Septima unidad (segunta ley de newton

Chapter 7 Newton’s Second Law

Physics, 6th Edition

Chapter 7. Newton’s Second Law
Newton’s Second Law
7-1. A 4-kg mass is acted on by a resultant force of (a) 4 N, (b) 8 N, and (c) 12 N. What are the resulting accelerations? (a) 1 m/s2 (b) 2 m/s2 (c) 3 m/s2

7-2. A constant force of 20 N acts on a mass of (a) 2 kg, (b) 4 kg, and (c) 6 kg. What are the resulting accelerations? (a) 10m/s2 (b) 5 m/s2 (c) 3.33 m/s2

7-3. A constant force of 60 lb acts on each of three objects, producing accelerations of 4, 8, and 12 N. What are the masses? 15 slugs 7.5 slugs 5 slugs

7-4. What resultant force is necessary to give a 4-kg hammer an acceleration of 6 m/s2? F = ma = (4 kg)(6 m/s2); F = 24 N

7-5. It is determined that a resultant force of 60 N will give a wagon an acceleration of10 m/s 2. What force is required to give the wagon an acceleration of only 2 m/s2? ; F = ma = (6 slugs)(2 m/s2); F = 12 N

7-6. A 1000-kg car moving north at 100 km/h brakes to a stop in 50 m. What are the magnitude and direction of the force? Convert to SI units: 100 km/h = 27.8 m/s

F = ma = (1000 kg)(7.72 m/s2); 65

F = 772 N, South.

Chapter 7 Newton’s Second Law The RelationshipBetween Weight and Mass

Physics, 6th Edition

7-7. What is the weight of a 4.8 kg mailbox? What is the mass of a 40-N tank? W = (4.8 kg)(9.8 m/s2) = 47.0 N ; = 4.08 kg

7-8. What is the mass of a 60-lb child? What is the weight of a 7-slug man? = 1.88 slugs ; W = (7 slugs)(32 ft/s2) = 224 lb

7-9. A woman weighs 180 lb on earth. When she walks on the moon, she weighs only 30 lb. What is theacceleration due to gravity on the moon and what is her mass on the moon? On the Earth? Her mass is the same on the moon as it is on the earth, so we first find the constant mass: mm = me = 5.62 slugs ;

Wm = mmgm

;

gm = 5.33 ft/s2

7-10. What is the weight of a 70-kg astronaut on the surface of the earth. Compare the resultant force required to give him or her an acceleration of 4 m/s2on the earth with the resultant force required to give the same acceleration in space where gravity is negligible? On earth: W = (70 kg)(9.8 m/s2) = 686 N ; Anywhere: FR = 280 N FR = (70 kg)(4 m/s2) = 280 N

The mass doesn’t change.

7-11. Find the mass and the weight of a body if a resultant force of 16 N will give it an acceleration of 5 m/s2. = 3.20 kg ; W = (3.20 kg)(9.8 m/s2) = 31.4 N66

Chapter 7 Newton’s Second Law

Physics, 6th Edition

7-12. Find the mass and weight of a body if a resultant force of 200 lb causes its speed to increase from 20 ft/s to 60 ft/s in a time of 5 s. = 25.0 slugs W = mg = (25.0 slugs)(32 ft/s2); W = 800 lb

7-13. Find the mass and weight of a body if a resultant force of 400 N causes it to decrease its velocity by 4 m/s in 3 s. ; W = mg =(300 kg)(9.8 m/s2); W = 2940 N m = 300 kg

Applications for Single-Body Problems: 7-14. What horizontal pull is required to drag a 6-kg sled with an acceleration of 4 m/s2 if a friction force of 20 N opposes the motion? P – 20 N = (6 kg)(4 m/s2); P = 44.0 N
6 kg 20 N

P

7-15. A 2500-lb automobile is speeding at 55 mi/h. What resultant force is required to stop the car in 200 ft on a levelroad. What must be the coefficient of kinetic friction? We first find the mass and then the acceleration. (55 mi/h = 80.7 m/s)

F = ma = (78.1 slugs)(-16.3 ft/s2);
k

F = -1270 lb = 0.508

67

Chapter 7 Newton’s Second Law

Physics, 6th Edition

7-16. A 10-kg mass is lifted upward by a light cable. What is the tension in the cable if the + acceleration is (a) zero, (b) 6 m/s2upward, and (c) 6 m/s2 downward? T Note that up is positive and that W = (10 kg)(9.8 m/s2) = 98 N. (a) T – 98 N = (10 kg)(0 m/s (b) T – 98 N = (10 kg)(6 m/s (c) T – 98 N = (10 kg)(-6 m/s and and T = 98 N T = 60 N + 98 N or T = 158 N T = 38.0 N
10 kg

W = mg

and T = - 60 N + 98 N or

7-17. A 64-lb load hangs at the end of a rope. Find the acceleration of the load if the tension in the cable is...