Announcements • EXAMS will be returned at the END of class today! • Homework for tomorrow… (Ch. 7, Probs 30 & 32) 7.6a) 5.0 kgm/sb) 10 kgm/s • Office hours… M 3-4 pm TWR 9-10 am F 1-2 pm • Tutorial Learning Center – Houston Hall 113 MWR 8 am-6 pm T 8 am-7 pm F 8 am-5 pm
Chapter 7 Translational Momentum (Newton’s Laws and Momentum Conservation)
Review… • Translational Momentum • Newton’s 2nd Law… …for a particle …for a system • Impulse , Impulse-Momentum Theorem
Review… • Impulse (averaging the force) • The impulse is equal to the area under a force vs time graph
Quiz Question 1 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
Prob 7.10 Until he was in his seventies, Henri LaMothe excited audiences by belly-flopping from a height of 12 m into 30 cm of water. Assuming that he reaches the bottom of the water and estimating his mass, find the magnitude of • the average force • the average impulse on him from the water.
Demonstration Will the egg break?
Section 7.5: Newton’s Laws & Momentum Conservation If no net external forces act on a system of particles, the total translational momentum of the system cannot change If then
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 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
Question 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.
Elastic & Inelastic Collisions 2 types of Collisions: • Elastic (Bouncy) Collision • bothMomentum and K.E. are conserved • Inelastic (Sticky) Collision • onlyMomentum is conserved • perfectly inelastic collision • both objects stick together (they have the same final velocity)
How to solve “Conservation of Momentum Problems”… 1. Choose a system 2. If so…. 3. Total momentum of the system
1D PerfectlyInelastic Collision (2 bodies) • Initially… • Finally… vA1 vB1 mB mA mB v2 mA
Quiz Question 2 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