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# Physics PowerPoint PPT Presentation

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).

Physics

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## 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)

### 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

### How do you calculate momentum?

• Multiple mass by velocity

• Momentum = mv

### What is the symbol and unit for momentum?

• p (actually rho-)

• kgm/s

### Define impulse.

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

### What two things must an object have to have impulse?

• Force

• Time interval

### 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.

### How do you calculate impulse?

• Multiply force applied by time of application

• Impulse = Ft

### What is the unit for impulse?

• N•s

• There is no symbol for impulse!

### Are momentum and impulse vector quantities?

• Yes

• They both have magnitude (size) and direction

### 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)

### 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).

### 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

### 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

### 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)

### 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

### 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

### 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

### State Newton’s Third Law of Motion

• For every action there is an equal and opposite reaction

### 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

### What is a closed system?

• A system where mass is not lost or gained.

### 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)

### What is an isolated system?

• When no external forces act upon the system.

### 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.

### 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

### 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)