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PHYS16 – Lecture 21

PHYS16 – Lecture 21. Ch. 9 Momentum and Collisions. Ch. 9 Momentum & Collisions. Linear Momentum Related to Newton’s second law Rocket Propulsion Momentum Conservation Collisions Elastic vs. Inelastic 1D and 2D Impulse. Momentum pre-question.

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PHYS16 – Lecture 21

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  1. PHYS16 – Lecture 21 Ch. 9 Momentum and Collisions

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

  3. Momentum pre-question • Consider two carts on a frictionless air track with masses m and 2m. If you push the lower mass cart for 3 s and then the other cart for the same length of time and with the same force, which cart undergoes the larger change in momentum? • Cart with mass m • Cart with mass 2m • Change in momentum is the same for both • There is not enough information

  4. Solve Problems with Collisions • Read problem carefully! • Draw a picture. • Write down given quantities and what you want to solve for. • Find the correct equation (conservation of momentum and possibly conservation of energy). • Do the math and solve!

  5. Perfectly Elastic Collision • A 1.00-kg ball traveling at 3.00 m/s collides with a 3.00-kg ball traveling at 0.500 m/s. If after a perfectly elastic collision the 1.00-kg ball is traveling at -0.750 m/s, how fast is the second ball going? How do we check to see if it is perfectly elastic?

  6. Perfectly Inelastic Collision • How fast would a 5-g fly have to be traveling to slow a 1900-kg car traveling at 55 mph to 50 mph if the fly just splatted across the windshield?

  7. Superball vs. Basketball • Drop a superball and basketball together. How high does the superball bounce? • Homework problem…

  8. For Problems with multiple parts… • Follow the steps for solving problems as before • When you get to what equation to use break the problem into parts – usually chronologically – and solve each part • Then do the math as before…

  9. Click-Clack • In a click-clack if all balls have equal mass and I take one ball out and put it at height 4 m, how high should the ball on the other side go? A) 4 m B) 2 m C) 1 m D) 0 m E) Not enough information http://www.brucegray.com/images/clickclack.jpg

  10. Click-Clack http://www.brucegray.com/images/clickclack.jpg

  11. Click-Clack • In a click-clack if all balls have equal mass and I take one ball out and put it at height 4 m, how high should the ball on the other side go? A) 4 m B) 2 m C) 1 m D) 0 m E) Not enough information http://www.brucegray.com/images/clickclack.jpg

  12. Smith & Wesson • On homework there is a problem about how far a chair slides when a bullet gets shot into it. What demo does this remind you of? • What are the parts you will need to know?

  13. 2D collisions – How to solve problems • Separate vectors into x and y components • Solve two equations • Conservation of momentum in x and • Conservation of momentum in y • If perfectly elastic get a third equation • Conservation of energy

  14. 2D Collisions • Need to add 2D vectors • Assume masses of two objects equal • Before • After A B C

  15. 2D Collisions – Predict vectors, assume masses are equal A B

  16. 2D Collision problem • A 0.25 kg hockey puck traveling at 1.5 m/s strikes a stationary puck with the same mass. If the first puck exits at 30 degrees and 0.75 m/s, what is the direction of the second puck?

  17. 3. Impulse • Impulse describes the change in momentum • Good for describing what happens during a collision • Example: • What is momentum of 0.5 kg ball dropped from 5 m just before it hits the floor? • If the velocity after it hits the floor is +8 m/s upward, what is the impulse? • If the interaction lasts 0.01 s, what was the average Force?

  18. During Collisions… • Baseball • Soccer ball • Water balloon? http://www.youtube.com/watch?v=pQ9NiazPYI8 http://www.youtube.com/watch?v=90VyvOhPmA0&NR=1&feature=fvwp http://www.youtube.com/watch?v=jjE8SQG8AwI&feature=related

  19. Discussion • Why does an airbag reduce injury? • What is better in bungee jumping- a stiff cable that won’t break at high forces or a stretchy cable? • Why should a boxer “ride the punch” and not stiffen her neck muscles?

  20. Momentum pre-question • A 0.50 kg ball accelerates from rest at 10.0 m/s2 for 2.0 s. It then collides with and sticks to a 1.0 kg ball that is initially at rest. After the collision, how fast are the balls going? A) 3.3 m/s B) 6.7 m/s C) 10 m/s D) 15 m/s E) None of the above.

  21. Momentum pre-question • Consider two carts on a frictionless air track with masses m and 2m. If you push the lower mass cart for 3 s and then the other cart for the same length of time and with the same force, which cart undergoes the larger change in momentum? • Cart with mass m • Cart with mass 2m • Change in momentum is the same for both • There is not enough information

  22. Conclusions • Momentum • Momentum Conservation • Elastic vs. Inelastic Collisions • Impulse

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