# Homework Help: Why is the momentum not conserved

1. Apr 28, 2012

### Probie1

1. The problem statement, all variables and given/known data

Two vehicles collide one has a velocity of .0246 m/s the other a velocity of 4.374 m/s at impact. After impact one has a velocity of .49 m/s and the other a velocity of .91 m/s.

2. Relevant equations

(p) = mv

3. The attempt at a solution

(p)1 = 13158 * -0.0246
(p)1 = -323.68

(p)2 = 1726 * 4.374
(p)2 = 7549.52

at impact momentum = 7225.84 = (p)1 + (p)2

(p)3 = 13158 * -0.49
(p)3 = -6447.42

(p)4 = 1726 * 0.91
(p)4 = 1570.66

after impact momentum = -4876.76 = (p)3 + (p)4

So Iam missing 12102.6 kg*m/s. Why is this like this...am I missing something?

2. Apr 28, 2012

### cepheid

Staff Emeritus
Your problem statement doesn't actually pose a question. It is literally just a statement.

It also doesn't give the mass of either vehicle. So we have no way of knowing if the masses you assumed are what was intended.

It also doesn't give any directions. Are the two vehicles initially travelling in the same direction or opposite directions?

Basically, we don't have the info we need to evaluate if your work is sensible or not.

3. Apr 28, 2012

### Probie1

Two vehicles collide one has a velocity of .0246 m/s the other a velocity of 4.374 m/s at impact. After impact one has a velocity of .49 m/s and the other a velocity of .91 m/s.

veh1

weight 13158kg
departure 179
impact velocity 0.0246 ms
departure velocity 0.49 ms

veh2

weight 1726 kg
departure 245
impact velocity 4.374 ms
departure velocity 0.91 ms

Veh 2 is heading toward veh1 slightly off of 180 deg.

Last edited: Apr 28, 2012
4. Apr 28, 2012

### Staff: Mentor

Momentum is a vector quantity. Compute the separate components of the momentum for each vehicle both before and after collision. What's the total momentum vector before? After?

5. Apr 28, 2012

### Probie1

Okay,

weight 13158kg
departure 179
impact velocity 0.0246 ms
departure velocity 0.49 ms

Momentum-in Veh1

p = mv
p= 13158 * 0.0246
p = 323.68

Out

p = mv
p= 13158 * 0.49
p = 6447.42

veh2

weight 1726 kg
departure 245
impact velocity 4.374 ms
departure velocity 0.91 ms

Momentum-in Veh2

p = mv
p= 1726 * 4.374
p = 7549.52

Out

p = mv
p= 1726 * 0.91
p = 1570.66

The total before is 7873.2 and after is 8018.08

okay, but I still see a difference in momentum. Am I to include the approach and departure anges in this as well?

Last edited: Apr 28, 2012
6. Apr 28, 2012

### Staff: Mentor

Of course! Momentum is a vector: it has both magnitude and direction; it has components.

Compute the momentum vector components for the vehicles before and after. Find the total momentum vector components for before and after. Then compare them.

7. Apr 28, 2012

### Probie1

weight 13158kg
departure 179
impact velocity 0.0246 ms
departure velocity 0.49 ms

Momentum-in Veh1

p = mv
p= 13158 * 0.0246 * (sin 0 = 0)
p = 0

Out

p = mv
p= 13158 * 0.49 * (sin 179 = .017)
p = 109.60

veh2

weight 1726 kg
departure 245
impact velocity 4.374 ms
departure velocity 0.91 ms

Momentum-in Veh2

p = mv
p= 1726 * 4.374 * (sin 190 = -0.173)
p = -1306.06

Out

p = mv
p= 1726 * 0.91 * (sin 245 = -0.906)
p = -1423.01

Well... this is big time messed up.

Last edited: Apr 28, 2012
8. Apr 28, 2012

### Staff: Mentor

That's the y-component. What happened to the x-component? Your collision is happening the the x-y plane, so vectors will have x and y components.
Again, there should be both x and y components. And where did this angle "49" come from?
I thought the "departure" angle was 179°?

So you still need to sort out the rest of the vector components...

9. Apr 28, 2012

### Probie1

weight 13158kg
departure 179
impact velocity 0.0246 ms
departure velocity 0.49 ms

Momentum-in Veh1

p = mv
p= 13158 * 0.0246 * (sin 0 = 0)
p = 0

Out

p = mv
p= 13158 * 0.49 * (cos 179 = -0.999)
p = -6446.43

veh2

weight 1726 kg
departure 245
impact velocity 4.374 ms
departure velocity 0.91 ms

Momentum-in Veh2

p = mv
p= 1726 * 4.374 * (sin 190 = -0.173)
p = -1306.06

Out

p = mv
p= 1726 * 0.91 * (cos 245 = -0.422)
p = -662.81

The 49 was just a idiot mistake.

10. Apr 28, 2012

### Staff: Mentor

Still waiting to see ALL the components for ALL the momenta. For example, I see a calculation for the y-component of the momentum of the first vehicle for before the collision, but I don't see the corresponding x-component... and I see an x-component calculation for the first vehicle's momentum after the collision, but I don't see its y-component....

11. Apr 28, 2012

### Probie1

weight 13158kg
departure 179
impact velocity 0.0246 ms
departure velocity 0.49 ms

Momentum-in Veh1

p = mv
p= 13158 * 0.0246 * (sin 0 = 0)
p = 0

Out

p = mv
p= 13158 * 0.49 * (sin 179 = 0.017)
p = 112.52

veh2

weight 1726 kg
departure 245
impact velocity 4.374 ms
departure velocity 0.91 ms

Momentum-in Veh2

p = mv
p= 1726 * 4.374 * (cos 190 = -0.98)
p = -7398.53

Out

p = mv
p= 1726 * 0.91 * (cos 245 = -0.422)
p = -662.81

Sorry this is taking so long to grasp.

12. Apr 28, 2012

### Staff: Mentor

Let's concentrate on one particular vector to begin with; How about the initial momentum of the first vehicle?

You've got the:

vehicle mass: m1 = 13158 kg
Speed: v1 = 0.0246 m/s (meters per second --- you had written ms, which is meter seconds)
direction: θ = 0°

The speed is not a velocity; Speed has magnitude but no direction. Velocity has both magnitude AND direction. You're given the direction angle for the velocity, so you can determine the TWO components (x and y) for the velocity, and hence the TWO components for the momentum.

What are the x and y components of the momentum for the vehicle?

13. Apr 28, 2012

### Probie1

weight 13158kg
departure 179
impact velocity 0.0246 m/s
departure velocity 0.49 m/s

P1x = m1v1 cos 0 = 13258kg * 0.0246 * 1 = 323.68 kg*m/s
P1y = m1v1 sin 179 = 13158kg * 0.49 * 0.017 = 109.60 kg*m/s

weight 1726 kg
departure 245
impact velocity 4.374 m/s
departure velocity 0.91 m/s

P2x = m2v2 cos 190 = 1726kg * .91 * -0.984 = -1545.52 kg*m/s
P2y = m2v2 sin 245 = 1726kg * 4.374 * -.906 = 1731.28 kg*m/s

I am confused about the x and y ...is the approach of each vehicle on the x and then the seperations on the y or is one veh totally y and the othe totally x ?

Last edited: Apr 28, 2012
14. Apr 28, 2012

### Staff: Mentor

Er, no. The initial velocity has only one angle, namely 0°. The angle 179° pertains to a different velocity, the departing velocity. We're only concerned with the initial velocity here.

Use 0° for BOTH the x and y component calculations for the initial velocity.
A vector in the plane has two components: an x-direction component and a y-direction component. Every velocity in the plane thus has two components. There is ONE angle associated with a given velocity; it's generally the angle between the velocity vector and the positive x-axis. Use that ONE angle to determine BOTH components for that velocity.

If V is the speed and θ is the angle, then Vx = V cos(θ) and Vy = V sin(θ).

15. Apr 28, 2012

### Probie1

weight 13158kg
departure 179
impact velocity 0.0246 m/s
departure velocity 0.49 m/s

Vx = .0246 * cos 0 = .0246
Vy = .49 * sin 179 = .0085

16. Apr 28, 2012

### Staff: Mentor

No!!! The initial velocity HAS ONLY ONE ANGLE. Forget the 179° for now. That comes later, after the collision. Only the 0° angle exists at the moment. An object cannot be heading in two directions at the same instant in time!

The initial velocity of the first vehicle has one angle: zero degrees. That's the ONLY ANGLE!

The initial velocity vector is comprised of two components: One is its x-component, and one is its y-component. The ONE ANGLE along with the magnitude (speed) define the two components. There is the component that is parallel to the x-axis, and the component that is parallel to the y-axis. Use the trig functions cos and sin to extract those two components USING THE SAME ANGLE.

17. Apr 28, 2012

### Probie1

weight 13158kg
departure 179
impact velocity 0.0246 m/s
departure velocity 0.49 m/s

Vx = .0246 * cos 0 = .0246
Vy = .49 * cos 0 = .49

18. Apr 28, 2012

### Staff: Mentor

Why are you using the speed from AFTER the collision? Why are you using the cosine function for both components? You want to find BOTH components of the INITIAL velocity (before the collision) for the first vehicle. The initial velocity has ONLY speed 0.0246 m/s at angle 0°. That's all. No other angles or speeds should enter into consideration here.

Perhaps you are unfamiliar with the use of vectors? In the following diagram a velocity vector V is shown on a set of Cartesian coordinate axes. The velocity vector has magnitude |V| which corresponds to the speed, angle θ with respect to the positive x-axis, and x and y components are depicted in red. Use the appropriate trig function of the angle to extract the individual x and y components of V.

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19. Apr 28, 2012

### Probie1

weight 13158kg
departure 179
impact velocity 0.0246 m/s
departure velocity 0.49 m/s

weight 1726 kg
departure 245
impact velocity 4.374 m/s
departure velocity 0.91 m/s

Vx = .0246 * cos 0 = .0246
Vx = 4.374 * cos 190 = -4.30

Vy = .49 * sin 179 =0.008
Vy = .91 * sin 245 = -0.824

20. Apr 28, 2012

### Staff: Mentor

Those would be the x-components of the initial velocities of vehicles 1 and 2. You should give them distinctive variable names in order to distinguish between them. Something like V1x and V2x.

Now, where are the initial y-components of those same velocities?
Those are the y-components of the final velocities of the of vehicles 1 and 2. You should give them distinctive variable names in order to distinguish between them. Something like U1y and U2y.

Now, where are the final x-components of these same velocities?

21. Apr 28, 2012

### Probie1

weight 13158kg
departure 179
impact velocity 0.0246 m/s
departure velocity 0.49 m/s

weight 1726 kg
departure 245
impact velocity 4.374 m/s
departure velocity 0.91 m/s

V1x = .0246 * cos 0 = .0246
V2x = 4.374 * cos 190 = -4.30
V3x = .49 * cos 179 = -0.489
V4x = .91 * cos 245 = -0.384

V1y = .0246 * sin 0 = 0
V2y = 4.374 * sin 190 = -0.759
V3y = .49 * sin 179 =0.008
V4y = .91 * sin 245 = -0.824

Last edited: Apr 28, 2012
22. Apr 28, 2012

### Staff: Mentor

Okay, I suppose that sorts out all the components. NOTE: you should probably keep an extra decimal place or two in the values of intermediate results to minimize truncation and rounding errors in further calculations.

Now you should be in a position to write the components of the momenta by multiplying the velocity components by the appropriate masses (p = mv).

23. Apr 28, 2012

### Probie1

I hope you mean drop everything before the = sign then multiply the weight with everything after the = sign?

weight 13158kg
departure 179
impact velocity 0.0246 m/s
departure velocity 0.49 m/s

weight 1726 kg
departure 245
impact velocity 4.374 m/s
departure velocity 0.91 m/s

V1x = 13158 * .0246 = 323.6868
V2x = 1726 * -4.30 = -7421.8
V3x = 13158 * -0.489 = -6434.262
V4x = 1726 * -0.384= -662.784

V1y = 0
V2y = 1726 * -0.759 = -1310.034
V3y = 13158 * 0.008 = 105.264
V4y = 1726 * -0.824 = -1422.224

I have to pack it in for the night... thanks for now, will be here tomorrow.
You are the greatest...

Thanks A LOT :)

24. Apr 28, 2012

### Staff: Mentor

After multiplying velocities by mass you get momenta, so you should change the names of the variables accordingly. So, P1y, P2y, P3y,... and so on.
No worries. Pleasant dreams.

25. Apr 29, 2012

### Probie1

Okay... changed everything to momenta (P)

weight 13158kg
departure 179
impact velocity 0.0246 m/s
departure velocity 0.49 m/s

weight 1726 kg
departure 245
impact velocity 4.374 m/s
departure velocity 0.91 m/s

P1x = 13158 * .0246 = 323.6868
P2x = 1726 * -4.30 = -7421.8
P3x = 13158 * -0.489 = -6434.262
P4x = 1726 * -0.384= -662.784

P1y = 0
P2y = 1726 * -0.759 = -1310.034
P3y = 13158 * 0.008 = 105.264
P4y = 1726 * -0.824 = -1422.224