physics
posted by sana .
Consider the Sun to be at the origin of an xycoordinate system. A telescope spots an asteroid in the xyplane at a position given by (1.8 1011 m, 3.1 1011 m) with a velocity given by (8.4 103 m/s, 6.3 103 m/s). What will be the magnitude of the asteroid's velocity and it's distance from the Sun at closest approach?

This is a very clever problem. Here is how to do it.
The cross product of the V and R vectors is the rate at which area is swept out by the asteroid, which (according to Kepler's second law) is a constant related to the angular momentum. Compute its value using the experimental measurements.
C1 = R*Vtheta
= [(3.1)(8.4)  (6.3)(1.8)]*10^11
= 14.7*10^11 m^2/s
Vtheta is the velocity component perpendicular to the position vector, R
Another constant of motion is the total energy, which is (1/2)mV^2 GMm/R.
Since m (the asteroid mass) is constant,
V^2/2  GM/R = C2 , another constant. Compute this constant using G, M(solar) and the observed values of the magnitudes of R and V.
At perihelion, the C1 constant is the product of the velocity Vmax and closest approach distance, Rmin
Vmax*Rmin = C1
Vmax^2/2  G*M/Rmin = C2
You now have two equations for the two unknowns, Vmax and Rmin. You will need to use the other known constants, M and G 
Could u rewrite the eqn that you used for C2

(V^2/2)  (GM/R) = C2
(Vmax^2/2)  (G*M/Rmin) = C2
It is the sum of kinetic and potential energy per unit mass. Using the first equation and the V and R magnitudes of the observation, you can compute the value of C2. 
i am unable to get the right answer..please help

Sorry. You'll have to fill in the blanks. I have explained how to do it. Make sure you are using the correct values of G and M, and that your equations are dimensionally consistent.

what drwls did makes no sense..