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制作 张昆实 Yangtze University

Bilingual Mechanics. Chapter 3 Force. 制作 张昆实 Yangtze University. 制作 张昆实 Yangtze University.

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制作 张昆实 Yangtze University

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  1. BilingualMechanics Chapter 3 Force 制作 张昆实 Yangtze University 制作 张昆实 Yangtze University

  2. Chinese astronauts Jing Haipeng(L), Zhai Zhigang(C) and Liu Boming wave hands during a press conference in Jiuquan Satellite Launch Center (JSLC) in Northwest China's Gansu Province, September 24, 2008. The Shenzhou VII spaceship will blast off Thursday evening from the JSLC to send the three astronauts into space for China's third manned space mission.

  3. China´s manned spacecraft Shenzhou-7 blasts off

  4. China´s manned spacecraft Shenzhou-7 blasts off

  5. Chinese taikonauts report they feel "physically sound"

  6. Astronauts assemble EVA suit for spacewalk

  7. China successfully launched its third manned spacecraft on Thursday with three astronauts on board to attempt the country‘s first-ever space walk. The spaceship Shenzhou-7 blasted off on a Long March II-F carrier rocket from the Jiuquan Satellite Launch Center in the northwestern Gansu Province at 9:10 p.m. after a breathtaking countdown to another milestone on China's space journey.

  8. Onboard pilots Zhai Zhigang, Liu Boming and Jing Haipeng are expected to orbit the earth for three days, when one of them will float out of the cabin about 343 kilometers above the earth. When they make it, China will become the third country in the world who is able to conduct extravehicular activity (EVA) in space following the former Soviet Union and the United States. The spaceship is scheduled to land in the central region of north China's Inner Mongolia Autonomous Region after completing the task.

  9. Congratulations to the successful launching of the Shenzhou-7 ! The fundamental principles of space flight is Mechanics ! Physics is the cradle of modern science and technology !

  10. BilingualMechanics Chapter 3 Force 制作 张昆实 Yangtze University 制作 张昆实 Yangtze University

  11. Chapter 3 Force 3-1 What Is Physics? 3-2 Newtonian Mechanics 3-3 Newton’s First Law 3-4 Force 3-5 Mass 3-6 Newton’s Second Law 3-7 Newton’s Third Law

  12. Chapter 3 Force 3-8 Applying Newton’s Laws 3-9 Some Particular Forces 3-10 Friction 3-11 The Drag Force and Terminal Speed 3-12 Uniform Circular Motion *3-13 Noninertial Reference Systemand Inertial Forces

  13. 3-1 What Is Physics （Kinematics） ★ We have discussed how motion is described in terms of velocity and acceleration. ★ Now wedeal withthe question ofwhy objects move as they do: What makes an object at rest begin to move? What causes a body to accelerate or decelerate? What is involved when an object moves in a circle? ★ In this chapter we learnwhat is physics through investigating the connection between force and motion, which is the subject called dynamics.

  14. 3-2 Newtonian Mechanics . ★ The relationship between a force and the acceleration it causes was first understood by Isaac Newton Newton published his book 《The Mathematical Principles of Natural Philosophy》in 1687. ★The study of that relation is called Newtonian Mechanics

  15. ★ Special theory of relativity (for very high speed speed of light) 3-2 Newtonian Mechanics Modifications are necessary: ★Quantum mechanics (for the scale of atomic structure) Newtonian Mechanics is viewed asaspecial case ofthese two more comprehensive theories.

  16. 3-3 Newton’s First Law ★ Observations: Send a puck sliding over a extremely slippery surface, over which the puck would hardly slow. ★Conclusion: a body will keep moving with constant velocity if no force acts on it. ★ Newton’s First Law: If no force acts on a body, then the body’s velocitycannot change; that is, the body cannot accelerate. at rest remain at rest; in motion move with constant velocity.

  17. 3-4 Force ★ Define the unit of force in terms of the acceleration that a force gives to a standard reference body (a mass of 1 kg). ★ A force (vector) is measured by acceleration it produces. magnitude; direction ★Principle of superposition for forces When two or more forces act on a body, a net force or resultant force can be found. The net force has the same effect on the body as all the individual forces together.

  18. F 0 = net 3-4 Force ★Newton’s First Law (restate): If nonet forceacts on a body( ),then the body’s velocity cannot change; that is, the body cannot accelerate. ★Inertial Reference Frames An inertial reference frame is one in which Newton’s laws hold. Example: the ground, any reference frame moving with constant velocity with respect tothe ground. ★Noninertial frame:a accelerating frame; a rotational frame.

  19. a F 0 m a F m x 3-5 Mass ★ What is mass ? the less massive baseball receives a largeracceleration Equal force the more massive bowling ball receives a smalleracceleration ★ Conjecture: The ratio of the masses of two bodies is equal to the inverse of the ratio of their accelerations when the same force is applied to both.

  20. a F 0 m a F m x F 1N = F 8N = 3-5 Mass ★ What is mass ? Equal force

  21. 3-5 Mass ★Mass is an intrinsic characteristic of a body a characteristic that automatically comes with the existance of the body. ★ The mass of a body is the characteristic that relates a force on the body to the resulting acceleration. ★Mass is a measure of the inertia of a body.

  22. (3-2) 3-6 Newton’s Second Law ★Newton's Second Law The net force on a body is equal to the product of the body’s mass and the acceleration of the body. (Newton's second law) (3-1) ★ Caution: is the mass of a body, is the vector sum of all the forces act on that body. ★Equivalent equations:

  23. If stays at rest move at constant velocity (5-2) 3-6 Newton’s Second Law ★The acceleration component along a given axis is caused only by the force componentalong that same axis, and not by force component along any other axis. (a) at rest (b) In motion the forces and the body: in equilibrium

  24. 3-6 Newton’s Second Law ★The SI unit of force: Newton (N) ★ the free-body diagram To solve problems with Newton’s second law, we often draw a free-body diagram in which only one body, represented by a dot, is considered. The external forces on the body are drawn. A coordinate system is usually included.

  25. 3-6 Newton’s Second Law ★external forces: any force on the bodies inside the system (a collection of two or more bodies) from bodies outside the system. ★internal forces: forces between two bodies inside the system (a collection of two or more bodies).

  26. B C ●Scalar form: ●Vector form: 3-7 Newton’s Third Law ★Newton’s Third Law : When two bodies interact, the forces on the bodiesfrom each other are always equal in magnitude and opposite in direction. ●A pair of action-reaction forces a third law force pair book B leans against crate C

  27. Table T Earth E Note: and are not ! 3-7 Newton’s Third Law ★Exp. Cantaloupe-table-earththree bodies A pair of action-reaction forces (a third law force pair) ●Cantaloupe-Earth interaction: Cantaloupe C ( gravitational force ) ●Cantaloupe-Table interaction:

  28. 3-8Applying Newton’s Laws ★ When you read the sample problems, pay attention to: Problem solving procedures; How to draw a free-body diagram with appropriate axes; How to useNewton's Laws to solve problems. P55 Sample problems 3-1

  29. Two blocks particles, Earth is also involved ; Cord massless, unstretchable; Pulley massless, only changes the cord’s orientation. 3-8Applying Newton’s Laws P55 Sample problem 3-1 frictionless pulley cord Earth is involved Solution Problem description equal in all sections of the cord.

  30. Substituting into Eq. 3-5 yields Block H (3-6) Solving for yields 3-8Applying Newton’s Laws draw free-body diagram Earth is involved apply Newton’s second law Block S (3-7) (3-5) (3-8)

  31. 3-8Applying Newton’s Laws P57 Sample problem 3-2 In fig.3-8a, a cord holds a 15 kg block stationary on a friction-less plane inclined at angle (a) What are the magnitudes of the force on the block from the cord and the normal force from the plane? (b) We now cut the cord. Does the block accelerateas it slides down the inclined plane? If so, what is its acceleration?

  32. free-body diagram (a) (b) Cutting the cord : 3-8Applying Newton’s Laws Solution Three forces are in equilibrium Use a coordinate system

  33. ★ A body (mass m) is in free fall with the free- fall acceleration of magnitude g. the only force acting on the body is the gravitational force (neglecting the effects of the air). 3-9Some Particular Forces ★ The Gravitational Force on a body is a pull by another body (Earth). For Earth, the force is directed down toward the ground, which is assumed to be a inertial frame. ★ choose a vertical y axis along the body’s path, with the positive directionupward.

  34. (3-14) (3-15) 3-9Some Particular Forces ★ The Gravitational Force Newton’s second law the vector form: ★ Weight (scalar) The weight W of a body is the magnitude of the net force requiredto provent the body from falling freely. upward force = grivatationalforce balanced keep the ball at rest the weight of the ball is the magnitude of the upward force 2N Upward force 2N

  35. 3-9Some Particular Forces In general: a body has relative to the ground (inertial frame), two forces acting on it are balanced. in vertical direction: (weight, with ground as inertial frame) Substituting for : (weight) The weight of a body is equal to the magnitude of the gravitational force on the body. (3-16) (3-17) (3-18)

  36. Spring scale Equal-arm balance 3-9Some Particular Forces Measuring the weight of a body: When the device is in balance, The gravitational force on the Body (L) is equal to the gravi- tational force on the reference bodies (R). The body stretches a spring, moving a pointer along a scale (mass or force units)

  37. apparent weight apparent weight 3-9Some Particular Forces ★The weight of a body must be measured when the body is not acceleratingvertically relative to the ground. ●apparent weight: elevator (lift) cab

  38. Weight Mass The magnitude of the gravitational force Measurement of inertial varies in different places latitude altitude Mass is constant (v<<C speed of light) Measured in kg Measured in N 3-9Some Particular Forces ★The weight of a body is not the mass of the body.

  39. y 0 (3-19) (3-20) 3-9 Some Particular Forces Normal force When a body presses against a surface, the surface ( even a seeminly rigid surface ) deforms and pushes on the body with a normal force that is perpendicular to the surface. ★The Normal Force Exp. A block rests on a tabletop

  40. 3-9Some Particular Forces Cord : massless, unstretchable Pulley ：massless, frictionless Tension: when the cord is bing pulled taut, The cord is under tension, it pulls on a body at each of its ends. The pulls at both ends of the cord have the same maglitude T and are directed along the cord.

  41. Direction of attempted slide 3-10 Friction ●A frictional force is the force on a body when the body slides or attempts toslide along a surface. ★ Friction ● The force is always parallel to the surface ●The force is directed so as to oppose the motion of the body. ★ Frictional forces exist everywhere.

  42. breakaway at rest r r Frictional force f f s s ,max ,max in motion breakaway time ●If the body does not slide, the frictional force is a static frictional force . ●If there is sliding, the frictional force is a kinetic frictional force . 3-10 Friction

  43. Property1. If the body does not move, then the static frictional force and the component of that is parallel to the surface balance each other. Property2. The magnitude of has a maximum velue : (3-21) is the coefficientof static friction If , the body begins to slide. 3-10Friction The frictional force has three properties:

  44. is the coefficient of kinetic friction. The coefficient and are dimensionless and must be determined experimentlly. 3-10 Friction Property3. If the body begins to slide along the surface, the magnitude of the frictional force rapidly decreases to a value (3-22) P65 Sample problem 3-5

  45. (3-28) 3-11 The Drag Force and Terminal speed When there is a relative motion between air ( or some other fluid ) and a body, the body experiences a drag force that opposes the relative motion and points in the direction In which the fluit flows relative to the body. The magnitude of is related to the relative speed by an experimentlly determined drag coefficient Caccording to (6-14)

  46. (3-28) 3-11 The Drag Force and Terminal speed Where is the air density; is the effective cross-sectional area of the body ( the area of a cross section taken perpendicular to the velocity ). The drag coefficient C can vary with the variation of , For simplicity, take it as a constant.

  47. (3-28) Falling body 3-11 The Drag Force and Terminal speed During the falling, Newton’s second law for a vertical y axis: If the body falls long enough, eventually equals the body falls at the terminal speed ( constant ). (3-29)

  48. 3-11 The Drag Force and Terminal speed If the body falls long enough, eventually equals the body’s speedno longer increases. The body then falls at a constant speed, called the terminal speed Find : (3-29) terminal speed (3-30)

  49. 3-11 The Drag Force and Terminal speed Skydiving

  50. 3-11 The Drag Force and Terminal speed Group Skydiving

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