复旦大学：《大学物理》课程教学资源（PPT课件，英文）Chapter 5 Applications of Newton’s Law

Chapter 5 Applications of Newton's law Sec 5-1 Force Laws Sec 5-2 Tension and normal forces Sec, 5-3 Friction forces Sec. 5-4 The dynamics of uniform circular motion x Sec. 5-5 Time-dependent force Sec. 5-6 Noninertial frames and pseudoforces Sec 5-7 Limitations of Newton's law

Chapter 5 Applications of Newton’s Law Sec. 5-1 Force Laws Sec. 5-2 Tension and normal forces Sec. 5-3 Friction forces Sec. 5-4 The dynamics of uniform circular motion Sec. 5-5 Time-dependent force Sec. 5-6 Noninertial frames and pseudoforces Sec. 5-7 Limitations of Newton’s law ★  ★ ★

Sec. 5-1 Force Laws Physicists have traditionally identified four basic forces (1)tne cirayiracional force (2)the elecirormacnetic force (3)the weak nuclear force, which causes certain radioactive decay processes and certain reactions among the fundamental particles (4)the strong force, which operates among the fundamental particles and is responsible for binding the nucleus together

Sec. 5-1 Force Laws Physicists have traditionally identified four basic forces: (1) the gravitational force (2) the electromagnetic force (3) the weak nuclear force, which causes certain radioactive decay processes and certain reactions among the fundamental particles. (4) the strong force, which operates among the fundamental particles and is responsible for binding the nucleus together

Two priors in Typical nucleus, for example, the relative strength of these forces would be strong(relative strength =1) electromagnetic( 10 weak(10); gravitational (103) In fact, everything we study about ordinary mechanical systems involves only two force cIralviry and elecirorgacnetisrn Tension forces Normal forces Friction forces

Two protons in typical nucleus, for example, the relative strength of these forces would be: strong (relative strength = ); electromagnetic( ); weak ( ); gravitational ( ). 2 10 − 38 10 9 − 10− 1 In fact, everything we study about ordinary mechanical systems involves only two force: gravity and electromagnetism.  Tension forces Friction forces Normal forces

Sec. 5-2 Tension and normal forces (张力与压力 )Tension force(such as in a stretched rope or string), arises because each small element of the string pulls on the element next to it If the mass of the rope is negligible, the values of the force exerted on the two ends of the rope must be nearly equal to each other

Sec. 5-2 Tension and normal forces (张力与压力) (1) Tension force (such as in a stretched rope or string), arises because each small element of the string pulls on the element next to it. m If the mass of the rope is negligible, the values of the force exerted on the two ends of the rope must be nearly equal to each other

(2)Normal force: Just like tension force the normal force is also contact force Both tension and normal forces originate with the atoms of each body ---each atom exerts a force on its neighbor. They belong to electromagnetic forces

(2) Normal force : Just like tension force, the normal force is also contact force. Both tension and normal forces originate with the atoms of each body --- each atom exerts a force on its neighbor. They belong to electromagnetic forces. N'  − N 

Sample problen 5-7 In a system, a block (of mass my= 9.5Kg) slides on a frictionless plane inclined at an angle 0=34. The block is attached by a string to a second block(of mass m2=2.6 Kg). The system is released from rest, Find the acceleration of the blocks and the tension in the string mA m2

5-7 In a system, a block (of mass m1 = 9.5 Kg) slides on a frictionless plane inclined at an angle . The block is attached by a string to a second block (of mass m2=2.6 Kg). The system is released from rest. Find the acceleration of the blocks and the tension in the string. Sample problem:  = 34 m2

Sec. 5-3 Friction forces x Friction is the force that oppose(反抗)the relative motion or the trend of relative motion of two solid surfaces in contact static friction Friction sliding friction V kinetic friction rolling Triction 1) The forces of static friction(静摩擦力) The frictional forces acting between surfaces at rest with respect to each other

Sec. 5-3 Friction forces ★ Friction is the force that opposes (反抗) the relative motion or the trend of relative motion of two solid surfaces in contact Friction static friction kinetic friction sliding friction rolling friction √ ➢1) The forces of static friction (静摩擦力) The frictional forces acting between surfaces at rest with respect to each other

Friction force can be measured by following exot. Fig 5-12 Friction force N∠ Accele rated motion k Unifo motion No mg sMax motion 22 mg fs F 0 0 8121620 g (g) rest moving The maximum force of static friction will be the same as the smallest applied forces necessary to start motion

The maximum force of static friction will be the same as the smallest applied forces necessary to start motion. Fig 5-12 Friction force fsMax rest moving Friction force can be measured by following expt. k f  s f 

The maximum force of static friction fsM between any pair of dry unlubricated surface follows these two empirical laws (1)It is approximately independent of the area of contact surfaces 2)It is proportional to the normal force s Max (57) Where N the magnitude of the normal force, us the coefficient of static friction s Max the maximum force of static friction f≤∫ S Max

The maximum force of static friction between any pair of dry unlubricated surface follows these two empirical laws : (1) It is approximately independent of the area of contact surfaces. (2) It is proportional to the normal force (5-7) where N the magnitude of the normal force, the coefficient of static friction, the maximum force of static friction. s s s Max f N =  sMax f sMax f s sMax f f 

2) The force of kinetic friction(动摩擦力, (滑动)) (5-8 Where uk is the coefficient of kinetic friction Usually, for a given pair of surfaces us >Lp The actual value of us and uk depend on the nature of both the surfaces in contact

➢ 2)The force of kinetic friction(动摩擦力, （滑动）): (5-8) where is the coefficient of kinetic friction. f k = k N k k  s   k  s Usually, for a given pair of surfaces . The actual value of and depend on the nature of both the surfaces in contact