AP Physics Chapter 6Momentum and Collisions.ppt

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1、AP Physics Chapter 6 Momentum and Collisions,Chapter 6: Momentum and Collisions,6.1 Linear Momentum 6.2 Impulse 6.3 The Conservation of Linear Momentum 6.4 Elastic and Inelastic Collisions 6.5 Center of Mass,Homework for Chapter 6,Read Chapter 6HW 6.A : p. 203: 3, 6, 7, 10, 11, 13, 18, 26, 29, 31, 3

2、3, 34, 35.HW 6.B: p. 205: 38, 40, 41, 43, 49. p.206: 53-56, 60, 62, 68, 70.,6.1 Linear Momentum 6.2 Impulse,Loosen Up! II,Impulse is found by multiplying the force acting on an object by the amount of time the force acts. The amount of impulse that an object experiences is equal to the amount that i

3、ts momentum changes. This means that the same change in momentum can be accomplished by exerting a large force for a short amount of time or a small force for a longer amount of time. Seat belts are designed to give a little when they are properly fastened. If not, they would not work as well at kee

4、ping you from injury in an accident or sudden stop. Use the concept of impulse equaling the change in momentum to explain why they need to give.,Physics Daily WarmUps # 91,Solution: The time to bring a person to stop is increased, thereby decreasing the force exerted on the person by the belt.,6.1 L

5、inear Momentum,6.1 Linear Momentum,The SI unit of momentum is the kgm/s.The total linear momentum of a system is the vector sum of the momenta of the individual particles.P = p1 + p2 + p3 + = pii,6.1 Linear Momentum,Example 6.1: Which has more linear momentum:a) a 1500 kg car moving at 25.0 m/s orb)

6、 a 40,000 kg truck moving at 1.00 m/s?,6.1 Linear Momentum,Example 6.2: Two identical 1500 kg cars are moving perpendicular to each other. One moves with a speed of 25.0 m/s due north and the other moves at 15.0 m/s due east. What is the total linear momentum of the system?,6.1 Linear Momentum: Chec

7、k for Understanding,6.1 Linear Momentum: Check for Understanding,6.1 Linear Momentum: Check for Understanding,6.1 Linear Momentum: Check for Understanding,6.2 Impulse,Ft,6.2 Impulse, In fact, Newton used the momentum form of his second law when he first started formatting the law., For a system in w

8、hich mass is not a constant, such as rocket propulsion, the momentum form should be used.,6.2 Impulse,Example 6.3: A 0.10 kg ball is dropped onto a table top. The speeds of the ball right before hitting the table top and right after hitting the table top are 5.0 m/s and 4.0 m/s respectively. If the

9、collision between the ball and the tabletop lasts 0.15 s, what is the average force exerted on the ball by the table top?,6.2 Impulse: Check for Understanding,1.,6.2 Impulse: Check for Understanding,6.2 Impulse: Check for Understanding,2.,6.2 Impulse: Check for Understanding,Answer: b,6.2 Impulse: C

10、heck for Understanding,3.,6.2 Impulse: Check for Understanding,J,Homework for Sections 6.1 & 6.2,HW 6.A : p. 203: 3, 6, 7, 10, 11, 13, 18, 19, 22, 26, 29, 31, 33, 34, 35.,6.3 The Conservation of Linear Momentum 6.4 Elastic and Inelastic Collisions,Collision Conservation,Momentum is always conserved

11、in any collision, as is energy. However, energy usually transformed into several different types as a result of the collision. In most collisions, a significant amount of the kinetic energy is converted into other forms, and the colliding objects movements are greatly affected. These are called inel

12、astic collisions. There are some collisions, like the colliding steel balls suspended by strings in a Newtons Cradle, in which kinetic energy is almost conserved. Since so little energy is lost in each collision, they seem to just go on bouncing. These collisions represent elastic collisions.Elastic

13、 collisions always seem to catch our attention. Because they are so rare, they seem unusual to us. Write down a personal experience where you witnessed an elastic collision.,Physics Daily WarmUps # 97,6.3 The Conservation of Linear Momentum,6.3 The Conservation of Linear Momentum,Momentum is conserv

14、ed in an isolated system. The motion in two dimensions may be analyzed in terms of the components of momentum, which is also conserved.,If m1 and m2 are equal, they will split apart at a 90 degree angle.,Conservation of Momentum Before the rifle is fired, the total momentum of the rifle and the bull

15、et is zero. During firing, there are equal and opposite internal forces, and the instantaneous total momentum of the rifle-bullet system remains zero. When the bullet leaves the barrel, the total momentum of the system is still zero.,6.3 The Conservation of Linear Momentum,Example 6.4: A 50 kg pitch

16、ing machine (excluding the baseball) is placed on a frozen pond. The machine fires a 0.40 kg baseball with a speed of 35 m/s in the horizontal direction. What is the recoil velocity of the pitching machine? (Assume negligible friction.),6.3 The Conservation of Linear Momentum,Example 6.5: A 10 gram

17、bullet moving at 300 m/s is fired into a 1.0 kg block. The bullet emerges (does not stay embedded in the block) with half of its original speed. What is the velocity of the block right after the collision?,6.3 The Conservation of Linear Momentum: Check for Understanding,6.3 The Conservation of Linea

18、r Momentum: Check for Understanding,6.3 The Conservation of Linear Momentum: Check for Understanding,6.3 The Conservation of Linear Momentum: Check for Understanding,6.4 Elastic and Inelastic Collisions,6.4 Elastic and Inelastic Collisions,6.4 Elastic and Inelastic Collisions,6.4 Elastic and Inelast

19、ic Collisions,Inelastic collisions In inelastic collisions, momentum is conserved but kinetic energy is not. Collisions like the ones shown here, in which the objects stick together , are called completely inelastic collisions. The maximum amount of kinetic energy lost is consistent with the conserv

20、ation of momentum.,6.4 Elastic and Inelastic Collisions,6.4 Elastic and Inelastic Collisions,Example 6.6: While standing on skates on a frozen pond, a student of mass 70.0 kg catches a 2.00 kg ball travelling horizontally at 15.0 m/s toward him.What is the speed of the student and the ball immediate

21、ly after he catches it?How much kinetic energy is lost in the process?,6.4 Elastic and Inelastic Collisions,Example 6.7: A rubber ball with a speed of 5.0 m/s collides head on elastically with an identical ball at rest. Find the velocity of each object after the collision.,6.4 Elastic and Inelastic

22、Collisions: Check for Understanding,6.4 Elastic and Inelastic Collisions: Check for Understanding,6.4 Elastic and Inelastic Collisions: Check for Understanding,6.4 Elastic and Inelastic Collisions: Check for Understanding,6.4 Elastic and Inelastic Collisions: Check for Understanding,6.4 Elastic and

23、Inelastic Collisions: Check for Understanding,6.4 Elastic and Inelastic Collisions: Check for Understanding,6.4 Elastic and Inelastic Collisions: Check for Understanding,6.4 Elastic and Inelastic Collisions: Check for Understanding,6.4 Elastic and Inelastic Collisions,Homework for Sections 6.3 & 6.4

24、,HW 6.B: p. 205: 38, 40, 41, 43, 49. p.206: 53-56, 60, 62, 68, 70.,6.5 Center of Mass,6.5 Center of Mass,The center of mass of a system is the point at which all the mass of the system may be considered to be concentrated.If the acceleration due to gravity, g, is a constant, then the center of gravi

25、ty, or the point at which al the weight of the system may be concentrated, is at the center of mass.The conservation of momentum allows us to analyze a system of particles; the system can be replaced by an equivalent single particle at the center of mass. The motion of the center of mass also obeys

26、Newtons second law, Fnet = ma. If the net external force on a system is zero, the total linear momentum of the center of the mass stays constant (or zero).,6.5 Center of Mass,How can you find the center of mass (or center of gravity) of an object?1) Symmetry (the center of the object) 2) Mathematically (beyond the scope of this course) 3) Experimentally (by suspension),