Transcript Momentum
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-) kgm/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 = Ft 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 = mv/t Ft = mv (fat mav) Write down the Impulse- Momentum Theorem. Explain it in words. Ft = p2 – p1 FΔt = mΔv Impulse (Ft) 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 (Ft = mv) 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)