physics problem stuck again (2 Viewers)

[austinwes]

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.

 

[NCSaint]

I have no idea but it seems the final velocity would be 0 in all cases. Just a guess.

 

[tomwaits]

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?

Velocity cannot be measured in m/second. Velocity In physics is defined as the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it.

We do not have the exact direction for this equation.

 

[wbbigtymer]

I have no idea but it seems the final velocity would be 0 in all cases. Just a guess.

Well, no--by "final" velocity, they're really asking for the velocity of both vehicles immediately after impact, and you know they're two different velocities because it's an elastic collision.

 

[wbbigtymer]

Velocity cannot be measured in m/second. Velocity In physics is defined as the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it.

We do not have the exact direction for this equation.

Direction is just signified by the sign convention in these problems. In the standard engineering style solution, you define your coordinates (e.g. "+" to the right and "-" to the left) and from there on out, that's how you communicate the vector's direction.

Of course, this is really just a 1-D problem. If there were an angle or something, the final velocity would have to be reported along with an angle with respect to an axis.

 

[NCSaint]

Well, no--by "final" velocity, they're really asking for the velocity of both vehicles immediately after impact, and you know they're two different velocities because it's an elastic collision.

But how do you define immediate then? Like like I said I really have no idea so just asking.

 

[wbbigtymer]

But how do you define immediate then? Like like I said I really have no idea so just asking.

Basically before friction and other conservative forces have a chance to affect the velocity after the collision.

 

[MackeyM]

But how do you define immediate then? Like like I said I really have no idea so just asking.

In this case, the inelastic collision, immediate would be when the velocity, and thus the kinetic energy, of the second mass is no longer zero.

tomwaits, you are correct, but in most fundamental mechanics problems, unless a direction is explicitly defined (thus denoting a vector quantity), then it's assumed to mean the magnitude of the vector quantity.

9.78% is the answer he's looking for in the inelastic collision problem.

 

[SWJJ]

In this case, the inelastic collision, immediate would be when the velocity, and thus the kinetic energy, of the second mass is no longer zero.

tomwaits, you are correct, but in most fundamental mechanics problems, unless a direction is explicitly defined (thus denoting a vector quantity), then it's assumed to mean the magnitude of the vector quantity.

9.78% is the answer he's looking for in the inelastic collision problem.

9.78 is correct, but I dont understand why.

 

[MackeyM]

9.78 is correct, but I dont understand why.

It is due to the ratio of KE before and after,

 

[2fya]

This is exactly why I am a business student.

same here. my first 2 years at UL Lafayette, i was in mechanical engineering. i couldnt pass physics 202 and calculus 3 (math 302) after 2 semesters of trying. i thought that any field i would study would be this hard. but i switched to finance 2 years ago, and it is an absolute breeze and i love it. after seeing what my friends in nursing and engineering have to go through, they should all be awarded $6,465,298 when they graduate. seriously, you have to be really smart and dedicated to the books to make it through those curriculums