Chapter 3 Force and Newton s laws
Chapter 3 Force and Newton’s laws
Section 3-1 Classical mechanics The approach to the dynamics we consider here is generally called classical mechanics Galileo Galilei Issac Newton (1564-1642)(1642-1727
Issac Newton (1642－1727） Galileo Galilei (1564－1642） Section 3-1 Classical mechanics • The approach to the dynamics we consider here is generally called classical mechanics
In this chapter, we will study in detail the bases of classical mechanics: Newtons three laws Classical mechanics was found not to describe well the motions in certain realms For ordinary objects, classical mechanics is important and very useful
• In this chapter, we will study in detail the bases of classical mechanics: Newton’s three laws. • Classical mechanics was found not to describe well the motions in certain realms. • For ordinary objects, classical mechanics is important and very useful
Section 3-2 Newtons first law What can cause the motion of a body? Force Take the apple's freely falling motion as an example What will be the states of the body if there is no any interactions between it and its environment? (an isolated system) At rest or 1D uniform motion Newton's first law Every body continues in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it
Section 3-2 Newton’s first law ▪ Newton’s first law : Every body continues in its state of rest or uniform motion in a straight line, unless it is compelled to change that state by forces impressed on it. ▪ What can cause the motion of a body? Take the apple’s freely falling motion as an example Force ▪ What will be the states of the body if there is no any interactions between it and its environment? (an isolated system) At rest or 1D uniform motion
1. Newton's first law tells us Consider a body on which no net force acts 1)If the body is at rest, it will remain at rest; 2)If the body is moving with constant velocity, it will continue to do so, no force is needed to keep it moving
1. Newton’s first law tells us: Consider a body on which no net force acts. 1) If the body is at rest, it will remain at rest； 2) If the body is moving with constant velocity, it will continue to do so, no force is needed to keep it moving
2 The correctness of newton 's laws is dependent on the reference frames! See an example in 动画库/学夹/2-01牛顿定律适用的参照系eXe1
2. The correctness of Newton’s laws is dependent on the reference frames! See an example in 动画库/力学夹/2-01牛顿定律适用的参照系.exe 1
3. nertial frames(惯性参考系) The reference frames to which newton s law applies(适用) are called" inertial frames The tendency of a body to remain at rest or in uniform linear motion is called“ inertia” No net force or No acceleration Can we find inertial frames in the nature?
3. Inertial frames(惯性参考系): No net force or No acceleration • The reference frames to which Newton’s law applies (适用) are called “inertial frames”. • The tendency of a body to remain at rest or in uniform linear motion is called “inertia”. • Can we find inertial frames in the nature?
地心系,地球赤道加速度a=34X102m/s2 日心系,地球公转轨道加速度a=6x103m/s2 银河系,太阳向心加速度a=3x1010m/s2 A frame that keeps rest or uniform linear motion elative to any inertial frames, is an inertial frame Newtons first law is often called the law of inertia See动画库/力学夹/2-01德行与惯性,exe
地心系, 地球赤道加速度 a=3.4x10-2m/s2 日心系, 地球公转轨道加速度 a=6x10-3m/s2 银河系, 太阳向心加速度 a=3x10-10m/s2 • Newton’s first law is often called the law of inertia. • A frame that keeps rest or uniform linear motion, relative to any inertial frames, is an inertial frame. ⚫ See 动画库/力学夹/2-01德行与惯性.exe
Section 3-3 Force Newtons first law tell us that force causes the change in the motion states( AD). F>a For a fixed body, a larger force applied to the body will generate a larger acceleration for the body The force is determined through the measure of acceleration the body gets under the force
Section 3-3 Force ▪ The force is determined through the measure of acceleration the body gets under the force. ▪ Newton’s first law tell us that force causes the change in the motion states (~ ). F a v → ▪ For a fixed body, a larger force applied to the body will generate a larger acceleration for the body
Section 3-4 Mass It is much easy to accelerate a bicycle than a car by pushing it Clearly same force produces different acceleration when applied to different bodies What makes the difference??? mass
Section 3-4 Mass It is much easy to accelerate a bicycle than a car by pushing it. Clearly same force produces different acceleration when applied to different bodies. What makes the difference??? Mass