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Physics

Physics. Chapter 9 Momentum and its Conservation. Momentum and its Conservation. Let’s check our knowledge (1-12). Define momentum. Momentum The quantity of motion The property of an object in motion. What two things must an object have to have momentum?. Mass (kg) Velocity (m/s).

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Physics

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  1. Physics Chapter 9 Momentum and its Conservation

  2. Momentum and its Conservation • Let’s check our knowledge (1-12)

  3. Define momentum. Momentum • The quantity of motion • The property of an object in motion

  4. What two things must an object have to have momentum? • Mass (kg) • Velocity (m/s)

  5. Explain how inertia is different than momentum. • Inertia is a property of any object with mass • It is the tendency of an object to resist a change in its velocity • Momentum is a property of an object in motion • A stationary object has no (O) momentum • An object must be moving to have momentum but not inertia

  6. How do you calculate momentum? • Multiple mass by velocity • Momentum = mv

  7. What is the symbol and unit for momentum? • p (actually rho-) • kgm/s

  8. Define impulse. • Force applied to an object over a time interval to change its momentum

  9. What two things must an object have to have impulse? • Force • Time interval

  10. Explain how impulse is different than momentum. • Momentum is the property of an object in motion. • Impulse is force applied for a certain time that causes an object’s momentum to change.

  11. How do you calculate impulse? • Multiply force applied by time of application • Impulse = Ft

  12. What is the unit for impulse? • N•s • There is no symbol for impulse!

  13. Are momentum and impulse vector quantities? • Yes • They both have magnitude (size) and direction

  14. Use Newton’s Second law of Motion and the definition of acceleration to derive the relationship between impulse and change in momentum. • F = ma and a = v/t • F = mv/t • Ft = mv (fat mav)

  15. Write down the Impulse- Momentum Theorem. Explain it in words. • Ft = p2 – p1 • FΔt = mΔv • Impulse (Ft) on an object causes its initial momentum (p1) to change to its final momentum (p2).

  16. If a tennis ball is hit by a racket for 0.15 s with a force of 23 N find its: • Beginning momentum • Impulse given it by the racket • Change in momentum • Final momentum

  17. If a tennis ball is hit by a racket for 0.15 s with a force of 23 N find its: • Beginning momentum: 0 kg*m/s • Impulse given it by the racket: 3.45 N*s • Change in momentum: 3.45 kg*m/s • Final momentum: 3.45 kg*m/s

  18. Explain using the terms momentum and impulse why automobile airbags save lives. • When a person stops there is the same change in momentum with or without the airbag. • The airbag increases the time for the force so it is less (Ft = mv)

  19. A 725 kg compact car is moving at 100 km/h towards the east. Find its momentum (kg*m/s) • 100 km/h = 27.8 m/s (?) • p = mv = (725)(27.8) = 20155 kg*m/s

  20. If the driver of the 725 kg car traveling at 27.8 m/s, applies the brakes for 2 s and the brakes apply an average force of 5000N. Find: • The impulse given to the car • The car’s change in momentum • The car’s final momentum • The car’s final velocity

  21. If the driver of the 725 kg car traveling at 27.8 m/s, applies the brakes for 2 s and the brakes apply an average force of 5000N. Find: • The impulse given to the car: - 10,000 N*s • The car’s change in momentum: - 10,000 kg*m/s • The car’s final momentum: 10,155 kg*m/s • The car’s final velocity: 14 m/s

  22. State Newton’s Third Law of Motion • For every action there is an equal and opposite reaction

  23. State the Law of Conservation of Momentum • The momentum of any closed, isolated system doesn't change • In a closed, isolated system, the initial momentum is equal to the final momentum

  24. What is a closed system? • A system where mass is not lost or gained.

  25. What is the difference between internal forces and external forces on a system? • Internal forces are those forces that act within the system. • External force are forces acting on the system from the outside (the surroundings)

  26. What is an isolated system? • When no external forces act upon the system.

  27. How is an isolated system different from a closed system? • A closed system can have an outside force acting upon it but an isolated system cannot.

  28. For the Law of Conservation of Momentum to be true, must a system be closed or isolated or both. Explain. • An isolated system is also closed, so it must be isolated (and thus also closed). • If only closed momentum can still be transferred to it by way of an outside force

  29. If there is no air in outer space for a rocket to push off of, explain how when a rocket fires its engines in space it can move. • The escaping gas pushes off the rocket and the rocket pushes off the gas with the same force. • This causes and equal and opposite change in momentum for both • So both move but in opposite directions (and speeds)

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