Momentum

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Why cant i download these powerpoints? wait....i figured it out! I just had to think outside the computer box!

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Help! I can't download the lecture notes for this Monday to print them out. It would be nice to have them before class. Makes getting frustrated a whole lot easier.

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Presentation Transcript

Announcements : 

Announcements Homework for tomorrow… (Ch. 6, Probs 8 & 20) Office hours… MTWRF 11-noon

Chapter 6 : 

Chapter 6 Momentum and Collisions

Section 6.1:Momentum & Impulse : 

Section 6.1:Momentum & Impulse Linear Momentum Vector quantity - direction is that of the velocity of the object Constant momentum  straight-line motion Units?

Newton’s 2nd Law, reformulated : 

Newton’s 2nd Law, reformulated Newton’s 2nd Law… Newton’s 2nd Law, reformulated… For constant mass only

Impulse - constant force : 

Impulse - constant force Newton’s 2nd Law Solving for The impulse delivered by a constant force over a time interval

Impulse - variable force : 

Impulse - variable force Newton’s 2nd Law Solving for The impulse delivered by a variable force over a time interval is equal to the area under a force vs. time graph

Quiz Question 1 : 

Quiz Question 1 A 2 kg object is acted upon by a single force in the x-direction in a manner described by the graph shown. The momentum acquired by the object is? 16 kg·m/s 30 kg·m/s 32 kg·m/s 40 kg·m/s 48 kg·m/s F(N) 4 8 12 2 4 6 8 t(s)

Impulse-momentum theorem : 

Impulse-momentum theorem The impulse of the force acting on an object equals the change in momentum of that object

Quiz Question 2 : 

Quiz Question 2 A ball hits a wall and rebounds with the same speed, as diagrammed below. The changes in the components of the momentum of the ball are: px > o, py > o px < o, py > o px = o, py > o px = o, py < o px > o, py < o y x  

Demo : 

Demo Drop your favorite book (Serway of course) on the floor from a height of 1 m… What is the average force of the floor on the book? Given: m = 1kg, t = 10 ms

Quiz Question 3 : 

Quiz Question 3 A person attempts to knock down a large wooden bowling pin by throwing a ball at it. The person has 2 balls of equal size and mass, 1 made of rubber and the other made of putty. The rubber ball bounces back, while the ball of putty sticks to the pin. Which ball is most likely to topple the pin? the rubber ball the ball of putty makes no difference need more info

Serway 6.15 : 

Serway 6.15 The front 1.20 m of a 1400-kg car is designed as a “crumple zone” that collapses to absorb the shock of a collision. If a car is traveling 25.0 m/s stops uniformly in 1.20 m, how long does the collision last, what is the magnitude of the average force on the car, and what is the acceleration of the car? Express the acceleration as a multiple of the acceleration of gravity.

Demo : 

Demo Will the egg break?

Quiz Question 4 : 

Quiz Question 4 A student’s life was saved in an automobile accident because an airbag expanded in front of his head. If the car had not been equipped with an airbag, the windshield would have stopped the motion of his head in a much shorter time. Compared to the windshield, the airbag: causes a much smaller change in momentum exerts a much smaller impulse causes a much smaller change in kinetic energy exerts a much smaller force does much more work

Section 6.2:Conservation of Momentum : 

Section 6.2:Conservation of Momentum

Question : 

Question When is ? When NO NET EXTERNAL FORCE acts on an isolated system, the total momentum of the system is constant.

Question : 

Question Which of these systems conserve momentum? While slipping on a patch of ice, a car collides with another car at rest (also on ice). System: both cars While slipping on a patch of ice, a car collides with another car at rest (also on ice). System: slipping car A single car slips on a patch of ice. System: slipping car

Question : 

Question Which of these systems conserve momentum? A car makes an emergency stop on a dry road System: car A car collides with a parked car on dry pavement. System: both cars A ball drops to Earth. System: ball A ball executes projectile motion. System: ball

Quiz Question 5 : 

Quiz Question 5 An 80 kg man is marooned on level frictionless ice. How can he save himself? In desperation, he hurls his 0.5 kg shoe to the right at 15 m/s. Find the resulting velocity (magnitude and direction) of the man.

Quiz Question 6 : 

Quiz Question 6 Think fast! You’ve just driven around a curve in a narrow, one-way street at 25 mph when you notice a car identical to yours coming straight toward you at 25 mph. You have only two options: hitting the car head on or swerving into a massive concrete wall, also head on. In the split second before the impact, you decide to hit the other car hit the wall hit either one - it makes no difference consult your lecture notes

Quiz Question 7 : 

Quiz Question 7 Suppose the entire population of the world gathers in one spot and, at the sounding of a prearranged signal, everyone jumps up. While all the people are in the air, does Earth gain momentum in the opposite direction? No; the inertial mass of Earth is so large that the planet’s change in motion is imperceptible Yes; because of its much larger inertial mass, however, the change in momentum of the Earth is much less than that of all the jumping people Yes; Earth recoils, like a rifle firing a bullet, with a change in momentum equal to and opposite that of the people. It depends.

Quiz Question 8 : 

Quiz Question 8 Suppose the entire population of the world gathers in one spot and, at the sounding of a prearranged signal, everyone jumps up. About a second later, 6 billion people land back on the ground. After the people have landed, Earth’s momentum is the same as what it was before the people jumped. different from what it was before the people jumped.

Section 6.3:Collisions : 

Section 6.3:Collisions 2 types of Collisions: Elastic Collision both Momentum and K.E. are conserved Inelastic Collision only Momentum is conserved Special Case: perfectly inelastic collision both objects stick together (they have the same final velocity)

Question : 

Question If energy is not conserved in an inelastic collision, where does the energy go?

1D Elastic Collision (2 bodies) : 

1D Elastic Collision (2 bodies) Initially… Finally… m1 m2 v1i v2i m1 m2 v2f v1f

1D Perfectly Inelastic Collision (2 bodies) : 

1D Perfectly Inelastic Collision (2 bodies) Initially… Finally… m1 m2 v1i v2i m1 m2 vf

Problem 1 : 

Problem 1 A 4000 kg freight car moving at 3 m/s bumps into a stationary 6000 kg freight car. The two cars couple together. Their speed just after the impact is: 0 1.2 m/s 1.9 m/s 2.0 m/s 3.0 m/s

Problem 2 : 

Problem 2 A 4000 kg freight car moving at 3 m/s bumps into a 6000 kg freight car moving at 1 m/s in the opposite direction. The two cars couple together. Their speed just after the impact is: 0 0.6 m/s 1.0 m/s 1.8 m/s 3.0 m/s

Newton’s Cradle : 

Newton’s Cradle What happens? m m m m m v

Newton’s Cradle : 

Newton’s Cradle Why not… m m m m

Serway 6.34 : 

Serway 6.34 Three carts of masses 4.0 kg, 10 kg, and 3.0 kg move on a frictionless horizontal track with speeds of 5.0 m/s, 3.0 m/s, and 4.0 m/s. The carts stick together after colliding. Does your answer require that all carts collide and stick together at the same time? Find the final velocity of the three carts. 4.0 kg 10 kg 3.0 kg 5.0 m/s 3.0 m/s 4.0 m/s

i.e. Ballistic pendulum : 

i.e. Ballistic pendulum A ballistic pendulum is an apparatus consisting of a block suspended from a massless rod. In an experiment, a ballistic pendulum is used to calculate the muzzle velocity of a projectile fired from a gun. The experiment consists of three steps Step 1: A metal bullet is fired from a pistol into the block at the end of the pendulum Step 2: The bullet embeds itself in the block before the pendulum has swung upwards. Step 3: The block and bullet swing up, and a ratchet system locks the pendulum at the highest point of its arc. M M+m m

Quiz Question 9 : 

Quiz Question 9 Which of the following is (are) conserved in the collision between the bullet and the block? I. Momentum II. Kinetic Energy III. Velocity I only II only I and II only I and III only

Serway 6.58 : 

Serway 6.58 Tarzan, whose mass is 80.0-kg, swings from a 3.00-m vine that is horizontal when he starts. At the bottom of his arc, he picks up 60.0 kg Jane in a perfectly inelastic collision. What is the height of the highest tree limb they can reach on their upward swing?

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