1. This Week Momentum Is momentum in basketball physics? Rockets and guns
How do spaceships work?
Collisions of objects
They get impulses!
Practical Propulsion
11/26/2018
Physics 214 Summer 2015

2. Momentum F What happens when a force acts on an object
We know v = v0 + at
So that mv – mvo = mat
mv – mvo = Ft
The quantity mv is known as the momentum p and it is a vector quantity in the same direction as the velocity. The result of applying a force for a time t results in a change of momentum.
So a body moving with velocity v has kinetic energy = 1/2mv2 and
momentum = mv F
11/26/2018
Physics 214 Summer 2015

3. Conservation of Momentum
If we have a collision between two objects then in general the velocity of each object changes and kinetic energy is lost in the form of heat, sound or in a permanent deformation of the bodies.
For a short interval of time t we have mv – mvo = Ft and
p = F t
This is the impulse equation F2 F1
F1 = -F2 and Δp1 + Δp2 = 0
Total momentum is conserved
11/26/2018
Physics 214 Summer 2015

4. 1N-06 Equality of Momentum
Two cylinders are exploded apart by a spring
What happens when the spring is released ?
First case: Both identical
Second case: One much heavier
Use Momentum conservation m1v1 = m2v2
The height reached is an indication of the initial speed since 1/2mv2 = mgh
v = sqrt (2gh)
THE SPRING FORCE THAT DRIVES THEM APART IS INTERNAL TO THE SYSTEM, SO THE NET MOMENTUM REMAINS ZERO.
SINCE THE METAL CYLINDER IS HEAVIER IT FLIES AWAY WITH A SMALLER VELOCITY TO CONSERVE MOMENTUM
11/26/2018
Physics 214 Summer 2015

5. What happens if we use the big ball?
1N-12 Fun Balls
An enlarged version of the Classic Toy - The Array of Steel Balls
First case pull back one ball and release
What happens if we use the big ball?
What happens when more of the balls are pulled back than are left at rest?
The collision is nearly elastic so we can use both momentum conservation and kinetic energy conservation
What happens for large ball ? It can be shown that if the larger ball is initially at rest and the smaller ball made to collide with it, the larger ball will come to rest every other collision. On the large ball, small ball collision, it can be shown algebraically that the results are independent of what the masses are. It’s just that when the masses are equal, the stop occurs every time, in addition to the every other time.
NO MATTER HOW MANY BALLS ARE PULLED BACK, THE SAME NUMBER RECOIL AT THE SAME SPEED.
11/26/2018
Physics 214 Summer 2015

6. 1N-04 Conservation of Linear Momentum
Two carts move under tension of weight on frictionless track
The initial momentum of the carts is zero and they each feel equal and opposite forces. So at any time the net momentum will be zero.
Is momentum conserved in this system ?
0 = mAvA – mBvB  mAvA = mBvB T
vA / vB = mB / mA
dA/dB = (vAtA) / (vBtB)
If, tA = tB dA/dB = mB / mA
IF THE MASSES ARE IN INVERSE RATIO TO THE INITIAL DISTANCES, THE CARTS WILL ARRIVE AT THE STOPS SIMULTANEOUSLY.
11/26/2018
Physics 214 Summer 2015

7. Isolated systems In each case m1v1 = - m2v2 11/26/2018
anim0010.mov
11/26/2018
Physics 214 Summer 2015

8. Momentum conservation
The conservation of momentum also is
connected to the fundamental physical laws
The laws of physics do not change under
translation or rotation in space
11/26/2018
Physics 214 Summer 2015

9. Impulse FΔt = Δp. FΔt is called an impulse
We have seen that a force F acting for a time Δt changes momentum
FΔt = Δp FΔt is called an impulse
There are many situations where it is more useful to use the impulse when two objects collide than equating the change in momentum of each. In the case shown below the momentum of the earth changes but that is too difficult to calculate. It is better to use the fact that the earth exerts a force for a short time.
11/26/2018
Physics 214 Summer 2015

10. Collisions
In a closed, isolated system containing a collision, the linear momentum of each colliding object can change but the total momentum of the system is a constant.
This statement is true even if energy is lost by the colliding bodies *
11/26/2018
Physics 214 Summer 2015

11. Types of collision Elastic - no energy is lost
Inelastic - Energy is lost (transformed)
Perfectly inelastic – objects stick together
In two dimensions momentum is conserved along the x and y axes separately y x
11/26/2018
Physics 214 Summer 2015

12. Perfectly Inelastic
Let initial velocity be vi and the mass of a car be m.Then
mvi = 3mvfinal and vfinal = vi/3
Kinetic Energy before = 1/2mvi2
Kinetic energy after = ½ x 3mv2final
So KEbefore/KEfinal = 3
11/26/2018
Physics 214 Summer 2015

13. Head on Elastic collisions
Pool ball collisions are close to being elastic v v
Pool balls v
Bowling ball hits tennis ball
Tennis ball hits bowling ball
11/26/2018
Physics 214 Summer 2015

14. 1N-10 Elastic & Inelastic Collisions
Elastic and Inelastic collisions of Two identical Carts on a Frictionless Track
How do the collisions compare?
Conservation of momentum
mvA + mvB = mvA’ + mvB’
Conservation of Energy (Elastic)
½ mvA2 + ½ mvB2 = ½ mvA’2 + ½ mvB’2
If vB = 0 then vA’ = 0 and vB’ = vA
Completely inelastic (two carts stick)
if vB = 0 then vAB = ½ vA
WE CAN MEASURE THE SPEED BY TIMING THE CARTS ACROSS A FIXED DISTANCE. For THE INELASTIC CASE HALF THE VELOCITY IMPLIES IT SHOULD TAKE TWICE THE TIME.
11/26/2018
Physics 214 Summer 2015

15. 1N-02 Collision of Two Large Balls
What happens when two large balls of equal mass collide one is at rest at the other has velocity v1?
Conservation of momentum
mv1 + mv2 = mv1A + mv2A
 v1 = v1A + v2A
Conservation of Energy (Elastic)
½ mv12 + ½ mv22 = ½ mv12 + ½ mv22
 v12 = v1A2 + v2A2 v1A = 0 & v2A = v1
Can we predict the velocities of each ball after a collision ?
Completely Inelastic collision (stick together)
mv1 + mv2 = (m + m)vA (v2 =0)
 v1 = 2vA vA = ½v1
In practice some energy is always lost. You can hear the noise when they hit and there will be some heat generated at impact
11/26/2018
Physics 214 Summer 2015

16. 1N-05 Elastic Collision (Magnets)
Two Magnets collide with like poles facing each other
We know that the magnets repel each other, so they will not touch. So is this a collision? S v
Rest
What does it mean for objects to ‘touch’ ?
MOMENTUM TRANSFER AND CONSERVATION REQUIRE ONLY THAT THERE BE A MUTUAL INTERACTION.
AT THE MICROSCOPIC LEVEL, ALL CONTACT INVOLVES ELECTROMAGNETIC INTERACTIONS.
11/26/2018
Physics 214 Summer 2015

17. Collisions of Particles
Both energy and momentum and charge are conserved in elementary particle interactions and are a powerful tool in analyzing the fundamental physics.
There are also other important conservation laws that for example prevent the proton from decaying.
11/26/2018
Physics 214 Summer 2015

18. Summary of Chapter 7 F1 = -F2 and Δp1 - Δp2 = 0
The action of a force changes the momentum of an object
mv – mvo = Ft p = mv and is a vector
When two bodies interact they feel equal and opposite forces
F1 = -F2 and Δp1 - Δp2 = 0
Total momentum is conserved
11/26/2018
Physics 214 Summer 2015

19. Summary: Impulse FΔt = Δp is the impulse equation and is
used to determine the momentum
change when a force acts
11/26/2018
Physics 214 Summer 2015

20. Practical Propulsion
A space vessel requires an engine and fuel or an external force. It has to be able to maneuver and be able to escape from the gravity of all objects that affect it’s path. Or, for example, to leave the moon after landing.
Conventional rockets are ~90% fuel by weight most of which is used escaping from the earth.
Nuclear engines are used in deep space probes. In this the radioactive decay of the fuel emits particles which eject “backwards” and give the probe momentum. In this case it takes a long time to achieve high velocities
Solar sails have been tested which use enormous sails pushed by the solar wind of particles. This technique has limited application
Remember the nearest star is 4 light years away that is ~ 2 x 1013 miles or ~200,000 times the distance to the sun (ONE WAY!!)
Voyager just left the solar system after 36 years!!
11/26/2018
Physics 214 Summer 2015

21. Light Sail Solar sail
11/26/2018
Physics 214 Summer 2015

22. Questions Chapter 7
Q5 Are impulse and momentum the same thing? Explain.
No impulse changes momentum
Q6 If a ball bounces off a wall so that its velocity coming back has the same magnitude that it had prior to bouncing:
A. Is there a change in the momentum of the ball? Explain.
B. Is there an impulse acting on the ball during its collision with the wall? Explain.
A. Yes momentum is a vector
B. Yes a force acts for a short time
11/26/2018
Physics 214 Summer 2015

23. Q9 What is the advantage of an air bag in reducing injuries during collisions? Explain using impulse and momentum ideas.
It increases the time over which the force acts.
It also spreads the force over a larger area
Q11 If you catch a baseball or softball with your bare hand, will the force exerted on your hand by the ball be reduced if you pull your arm back during the catch? Explain.
Yes. The impulse is the same but the impact time is longer.
From a work point of view the kinetic energy = Fd so increasing d
reduces F
11/26/2018
Physics 214 Summer 2015

24. Q17 A compact car and a large truck have a head-on collision
Q17 A compact car and a large truck have a head-on collision. During the collision, which vehicle, if either, experiences:
A. The greater force of impact? Explain.
B. The greater impulse? Explain.
C. The greater change in momentum? Explain.
D. The greater acceleration? Explain.
A. The forces are equal and opposite
B. The impulse for each is the same
C. The momentum changes are equal and opposite
D. F = ma so a is larger for the compact car
Q22 Is it possible for a rocket to function in empty space (in a vacuum) where there is nothing to push against except itself?
Yes. It ejects material at high velocity and momentum conservation
means the rocket recoils
11/26/2018
Physics 214 Summer 2015

25. Q23 Suppose that you are standing on a surface that is so slick that you can get no traction at all in order to begin moving across this surface. Fortunately, you are carrying a bag of oranges. Explain how you can get yourself moving.
Throw the oranges opposite to the direction you wish to move
Q24 A railroad car collides and couples with a second railroad car that is standing still. If external forces acting on the system are ignored, is the velocity of the system after the collision equal to, greater than, or less than that of the first car before the collision?
The velocity after is exactly half
11/26/2018
Physics 214 Summer 2015

26. Ch 7 E 2 What is the momentum of a 1200 kg car traveling at 27 m/s?
P= mv = (1200 kg)(27 m/s)
P = kg m/s M v
11/26/2018
Physics 214 Summer 2015

27. Ch 7 E 6 A ball experiences a change in momentum of 9.0 kg∙m/s.
What is the impulse?
b) If the time of interaction = 0.15 s, what is the magnitude of the average force on the ball?
a) Impulse = p = 9.0 kg m/s
b) Impulse = Ft, F = 9/0.15 = 60N
11/26/2018
Physics 214 Summer 2015

28. Ch 7 E 8
A ball has an initial momentum = 2.5 kg m/s, it bounces off a wall and comes back in opposite direction with momentum = -2.5 kg m/s
a) What is the change in momentum of the ball?
b) What is the impulse?
a) Δp = pf – pi = -2.5 – (+2.5) = - 5kgm/s
Pi = 2.5kgm/s
Pf = -2.5kgm/s F +
b) Impulse = Δp = - 5kgm/s
11/26/2018
Physics 214 Summer 2015

29. Ch 7 E 10 M1 and M2 collide head on
a) Find initial momentum of M1 and M2
b) What is the total momentum of the system before collision?
c) Ignore external forces, if they stick together after collision, which way do the masses travel?
6.0m/s
3.5m/s
M1 = 100kg
M2 = 80kg
east
west
a) p1 = -100 x 3.5 = - 350kgm/s p2 = 80 x 6 = 480kgm/s
b) Total momentum = 480 – 350 = 130kgm/s east
c) The masses will travel east with p = 130kgm/sec
11/26/2018
Physics 214 Summer 2015

30. Ch 7 E 16 M1 = 4000kg v1 = 10m/s due north
M2 = 1200kg v2 = 20m/s due south
Masses collide and stick.
Find initial momentum of each mass.
Find size and direction of momentum after collision. N S M2 M1 v2 v1 +x
Call due north the +x direction
p1 = m1 v1 = 4000 kg (10m/s) = kg m/s
p2 = m2 v2 = 1200 kg (-20m/s) = kg m/s
p = p1 + p2 = kg m/s
This is momentum of system before collision, but momentum is conserved. So after masses stick:
p = kg m/s due North.
11/26/2018
Physics 214 Summer 2015

31. Ch 7 E 18 A truck of mass 4000kg and speed 10m/s collides at right
angles with a car of mass 1500kg and a speed of 20m/s.
Sketch momentum vectors before collision
Use vector addition to get total momentum of system before collision. y x
p1 = 40000kgm/s + y
p2 = 30000kgm/s + x b)
p1 = 40000
p2 = 30000 p
p2 = p12 + p22
P = 50000kgm/s
11/26/2018
Physics 214 Summer 2015

32. Ch 7 CP 2
A bullet is fired into block sitting on ice. The bullet travels at
500 m/s with mass kg. The wooden block is at rest with a
mass of 1.2 kg. Afterwards the bullet is embedded in the block.
a) Find the velocity of the block and bullet after the impact (assume momentum is conserved).
b) Find the magnitude of the impulse on the block of wood.
c) Does the change in momentum of the bullet equal that of wood?
pfinal = pinitial = (0.005 kg)(500 m/s)
pfinal = (Mbullet + Mwood)v = 2.5 kg m/s
v = (2.5 kg m/s)/(1.205 kg) = 2.07 m/s m M
No friction (ice)
b) Impulse = Δp = pfinal – pinitial
= (1.2 kg)(2.07 m/s) – 0 = 2.50 kg m/s
Δp for bullet = (0.005 kg)(500 m/s) – (0.005 kg)(2.07 m/s)
= 2.50 kg m/s
Momentum is conserved, so momentum lost by bullet is gained by wood.
11/26/2018
Physics 214 Summer 2015

33. Ch 7 CP 4 Car travels 18 m/s and hits concrete wall. Mdriver = 90 kg.
Find change in momentum of driver.
What impulse produces this change in momentum?
Explain difference between wearing and not wearing a seat belt.
a) When driver comes to a stop his p = 0.
p = 0 – (18 m/s)(90 kg) = kg m/s v p
b) Impulse = p = kg m/s
Impulse = Ft
With seat belt: t is large and F is spread over torso of driver
Without seat belt: t is small. This makes F much larger (F = Impulse/t).
11/26/2018
Physics 214 Summer 2015