Text questions that are similar to the ones below:
P: 4.33, 4.37, 4.40, 4.41, 4.46, 4.55, 4.56, 4.60, 4.64
Conceptual Questions:
1. Describe a two examples in which the force of friction exerted on an object is in the direction of motion of the object (unlike the case of a sliding box on a ramp).
2. Why should the friction force be proportional to the normal force, rather than the weight?
3. (A bit more deep...) According to the given formulae for friction on sliding surfaces, the only important parts are the normal force and the coefficient of friction. So, if we have a rectangular box on a ramp, the orientation of the box is apparently unimportant. If the box has 3 different dimensions a x b x c, and so has sides with three different surface areas, it doesn't matter which side is on the ramp, or which way the box faces as it slides. Why not? Give an example where the orientation is important.
Numerical Problems:
1. Two blocks joined by a string have masses of 6 and 9 kg. They rest on a frictionless horizontal surface. A 2nd string, attached only to the 9 kg block, has horizontal force = 30 N applied to it. Both blocks accelerate. Find the tension in the string between the blocks. (P-4.55)
2. A 10 kg block and a 2.0 kg block are connected by a light string over a massless, frictionless pulley. If g = 9.8 m/s2, what is the acceleration of the system when released? (P-4.55)
3. A boxcar of mass 200 tons at rest becomes uncoupled on a 10° grade. If the track is considered to be frictionless, what velocity does the boxcar have after 10 seconds? (P-4.37)
ans: 17 m/s
4. A trapeze artist, with swing, weighs 800 N; he is momentarily held to one side by his partner so that the swing ropes make an angle of 30° with the vertical. In such a condition of static equilibrium, what is the horizontal force being applied by the partner? (P-4.56)
5. A sled weighs 100 N. It is held in place on a frictionless 20 degree slope by a rope attached to a stake at the top; the rope is parallel to the slope. Find the tension in the rope. (P-4.37)
ans: 34 N
6. A 500 N tightrope walker stands at the center of the rope such that each half of the rope makes an angle of 10° with the horizontal. What is the tension in the rope? (P-4.46)
7. Hector drives a pickup truck horizontally at 15 m/s. He is transporting a crate of delicate lead crystal. If the coefficient of static friction between the crate and the truckbed is 0.40, what is the minimum stopping distance for the truck so the crate will not slide? (P-4.33)
ans: 29 m
8. A worker pulls a 200 N packing crate at constant velocity across a rough floor by exerting a force F = 55 N at an angle of 35° above the horizontal. What is the coefficient of kinetic friction of the floor? (P-4.60)
9. A hockey puck moving at 7 m/s coasts to a halt in 75 m on a smooth ice surface. What is the coefficient of friction between the ice and the puck? (P-4.64)
10. The coefficient of static friction between the tires of a car and the street is µs = 0.77. What is the steepest angle of a street on which a car can be parked (with wheels locked) without slipping? (P-4.41)
11. A 9 kg hanging weight is connected by a string over a pulley to a 5 kg block sliding on a flat table. If the coefficient of sliding friction is 0.2, find the tension in the string. (P-4.55)
12. A 250 kg crate is placed on an adjustable inclined plane. If the crate slides down the incline with an acceleration of 0.7 m/s2 when the incline angle is 25°, then what should the incline angle for the crate to slide down the plane at constant speed? (P-4.40)
ans: 21°