advanced/notes 7.1
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Linear MomentumChapter 7
Momentum and its Relation to Force
• How do we define momentum?
• How is momentum related to force?
• How can we apply the conservation of momentum?
Momentum
• http://www.youtube.com/watch?v=0LnbyjOyEQ8&feature=related
Momentum
• Linear Momentum: the product of an objects mass and its velocity
• Symbol is p
• Units are kg·m/s
Momentum
• Since v is a vector, …
• A big truck can have the same momentum as a small car if…
• Stopping force?
• Damage?
Momentum
• A smaller football player can have the same momentum as a larger one, if…
• Application?
• Direction change?
Relation to Newton’s 2nd
• Rate of change of momentum is equal to net force:
• Newton originally wrote the second law in terms of momentum
• Proof Page 168
Force of a tennis serve
• For a top player, a tennis ball may leave the racket on the serve with a speed of 55 m/s (about 120 mph). If the ball has a mass of 0.060 kg and is in contact with the racket for about 4 x 10-3 s, estimate the average force on the ball.
• Would this force be large enough to lift a 60 kg person?
Water
• Water leaves a hose at a rate of 1.5 kg/s with a speed of 20 m/s and is aimed at the side of a car, which stops it. What force is exerted by the water on the car?
Conservation of Momentum
• During a collision, measurements show that the total momentum does not change
Conservation of Momentum
• Since mass doesn’t change, the velocities change
The total momentum of an isolated system remains constant
Railroad cars
• A 10,000 kg railroad car, A, traveling at a speed of 24.0 m/s strikes an identical car, B, at rest. If the cars lock together as a result of the collision, what is their common speed just afterward?
Systems
• Only applies to an isolated system
• Only forces are between objects in the system
• System of rock falling by itself will not conserve momentum, because an external force, gravity
• If we expand the system to include the Earth, then we can conserve momentum
Systems
• Momentum conservation works for a rocket as long as we consider the rocket and its fuel to be one system, and account for the mass loss of the rocket.
Homework
• Read Sections 7.1 and 7.2• Do Problems # 1 – 12, Page 188