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Physics

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Physics

Chapter 9

Momentum and its Conservation

- Let’s check our knowledge (1-12)

Momentum

- The quantity of motion
- The property of an object in motion

- Mass (kg)
- Velocity (m/s)

- 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

- Multiple mass by velocity
- Momentum = mv

- p (actually rho-)
- kgm/s

- Force applied to an object over a time interval to change its momentum

- Force
- Time interval

- 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.

- Multiply force applied by time of application
- Impulse = Ft

- N•s
- There is no symbol for impulse!

- Yes
- They both have magnitude (size) and direction

- F = ma and a = v/t
- F = mv/t
- Ft = mv (fat mav)

- Ft = p2 – p1
- FΔt = mΔv
- Impulse (Ft) on an object causes its initial momentum (p1) to change to its final momentum (p2).

- Beginning momentum
- Impulse given it by the racket
- Change in momentum
- Final momentum

- 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

- 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 (Ft = mv)

- 100 km/h = 27.8 m/s (?)
- p = mv = (725)(27.8) = 20155 kg*m/s

- The impulse given to the car
- The car’s change in momentum
- The car’s final momentum
- The car’s final velocity

- 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

- For every action there is an equal and opposite reaction

- The momentum of any closed, isolated system doesn't change
- In a closed, isolated system, the initial momentum is equal to the final momentum

- A system where mass is not lost or gained.

- Internal forces are those forces that act within the system.
- External force are forces acting on the system from the outside (the surroundings)

- When no external forces act upon the system.

- A closed system can have an outside force acting upon it but an isolated system cannot.

- 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

- 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)