1. Unit 5: Conservation of Momentum
Force and Momentum and Conservation of Momentum (9-1,9-2, 9-3)
Collisions (9-4, 9-5, 9-6)
Collisions, Center of Mass (9-7,9-8)
Catch-up, Rocket Propulsion, and Quiz Three Review (9-9,9-10)
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2. No class Friday, March 30th! Next test Friday, April 6th
Schedule
No class Friday, March 30th!
Next test Friday, April 6th
Please, work on your problems sets and extra credit.
If you need information to compute your grade send or see me at my office!
11/24/2018
Physics 253

3. Conservation Laws
There are many conservation laws, we’ve already discussed Conservation of Energy.
In this course we will also discuss
Conservation of Momentum
Conservation of Angular Momentum
Many others: charge, baryon number, lepton number…all are a consequence of some fundamental symmetry of nature.
When we combine the conservation of energy, momentum, and angular momentum we can beautifully describe complex systems of objects.
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4. Momentum
Linear momentum is a deceptively simple quantity equal to the product of an object’s mass, m, and velocity, v:
p=mv
Properties of momentum
A vector with same direction as the velocity, which requires a reference frame
Magnitude equal to mv, increases linearly with m and with velocity.
SI unit is kg-m/s but carries no explicit name.
Newton actually did have a name he called it “the quantity of motion”. (Easy to see why it didn’t stick!)
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5. Force and Momentum
A force is required to change magnitude or direction of momentum with respect to time.
Actually this is similar to Newton’s original statement of the 2nd Law: The rate of change of momentum of an object is equal to the net force applied to it.
Or in symbols:
A quick derivation shows the two versions of the 2nd law are equivalent:
Note we assumed that the mass was constant!
Actually the equality of the force to the change of momentum is more general and useful. For instance, if we allow the mass to change we can describe propulsion …
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6. Example: Water Hitting a Car
Water leaves a hose and hits a car at a rate of 1.5 kg/s and a speed of +20 m/s.
What is the force exerted by the water on the car?
What if the water splashes off at -5m/s? Is the force greater or less?
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9. Experimental Conservation of Momentum
Consider head-on collision of two hard balls
Assume no net external forces.
In the 17th century and predating Newton it was found that:
Individual momentum can change
Vector sum of momentum was observed to be constant.
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10. Theoretical Conservation of Momentum
The experimental result can be explained using Newton’s Laws.
Consider two colliding objects
Initial momentum p1 and p2
Final momentum p’1 and p’2
Object 1 exerts a force F on object 2
Object 2 exerts force –F on object 1.
No other significant forces involved.
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12. Example: Goal Line Stand
A 90-kg fullback attempts to dive over the goal line with a velocity of 6.00 m/s. He is met at the goal line by a 110-kg linebacker moving at 4.00 m/s in the opposite direction.  The linebacker holds on to the fullback. 
Does the fullback cross the goal line?
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14. Example: The Kick of a Rifle
What is the recoil velocity of a 5.0-kg rifle that shoots at kg bullet at a muzzle velocity of 120m/s?
What would be the initial velocity of a 75-kg rifleperson?
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16. Example: Billiard Ball Collision in 2-D
A billiard ball moving at 3.0m/s in the x direction strikes a ball of equal mass initially at rest. The two balls move off at 45o wrt to the x axis as shown.
What are the speeds of the two balls after the collision?
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19. Generalization to a System of Objects
The derivation can be extended to any number of objects.
The total initial momentum is:
P = m1v1+m2v2+…+mnvn=Smivi = Spi
Which can be differentiated with respect to time:
dP/dt = d(Spi)/dt = SFi
Where Fi is the net force on the ith object. There are internal & external forces. But remember that internal forces come in equal and opposite pairs so they will cancel in the summation. Thus: dP/dt = SFexternal
Now if the net external forces are zero then the change in momentum of the system is zero and P=constant!
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20. The Law of Conservation of Momentum:
The total momentum of an isolated system of bodies remains constant.
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21. Example: An Exploded Firecracker
A firecracker with a mass of 100g, initially at rest, explodes into 3 parts.   One part with a mass of 25g moves along the x-axis at 75m/s.  One part with mass of 34g moves along the y-axis at 52m/s. 
What is the velocity of the third part? 
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23. Impulse
Billiard balls interact almost instantaneously, certainly in a fraction of a second.
As shown in the figures, when two objects interact the contact force rises rapidly from zero to a maximum and just as quickly falls to zero.
This occurs in a small time interval Dt, during which there is an impulse of force.
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25. Equal to the area under the force versus time curve.
Properties of Impulse
Units are N-s or kg-m/s.
Equal to the area under the force versus time curve.
Convenient to calculate in terms of the average force during an event. Where the average force is defined as the constant force which, if acting over the time interval of the interaction would produce the same impulse and momentum change:
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26. Example: The Impulse of a Jump
Calculate the impulse on a 70-kg person when landing on the ground after jumping from 3.0m.
Estimate the average force exerted on the person’s feet by the ground if landing is
Stiff legged (body moves 0.01m)
With bent knees. (body moves (0.50m)
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27. The final velocity is zero!
Remember:
We can’t get the impulse by the time integral of the force, but we can get it by calculating the momentum.
The final velocity is zero!
The initial velocity can be determined by using energy conservation:
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29. Collisions: Elastic, Inelastic, 2 and 3Dimensional
Next
Collisions: Elastic, Inelastic, 2 and 3Dimensional
No class Friday, March 30th!
Next test Friday, April 6th
11/24/2018
Physics 253