friction between steel and
wood is 0.5.