What is Momentum? An object with a lot of momentum is also hard to stop Momentum = p = mv Units: kg∙m/s m=mass v=velocity Momentum is also a vector (it has direction)
How hard is it to stop a moving object? To stop an object, a force must be applied for a period of time. This is called Impulse (J) J = FΔt Units: N∙s F = force (N) Δt = time elapsed (s)
How hard is it to stop a moving object? Using Newton’s 2nd Law we get FΔt= mΔv Which means Impulse = change in momentum
Why does an egg break? or not break? An egg dropped on a tile floor breaks, but an egg dropped on a pillow does not. Why? FΔt= mΔv In both cases, m and Δv are the same. If time (Δt) increases, what happens to the amount of force (F)? Answer: Force decreases! When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time needed to stop the egg; therefore, less force is applied to the egg.
Stand in front of a 1000 kg. truck moving at 1 m/s, or A Thought Experiment: Suppose that you were captured by an evil physicist who gave you the following choice: You must either: Stand in front of a 1000 kg. truck moving at 1 m/s, or Stand in front of a 1 kg. frozen meatball moving at 1000 m/s. …think…
Truck: Truck's momentum = mv = (1000 kg)(1 m/s) = 1000 kg m/s Truck's kinetic energy = 0.5 mv2 = (0.5)(1000 kg)(1 m/s)2 = 500 Joules Meatball: Meatball's momentum = mv = (1 kg)(1000 m/s) = 1000 kg m/s Meatball's kinetic energy = 0.5 mv2 = (0.5)(1kg) (1000 m/s)2 = 500 000 Joules
Using vectors to understand A 120 kg lineman moving west at 2 m/s tackles an 80 kg fullback moving east at 8 m/s. After the collision, both players move east at 2 m/s. Draw vector diagrams for the before AND after situations; draw two diagrams.
Answer to previous slide question www.physicsclassroom.com
impulse = change in momentum www.physicsclassroom.com
Bouncing increases impulse explain... www.physicsclassroom.com
Impulse= change in momentum www.physicsclassroom.com
Elastic collisions: momentum is redistributed or transferred without loss. m1v1o + m2v2o = m1 v1f +m2v2f
Inelastic collisions: when two objects become entangled m1v1o + m2 v2o = (m1+m2)vf
mredvred + mbluevblue = m(red +blue) vf Inelastic Collisions vi = 10m/s vi = 0 (at rest) M M Before Collision p = mv vf = 5m/s M After Collision p = 2mv mredvred + mbluevblue = m(red +blue) vf
Granny & Little Timmy: calculate the final velocity. www.physicsclassroom.com
Law of Conservation of Momentum In the absence of an external force, the momentum of a system does not change. www.physicsclassroom.com
Stopping a Truck Suppose a large truck and a small truck are moving with equal momentum. Which will require more work to stop? Which will require more force to stop in a given distance? http://hyperphysics.phy-astr.gsu.edu/hbase/truckc2.html#c1
More Force to Stop the Little Truck!?? Since the little truck is moving substantially faster it will have more Kinetic Energy!
Impulse http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
YIKES! If the boulder and the boy have the same momentum, will the boulder crush the boy? …do not try this at home! http://sol.sci.uop.edu/~jfalward/physics17/chapter3/chapter3.html
Truck Collision In a head-on collision: Which truck will experience the greatest force? Which truck will experience the greatest impulse? Which truck will experience the greatest change in momentum? Which truck will experience the greatest change in velocity? Which truck will experience the greatest acceleration? Which truck would you rather be in during the collision? http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html#c1
Ride in the BIG truck! http://hyperphysics.phy-astr.gsu.edu/hbase/truckc.html#c2