physics problem stuck again (1 Viewer)

SWJJ

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A 1040-kg van, stopped at a traffic light, is hit directly in the rear by a 797-kg car traveling with a velocity of +3.50 m/s. Assume that the transmission of the van is in neutral, the brakes are not being applied, and the collision is elastic. What is the final velocity of (a) the car and (b) the van?

Ok so I have conservation of momentum and conservation of KE

so:

MvVv1 + McVc1 = MvVv2 + McVc2

The Vv1 is 0 so:

McVc1 = MvVv2 + McVc2
-McVc2 -McVc2
----------------------------------------------

Mc ( Vc1 - Vc2) = MvVv2
/Mv /Mv


so:
Vv2 = Mc/Mv (Vc1 - Vc2)



and I can sub Vv2in the KE conservation formula:
Mv(Vv1) ^2 + Mc(Vc1)^2 = Mv(Vv2)^2 + Mc(Vc2)^2

Vv1 = 0

so when I sub Vv2 what does that give me?
 

Eeyore

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This is exactly why I am a business student.
 

superchuck500

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I just want to say that I enjoy these threads. My knowledge of physics and the equations is nil but I think its cool that someone on here always knows the answer and can explain it.
 
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SWJJ

SWJJ

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One of the many reasons I love SR.com, Any question can be answered on SR.com
 

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There is a house.

Some snow is on the roof.

The sun is shining brightly above the house.

A very large bird flies overhead.

<font size="5">Draw it!</font>
 

Richard

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The final velocity of both the car and the van are zero. After the crash, they come to a stop until the cops show up. Guess you can tell the guys who took logic instead of physics. :ezbill:
 
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SWJJ

SWJJ

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I was able to discover a formula Vc2 = (Mc-Mv / Mc+Mv) (Vc1)
and Vv2 = (2Mc / Mc + Mv ) (Vc1)


It gives me the correct ancswers but I have NO IDEA how the formulas were derived...



next stumper:

A projectile (mass = 0.271 kg) is fired at and embeds itself in a target (mass = 2.50 kg). (The target is initially stationary). The target (with the projectile in it) flies off after being struck. What percentage of the projectile's incident kinetic energy does the target (with the projectile in it) carry off after being struck?
 

Jonesy77

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First, you have to assume a spherical van. Then plug in what you know into F=MA. Simple
 

saintsfan342000

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You have to conserve momentum and energy in this problem, because an elastic collision, by definition, is one in which energy is conserved. Momentum is conserved in all ideal collisions.

So write your basic momentum conservation eqn which it looks like you've done, and write your kinetic energy conservation equation as well. You wind up with a 2 unknown, 2 equation, and easily solvable non-linear system. Solve it algebraically - should be simple enough, or if you have a graphing calculator, solve it graphically.
 

wbbigtymer

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You have to conserve momentum and energy in this problem, because an elastic collision, by definition, is one in which energy is conserved. Momentum is conserved in all ideal collisions.

So write your basic momentum conservation eqn which it looks like you've done, and write your kinetic energy conservation equation as well. You wind up with a 2 unknown, 2 equation, and easily solvable non-linear system. Solve it algebraically - should be simple enough, or if you have a graphing calculator, solve it graphically.
Yup, this sounds about right. It's been a year since I've taken dynamics, but this is a pretty simple problem. Use conservation of momentum and remember your sign convention.
 
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SWJJ

SWJJ

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I already solved it

now I need:
A projectile (mass = 0.271 kg) is fired at and embeds itself in a target (mass = 2.50 kg). (The target is initially stationary). The target (with the projectile in it) flies off after being struck. What percentage of the projectile's incident kinetic energy does the target (with the projectile in it) carry off after being struck?
__________________
 

Devildog

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My head just asploded.

And I thought it was hard figuring out Ohm's Law.
 

Superfan

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I need the formulas for the second problem. You have webassign or something or so sort of internet homework due?
 

austinwes

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You have to conserve momentum and energy in this problem, because an elastic collision, by definition, is one in which energy is conserved. Momentum is conserved in all ideal collisions.
In all processes, ideal or real, both momentum and energy are conserved, so says modern physics. The question is what form is the energy in and where does it go.

As for the projectile problem, start with conservation of mass. Mp*Vp+Mt*Vt=(Mp+Mt)*Vboth because they are merged after the collision, thus thier velocities are the same. Vp is known, Vt is zero, thus you are left with one variable Vboth (the velocity of both after the collision) which you can solve for.

Then to find the kenetic energy transfered to the target, you know both its mass and velocity, so its 1/2MV^2. You know the kenetic energy of the projectile previous to the collision the same way. From there you can get the percentage.
 
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