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Introduction to Physical Science Monday, Wednesday, Thursday Tom Burbine [email protected] Quiz #1 Momentum • Momentum is inertia in motion • Momentum = mass x velocity • Momentum = mv • When you don’t care about direction: • Momentum = mass x speed Scalars and Vectors Momentum • Moving object can have large momentum if it has a large mass, large velocity, or both Momentum • To change momentum, you need to change the velocity or the mass of the object Momentum • To change the velocity of an object, you need to apply a force, which accelerates an object Momentum • The longer the force is applied, • The greater the velocity changes, • The greater the momentum changes Impulse • Impulse = Force x time interval • Impulse = Ft Questions: • Compare the momentum of a 1 kg cart moving at 10 m/s with that of a 2 kg cart moving at 5 m/s. • Does the moving cart have impulse? • Does a moving cart have momentum? • For the same force, which cannon imparts a greater impulse to a cannonball – a long cannon or a short one? Answers: • Compare the momentum of a 1 kg cart moving at 10 m/s with that of a 2 kg cart moving at 5 m/s. • Same momentum • Does the moving cart have impulse? • No. Impulse is something the cart experiences • Does a moving cart have momentum? • Yes • For the same force, which cannon imparts a greater impulse to a cannonball – a long cannon or a short one? • Long cannon. Force acts over a longer time Impulse and Momentum • Impulse = change in momentum • Ft = ∆(mv) Increase Momentum • Increase the momentum of an object as much as possible, apply the greatest force as long as possible • For example, a golfer and a baseball player will swing as hard as possible and follow through • Following through extends the time of contact Exercise Decrease Momentum • Why is it better to hit a haystack with a runaway car or a concrete wall to stop? • You need the same impulse to stop the car Answer: • Why is it better to hit a haystack with a runaway car or a concrete wall if you want to stop? • You need the same impulse to stop the car • By hitting the haystack instead of the wall, you extend the time during which your momentum is brought to zero Decrease Momentum • Break bricks by a large impulse over a short time • http://www.youtube.com/watch?v=xwAu0KXTuj Law of conservation of momentum • In the absence of an external force, the momentum of a system remains unchanged • http://www.youtube.com/watch?v=UwWVEpjAI Dg&feature=related Net Momentum • Net momentum (before collision) = Net momentum (after collision) Types of collisions • Elastic collision – all the momentum is transferred to another object during a collision • Inelastic collision – Colliding objects become entangled Recoil Velocity of a gun • The recoil velocity caused by a gun exactly balances the forward momentum of the projectile • http://www.youtube.com/watch?v=X8imjiuOx1c Exercise Question: • If a freight car travelling at 10 m/s hits a car at rest hits another freight car with the same mass and the frieght cars become stuck together, what will be the velocity of the stuck cars? Answer: • If a freight car travelling at 10 m/s hits a car at rest hits another freight car with the same mass and the frieght cars become stuck together, what will be the velocity of the stuck cars? • (net mv) (before) = (net mv) (after) • (m x 10 m/s) = (2m x v) • v = 5 m/s • http://dev.physicslab.org/Document.aspx?doctype =5&filename=Compilations_NextTime_Moment um.xml Homework • http://www.youtube.com/watch?v=DI3rPT_9rW0 • http://www.youtube.com/watch?v=DYMTAZrKL cM Any Questions?