PHYS16 – Lecture 19 Ch. 9 Momentum and Collisions

Announcements Meet in parking lot behind Merrill on Wednesday! We will do a demo then come back to class…

Energy Post-question Two balls have the same mass. Ball 1 is thrown upward at 3 m/s and Ball 2 is thrown downward with 3 m/s. Which Ball reaches the ground with more kinetic energy? A)Ball 1 B)Ball 2 C)Both balls have the same kinetic energy D)There is not enough information

Energy Post-question If a car engine for a 1400 kg car outputs 1000 kW of average power, how much time does it take to accelerate from 0 to 25 m/s? A)0.4 s B)1 s C)0.2 s D)0.8 s E)10 s

Energy Post-question The potential energy, U (x), is shown as a function of position, x, in the figure. In which region is the magnitude of the force the highest? A) B) C) D)

Energy Post-question Clyde, the dinosaur, is sick of his doll-sized roller coaster and decides to ride the 20-m tall roller coaster at Ohio’s Cedar Point. Since Clyde is a little scared he decides to start from a location on the track that is 10 m lower than the top. How does Clyde’s speed at the bottom compare to a rider that started from the top of the hill? A) It is slower by a factor of 4. B) It is slower by a factor of 2√2. C) It is slower by a factor of 2. D) It is slower by a factor of √10 E) None of the above

Linear Momentum – Related to Newton’s second law – Rocket Propulsion Momentum Conservation Collisions – Elastic vs. Inelastic – 1D and 2D Impulse Ch. 9 Momentum & Collisions

Momentum Momentum = mass times velocity – ↑ mass, then ↑ momentum – ↑ velocity, then ↑ momentum Vector quantity Units are (kg∙m/s)

Discussion Question If two objects have the same inertia, do they have the same momentum? Two cars of equal mass are traveling around a round-a-bout. If object 1 has twice the momentum of object 2, what is the ratio of the centripetal force acting on each? What about the ratio of their kinetic energies? If we have inertia and kinetic energy, why do we need momentum?

1. Momentum and Newton’s Second Law In Newton’s second law F≠ ma!

Rockets…

2. Momentum Conservation According to Newton’s third law momentum is conserved in an isolated system

Medicine Ball and Skateboard… University of Maryland Physics Demo Facility

Elastic and Inelastic Collisions Perfectly Elastic – no losses due to the interaction – Objects bounce perfectly off one another – Ex. pool balls, gliders on air track Inelastic – there are losses – Objects don’t perfectly bounce – Ex. basketball hitting ground Perfectly Inelastic – objects adhere – Ex. clay ball with floor

Conservation Laws Perfectly Elastic – both momentum and energy conserved Inelastic – only momentum conserved Perfectly Inelastic – only momentum conserved

Conclusions Momentum Momentum Conservation Elastic vs. Inelastic Collisions